Tracking Detectors

Transcrição

Tracking Detectors

FAIR/NUSTAR/R3 B/TDR Tracking Detectors
Technical Report for the Design,
Construction and Commissioning of the
Tracking Detectors for R3B
November 2014
Si
Fiber 4
Fiber 5
Target
GLAD
Strawtubes
TOF13
R3 B
Name
E-Mail
Thomas Aumann
[email protected]
Thomas Nilsson
[email protected]
Bjoern Jonson
[email protected]
Roy Lemmon
[email protected]
Heiko Scheit
[email protected]
Olof Tengblad
[email protected]
Roman Gernhäuser
[email protected]
Haik Simon
[email protected]
Tracking Detectors WG Convener
Stefanos Paschalis
[email protected]
Deputy
Anatoly Krivshich
[email protected]
Spokesperson
Deputy
Scientific Director
Deputy
Project Manager
Technical Coordinator
Deputy
GSI Contact
The R3B Collaboration
Canada
Saint Mary University: Rituparna Kanungo
Croatia
RBI Zagreb: Zoran Basrak, Igor Gasparic
France
CEA Bruyeres le Chatel: Audrey Chatillon, Benoit Laurent, Julien Taieb
CEA Saclay: Anna Corsi, Wolfram Korten, Alexandre Obertelli, Emauel Pollacco,
Clementine Santamaria
GANIL: Olivier Sorlin, Marine Vandebrouck
IPN Orsay: Laurent Audouin, Thomas Gorbinet, Laurent Tassan-Got
Germany
Extreme Matter Institute: Johann Isaak, Deniz Savran, Joel Silva
GSI Darmstadt: Konstanze Boretzky, Christoph Caesar, Peter Egelhof, Hans Geissel, Michael Heil, Aleksandra Kelic-Heil, Oleg Kiselev, Nikolaus Kurz, Daniel Körper,
Ralf Plag, Dominic Rossi, Haik Simon, Felix Wamers
Goethe University Frankfurt: Clemens Beinrucker, Anne Endres, Jan Glorius, Kathrin
Göbel, Tanja Heftrich, Christoph Langer, Moritz Pohl, Rene Reifarth, Kerstin
Sonnabend
Helmholtz-Zentrum Dresden-Rossendorf: Daniel Bemmerer, Thomas Cowan, Stefan
Reinicke, Andreas Wagner
TU Darmstadt: Leyla Atar, Thomas Aumann, Gregor Dentinger, Marc Duchêne,
Guillermo Fernández Martı́nez, Anna-Lena Hartig, Sebastian Heil, Marcel Heine,
Matthias Holl, Ilja Homm, Andrea Horvat, Alexander Ignatov, Stoyanka Ilieva, Jacob Johansen, Julian Kahlbow, Robert Kissel, Thorsten Kröll, Bastian Löher, Kenjiro Miki, Alina Movsesyan, Valerii Panin, Stefanos Paschalis, Marina Petri, HanBum Rhee, Heiko Scheit, Fabia Schindler, Philipp Schrock, Ina Syndikus, Joachim
Tscheuschner, Hans Törnqvist, Mirko von Schmid
TU Dresden: Tobias Reinhardt, Kai Zuber
2
TU München: Michael Bendel, Roman Gernhäuser, Benjamin Heiss, Philipp Klenze,
Tudi Le Bleis, Dennis Mücher, Sebastian Reichert, Patrick Remmels, Max Winkel
University of Cologne: Andreas Hennig, Jan Mayer, Lars Netterdon, Simon Pickstone, Andreas Zilges
Hungary
ATOMKI Debrecen: Zoltan Elekes, Attila Krasznahorkay
Eötvös Lóránd University: Akós Horváth
Lithuania
University of Vilnius: Arnoldas Deltuva
Netherlands
KVI-CART: Muhsin Harakeh, Nasser Kalantar-Nayestanaki, Catherine Rigollet
Portugal
Instituto Superior Tecnico, University of Lisboa: Raquel Crespo
Nuclear Physics Center, University of Lisbon: Daniel Galaviz Redondo, Ana Henriques, Pamela Teubig
Romania
Institute of Space Sciences: Maria Haiduc
Russia
JINR Dubna: Andrey Bezbakh, Andrey Fomichev, Mikhail Golovkov, Alexander
Gorshkov, Sergey Krupko
NRC Kurchatov Institute Moscow: Leonid Chulkov, Alexey Korsheninnikov, Evgeny
Kuzmin, Evgenii Nikolskii, Victor Sarantsev, Vasily Volkov
PNPI Gatchina: Georgij Alkhasov, Dmitri Balin, Leonid Batist, Alexander Dobrovolsky, Andrey Fetisov, Nikolay Gruzinsky, Alexander Inglessi, Alexey Khanzadeev,
Guerman Korolev, Anatoly Krivshich, Viacheslav Kuznetsov, Evgeny Maev, Dmitrii
Maisuzenko, Evgeny Orishchin, Lev Uvarov, Vladimir Vikhrov, Andrey Zhdanov
Spain
3
CSIC Madrid: Marı́a José Borge, Eduardo Garrido, Alejandro Garzon Camacho,
Irene Marroquı́n Alonso, Enrique Nacher, Angel Perea, Guillermo Ribeiro, Olof
Tengblad
Universidad Complutense de Madrid: Luis Fraile
Universidad de Vigo: Enrique Casarejos
University of Santiago de Compostela: Hector Alvarez-Pol, Jose Benlliure, Pablo
Cabanelas Eiras, Dolores Cortina-Gil, Benjamin Pietras
Sweden
Chalmers University of Technology: Andreas Heinz, Håkan Johansson, Björn Jonson, Simon Lindberg, Thomas Nilsson, Ronja Thies
Lund University: Vladimir Avdeichikov, Joakim Cederkall, Claes Fahlander, Pavel
Golubev
United Kingdom
STFC Daresbury Laboratory: Marcello Borri, Alan Grant, Moschos Kogimtzis,
Marc Labiche, Ian Lazarus, Roy Lemmon, Victor Pucknell
University of Birmingham: Martin Freer
University of Edinburgh: Thomas Davinson, Alfredo Estrade, Claudia Lederer, Phil
Woods
University of Liverpool: Marielle Chartier, Scott Lindsay, William Powell, Jim
Thornhill, David Wells
University of Surrey: Wilton Catford
United States of America
Texas A&M University-Commerce: Carlos Bertulani
4
Tracking Detectors Working Group Members
Convener: Stefanos Paschalis, TU Darmstadt, Germany
Deputy: Anatoly G. Krivshich, PNPI Gatchina, Russia
Germany
GSI Darmstadt: C. Caesar, M. Heil, A. Kelic-Heil, O. Kiselev, J. Litvinov, R. Plag,
H. Simon, F. Wamers
TU Darmstadt: T. Aumann, M. Holl, J. Johansen, J. Marganiec, A. Movsesyan,
S. Paschalis, M. Petri, H. Scheit, P. Schrock, I. Syndikus, H. Toernqvist, J. Tscheuschner
TU Munich: R. Gernhaeuser
Russia
PNPI Gatchina: V. Andreev, A. Fetisov, G. Gennady, A.G. Krivshich, D. Maisuzenko
France
CEA/DAM Bruyères-le-Châtel: J. Taieb
5
6
Contents
Executive Summary
11
1 Introduction and Overview
13
1.1
1.2
The
R3 B
setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Tracking Detectors – The backbone of the
R3 B
13
setup for identification
and tracking of beam and beam-like charged particles . . . . . . . .
15
1.2.1
Tracking detectors design goals – general remarks . . . . . . .
15
1.2.2
Tracking detector types . . . . . . . . . . . . . . . . . . . . .
18
2 Physics scenarios: Requirements and design goals for the tracking detectors
21
2.1
High resolution mode: Tracking of heavy ions . . . . . . . . . . . . .
21
2.2
High-acceptance mode: Tracking of light ions - multi-particle decays
24
2.3
High-rate mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
3 Summary of Prototype Results
3.1
Si detector
3.1.1
3.3
3.4
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
In-beam measurements from past LAND/ALADIN experiments
33
3.1.2
Measurements with an α-particle source and calculations . .
36
3.1.3
In-beam measurements from April 2014 and September 2014
runs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
51
Conclusions
. . . . . . . . . . . . . . . . . . . . . . . . . . .
55
Fiber detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
58
3.2.1
Fiber detector with MPPC . . . . . . . . . . . . . . . . . . .
60
3.2.2
Fiber detector with multianode PMT . . . . . . . . . . . . .
65
3.2.3
Comparison between the two fiber detectors . . . . . . . . . .
67
Time-of-flight plastic scintillator wall
. . . . . . . . . . . . . . . . .
68
3.3.1
Prototype developments and results . . . . . . . . . . . . . .
70
3.3.2
Time-resolution simulations . . . . . . . . . . . . . . . . . . .
79
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
88
4 Technical Specifications and Design Details of Tracking Detectors
4.1
27
(before 2013) . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.4
3.2
27
89
Si detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
89
4.1.1
90
Detector size and thicknesses . . . . . . . . . . . . . . . . . .
7
4.2
4.3
4.1.2
Detector technology . . . . . . . . . . . . . . . . . . . . . . .
90
4.1.3
Operation and radiation hardness . . . . . . . . . . . . . . . .
92
4.1.4
Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
4.1.5
Sumarry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
92
Fiber detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
93
4.2.1
The structure of the first four detectors . . . . . . . . . . . .
94
4.2.2
The fifth fiber detector . . . . . . . . . . . . . . . . . . . . . .
95
Time-of-flight plastic scintillator wall
4.3.1
4.4
4.5
. . . . . . . . . . . . . . . . .
96
Technical specifications and design details . . . . . . . . . . .
96
Proton-arm detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
4.4.1
The straw layout in the PAS . . . . . . . . . . . . . . . . . . 102
4.4.2
Assembly, positioning, alignment and commissioning of the
STW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.4.3
Straw Tube Description . . . . . . . . . . . . . . . . . . . . . 107
4.4.4
Detector material budget . . . . . . . . . . . . . . . . . . . . 112
4.4.5
Gas mixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
4.4.6
Influence of magnetic field on the R3 B straw detector operation114
4.4.7
The Gas System . . . . . . . . . . . . . . . . . . . . . . . . . 115
4.4.8
Front-end electronics and read-out . . . . . . . . . . . . . . . 116
4.4.9
The plastic scintillator wall of the proton arm . . . . . . . . . 118
Vacuum chambers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.5.1
All-in-one vacuum chamber for the detectors before the target 119
4.5.2
Vacuum pipe for the fragment arm . . . . . . . . . . . . . . . 119
5 Monte Carlo Simulations
5.1
5.2
121
Monte Carlo Simulations . . . . . . . . . . . . . . . . . . . . . . . . . 121
5.1.1
Simulation
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
5.1.2
Tracking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.1.3
Mass and relative energy
5.1.4
Acceptance for heavy fragments . . . . . . . . . . . . . . . . . 125
5.1.5
Comparison to the existing LAND/ALADIN setup
5.1.6
Efficiency and acceptance of protons . . . . . . . . . . . . . . 127
. . . . . . . . . . . . . . . . . . . . 123
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6 Radiation Environment and Safety Issues
133
6.1
Radiation environment . . . . . . . . . . . . . . . . . . . . . . . . . . 133
6.2
Safety issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133
7 Cost estimate and Funding scheme
135
7.1
Cost Estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
7.2
Funding Scheme . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136
8 Time Schedule Table and Milestones
8
. . . . . 126
137
Bibliography
139
9
10
Executive Summary
Over the past decades new quantum phenomena such as neutron halos and neutron
skins have been observed when the neutron to proton ratio changes dramatically as
compared to that in the valley of β stability. The R3 B (Reactions with Relativistic
Radioactive Beams) collaboration aims at exploring such quantum systems in great
detail and at expanding our understanding of nuclear structure, astrophysics, and
nuclear reactions by probing nuclei at the extremes of isospin and systems beyond
the drip lines. The collaboration is working towards the realisation of the R3 B experiment at FAIR which is designed to perform kinematically complete measurements
of reactions with relativistic radioactive beams with unprecedented efficiency and
resolution. This will open up the way to fully exploit the rarest of the isotopes that
will be available at the FAIR facility.
The R3 B apparatus is located at the high-energy branch behind the Super Fragment
Separator (Super-FRS). The fully stripped ions ranging from Helium up to Uranium
and moving at energies of about 1 AGeV are first selected and identified by the fragment separator before impinging on the secondary target at the entrance of the R3 B
setup. A large acceptance superconducting dipole magnet (GLAD) is responsible
for bending the rigid beams and dedicated detection systems are currently being
developed to surround the target area and measure the target-recoil fragments (Si
tracker, CALIFA callorimeter). A large time-of-flight neutron spectrometer (NeuLAND detector) is foreseen at forward angles to measure the evaporated neutrons
emitted from highly excited or unbound systems.
The R3 B detectors are designed to allow for a greatly versatile system capable of
performing a wide range of experiments. Although the R3 B setup to a large extent
resembles that of the LAND/ALADIN setup at GSI it is designed to deliver not
only significantly increased detection efficiency and resolution but also high-rate
capability and larger rigidity. Over the past decade the collaboration has been
exploring state-of-the-art detector technologies to equip the R3 B setup. Certain
new technologies have been adopted where needed and in other cases in order to
avoid cost intensive and time consuming R&D, we work on optimising established
technologies and expertise already available within the collaboration.
The tracking of the incoming beam as well as that of the outgoing beam-like fragments and the evaporated protons is accomplished with a series of detectors placed
11
before and after the large acceptance dipole magnet. The combined system is referred to as in-beam tracking detectors and constitutes the backbone of the experimental setup delivering charge and mass identification and a full momentum measurement of the beam and beam-like charged particles. The design is largely based
on the existing knowledge and detector technology used in the LAND/ALADIN
setup but with significant upgrades and improvements that will allow to meet the
challenging requirements set by the broad physics program and the intense, highrigidity beams delivered by the Super-FRS. The tracking system after the dipole
magnet can be viewed as consisting of two distinct arms, the heavy-ion fragment
arm and the proton arm. The detector types planned for the tracking system are
silicon detectors for energy-loss and position measurement, thin plastic scintillator
fiber detectors for position measurements, fast scintillator detectors for timing and
energy loss measurements and large-area straw-tube gas detectors for evaporated
protons flying at forward angles through the spectrometer into the proton arm.
The main design goals of the in-beam tracking detectors are to measure the charge
of heavy fragments with a resolution of ∆Z/Z ∼ 0.5% (σ) and their mass and
momentum with a resolution ∆A/A ∼ ∆P/P ∼ 10−3 (σ), to be capable of operating
in a high-rate mode of up to 1 MHz and in a multi-hit mode with large acceptance.
In addition, the detection efficiency of the combined system should exceed 85% for
particles flying within the geometrical acceptances of the detectors.
The present Technical Design Report is organised in the following way. An introduction of the R3 B setup and its main detector systems are presented in Chapter 1,
which also discusses the general design goals of the in-beam tracking detectors system and introduces the different detector types. In Chapter 2 we discuss physics
cases, which highlight the importance of the detection system and justify its flexible
and versatile nature. Prototype results and experience from past LAND/ALADIN
experiments are discussed in Chapter 3 for the different detector types. The technical specifications and the detailed design goals of each detector type in the tracking
system are given in Chapter 4. In Chapter 5 we present the performance results
of the complete system as they are obtained in a detailed Monte-Carlo simulation
for different physics scenarios. Chapter 6 discusses the radiation environment that
the detectors need to withstand and the safety issues. The cost estimate and the
foreseen funding scheme is presented in Chapter 7. Finally, in Chapter 8 we present
the time schedule for completing and commissioning the tracking detectors.
12
1 Introduction and Overview
1.1 The R3 B setup
The Reactions with Relativistic Radioactive Beams (R3 B) experiment is part of the
NUclear STructure, Astrophysics and Reactions (NUSTAR) pillar at the Facility for
Antiproton and Ion Research (FAIR). The key physics investigations of the R3 B program include probing the structure of exotic nuclei via Quasi-Free Scattering (QFS)
reactions on a hydrogen target, studying the evolution of the collective response
of exotic nuclei, exploring quantum phenomena such as neutron halos and neutron
skins, accessing information on unbound systems via the complete kinematical measurement of their decay products.
The strength of the R3 B experiment is the kinematically complete measurement of
reactions with relativistic short-lived ions with energies of up to 1 AGeV. These rare
radioactive isotopes are produced at the beginning of the Super Fragment Separator
(Super-FRS) when the high-energy primary beam is fragmented on the production
target. The separator selects and identifies on an event-by-event basis the ions of
interest and defines the limits of momentum resolution measurement on the order
of 10−4 and the maximum rigidity of the beam at 20 Tm. The R3 B experiment is
planned at the high-energy branch of the Super-FRS.
The key constituents of the R3 B setup, shown in Fig. 1.1 are:
• the large-acceptance superconductiong dipole magnet (GLAD)
• the New Large Area Neutron Detector (NeuLAND)
• the silicon tracker (R3 B-Si-Tracker)
• the photon and particle calorimeter and spectrometer (CALIFA) and
• in-beam tracking detectors for the heavy fragments and evaporated protons
The construction of the GLAD magnet is completed at CEA Saclay and will be
installed in cave C at GSI with all its cryogenic components beginning of 2015. The
NeuLAND detector is currently being build and 20% is completed already. When
fully built it will provide an unprecedented detection efficiency of more than 50% for
four neutrons evaporated from the fast moving fragment. This enables the study of
13
the most neutron rich systems and of multi-neutron correlations. The silicon tracker
has all components developed and is expected to be fully completed by the end of
2015. It is designed to offer, for example, precise tracking and vertex reconstruction
by detecting the two protons originating from a (p,2p) quasi-free scattering reaction.
The CALIFA calorimeter is also under construction with the goal to reach 20%
in the year 2015. Its high granularity, large geometrical coverage and minimum
dead layers between the crystals will offer high efficiency and high resolution for
detecting γ rays and light particles scattered at large angles. For more details on
these systems the reader is referred to the corresponding Technical Design reports
of each system [Neu11, Cal11] and also to the R3 B Technical Proposal [R3B05].
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Figure 1.1: The R3 B setup in its startup version of 2017 with its main components: the silicon tracker R3 B-Si-TRACKER, the calorimeter CALIFA,
the dipole magnet R3 B-GLAD and the neutron time-of-flight spectrometer NeuLAND.
In this report we concentrate on the upgrade of the detection system required for
tracking the incoming beam and the outgoing beam-like charged particles including evaporated protons. As will be discussed in more detail in the next section,
the required momentum resolution (∆P/P ) of the tracking system is about 10−3 .
This enables a mass identification up to the heaviest ions. The resolution of the
total momentum measurement will be adequate to perform nuclear structure studies (∼ 100 MeV/c) for elements up to 1 AGeV Ni (∼ 100 GeV/c). In order to
reach momentum resolutions that match that of the incoming beam from Super-FRS
(∆P/P∼10−4 ) a high-resolution spectrometer is required after the GLAD magnet,
which is beyond the scope of this report.
14
1.2 Tracking Detectors – The backbone of the R3 B setup
for identification and tracking of beam and beam-like
charged particles
The identification and tracking of the incoming beam and the outgoing charged
fragments and protons is obtained using charged-particle detectors for energy-loss,
position and time-of-flight measurements before and after the GLAD magnet. In
the following sections we discuss the general performance requirements for such a
system and the detector types that are used. A schematic layout of the setup is
shown in Fig. 1.2.
1.2.1 Tracking detectors design goals – general remarks
The mass over charge ratio A/Z of the fragment can be calculated combining the
trajectory measurement through the dipole field, giving the magnetic rigidity Bρ,
and the time-of-flight (T oF ) measurement between the start- and stop-time detector,
yielding the velocity β, according to equation:
A
e Bρ
=
·
,
Z
uc βγ
(1.1)
where γ is the Lorentz factor, e is charge of the electron, u is the atomic-mass
unit and c is the speed of light. Once the nuclear charge Z is obtained from the
energy-loss measurements, the relative uncertainty in the mass determination can
be calculated as:
∆A
A
2
=
∆(Bρ)
Bρ
2
4
+γ ·
∆(T oF )
T oF
2
.
(1.2)
As already mentioned above, the Super-FRS will deliver fully stripped ions up to
the Pb-U region and consequently, the challenge will be to separate neighboring
masses in the mass region 200, where the relative difference in mass between two
neighboring nuclei amounts to ∼ 5·10−3 . In order to resolve 1 the masses the relative
uncertainty in mass must be on the order σ < 2 · 10−3 . In other words, in order to
obtain the required mass resolution the uncertainty of the magnetic rigidity (which
can be obtained via trajectory reconstruction of charged fragments) should be less
than 10−3 (σ) and for typical beam energies of 1 AGeV (γ ∼ 2 ) the time-of-flight
uncertainty should be less than 2.5·10−4 (σ). The time-of-flight is measured between
the start detector close to the target and the last detector at the end of the fragment
arm.
1
Assuming a gaussian shape, the masses or charges can be separated if the FWHM is less than
the distance between the two peaks.
15
:&-;8$<.-/,&1'9$
&6$'6$76$8$
1&('.319$
&6$'6$76$8$
/$5&-,$
0123,41
$
&6$'6$76$8
*+$,$
).3'319$
&6$'6$76$8$
!"#$%&'()$
'-./&'$
Figure 1.2: Drawing of the R3 B setup which highlights the heavy-ion tracking detectors and the proton arm detectors. The nominal bending angle of the
heavy fragments is 18 deg and that of the protons is 40 deg.
Knowing the velocity, the trajectory reconstruction of the charged particles can be
achieved with a set of position measurements before and after the dipole magnet.
A typical compromise between resolution, material budget and redundancy in the
measurements leads to a position measurement with a resolution of σx ∼ 200 µm
16
on the target, a position measurement between the target and the magnet with
σx ∼ 100 µm to determine the scattering angle and two position measurements
after the magnet with σx ∼ 100 µm.
Similar arguments hold for tracking of the protons through the proton arm. The
momentum resolution should also be on the order of 10−3 and the material of the
first detector station is kept to a minimum while maintaining some redundancy and
a σx ∼150 µm position resolution.
General remarks on the design goals can be made by the following considerations
and first order estimates based on typical values for the R3 B setup. For a more
detailed investigation of the required detector resolutions, material thicknesses and
the overall obtained performance the reader is referred to Chapter 5.
The determination of the magnetic rigidity is dominated by the angular measurement. For an order of magnitude estimate, we consider a typical bending angle of
the heavy fragments of about 300 mrad; in order to maintain the 10−3 resolution,
an angular resolution better than 0.3 mrad is required. As will be discussed in the
following Chapters, the main contribution to the angular straggling originates from
the detector materials. In addition, the velocity must be measured with adequate
precision. A typical flight path of ∼ 20 m results in a ToF resolution requirement
of σToF < 20 ps.
The nuclear charge of the fragment is calculated from energy-loss measurements
through the detector materials. To first order one obtains from the Bethe-Bloch
formula that the energy loss for fully stripped ions through matter is proportional
to the square of the charge of the ion and inversely proportional to the square of its
velocity. For a charge identification of the heaviest fragments, such as Pb, where Z
and Z-1 are separated (in energy loss) by 2.4% an energy loss measurement with a
resolution of σ∆E <1% is required.
Moreover, the FAIR facility and the Super-FRS will also provide beams with higher
intensities (up to a factor ≈ 100) than presently available. This means that in
certain experiments intensities of up to about 1 MHz could be reached. In the
R3 B setup, the beam tracking detectors “see” the full beam intensity since both
unreacted beam and fragments hit the detectors. Thus, in order to fully exploit the
potential of FAIR beams at the R3 B setup , i.e. heavy beams and high intensities,
the planned detectors must cope with such new and challenging conditions while
maintaining their optimum performance.
Finally, certain experiments require also a multi-hit capability for the detectors after the target. This allows to study multi-particle decays in which the populated
unbound state decays into two or more charged particles. Full four-momentum
17
reconstruction of all decaying particles enables then the reconstruction of the excitation energy. Often the in-flight decaying particles of interest are protons, which
are bent more and deflected to the proton arm after the GLAD magnet where they
are tracked. Typical bending angles are on the order of 0.7 - 0.9 rad and to maintain
again a momentum resolution on the order of 10−3 the angle of the protons must
be measured with a resolution of better than 1 mrad. As for the heavy fragments,
here also the detector material is the dominant factor to the angular resolution.
In order to meet the challenging requirements and serve the wide range of physics investigations planned at R3 B, it is necessary to design a versatile and flexible system.
The tracking system is designed to operate in the following modes: high-resolution
mode, high-rate mode, high-acceptance and multi-hit mode. This often requires the
combination of different detectors and adjustments in their configuration. Particular
cases will be discussed in more detail in the next chapter.
1.2.2 Tracking detector types
The following detector types to be used in the tracking system are based on existing
know-how obtained from the detectors used in the LAND/ALADIN setup but with
significant improvements in terms of resolution, size and rate capability.
Fast plastic scintillator detectors are used for fast-timing purposes. The shape and
type of the plastic scintillator vary depending on the detectors size, which in turn
depends on its position along the beam line. Position measurements are obtained
with a combination of position sensitive Si detectors and plastic scintillator fiber detectors. The energy loss is measured by Si and plastic scintillator detectors. Finally,
a large area gas detector based on the strawtube technology is used for the detection
of evaporated protons.
In front and close to the target a small and thin plastic scintillator (LOS) acts as the
start timing detector whose time resolution is well below any other detector used
in the R3 B setup. For example, recent results showed that a time resolution well
below 10 ps can be achieved for medium-mass ions with a 0.5 mm thick detector.
Its square-shape size of 5×5 cm2 is optimised for direct coupling with 2-inch photomultiplier tubes for maximum light collection. The thickness is carefully chosen
for each experiment to minimise the material in the beam line while maintaining
sufficient light output and excellent time resolution. This detector development has
already been completed and it meets our performance requirements. Therefore, the
details are not discussed further in this report.
Thin Si detectors are used along the beam line before and after the target. Their
purpose is twofold: 1) To obtain with minimum material adequate charge identification and 2) to obtain precise tracking information of the ions that impinge on or
18
emerge from the target. Based on our experience from the LAND/ALADIN setup
the detector types that will be used are position sensitive Si detectors based on
charge division and Si micro-strip detectors. Of particular interest is the Si detector
between the target and the dipole magnet, whose size needs to be 10×10 cm2 in
order to cover the full ±80 mrad acceptance of the magnet, but its thickness should
be kept small, on the order of 100-200 µm, to minimise angular straggling. These
silicon detectors are typically placed at distances less than 1 m from each other.
Their position resolution requirement is on the order of 100 µm (σ) in order to obtain an angular resolution much below the straggling introduced by their materials
(∼0.3 mrad) .
In conditions where the radiation hardness and rate capability of the Si detectors
are inadequate, the position measurement will be performed by a set of plastic fiber
detectors placed perpendicularly to each other. In this case, the energy-loss information just after the target is lost and one relies only on the energy-loss measurement
at the end of the fragment arm. The energy-loss measurement before the target is
performed by a LOS-type detector of adequate thickness to deliver a satisfactory
energy-loss information (1-5 mm thick).
A very thin fiber detector in a single layer (x-position only) is used for the very critical first position measurement after the magnet, which serves as the starting point
for the determination of the deflection angle. The distance between this detector
and the subsequent position measurement is typically several meters, such that the
angular measurement can be very precise even with moderate position resolutions.
However, once again, the angular straggling on the material of this detector is the
dominant factor for the angle measurement.
The time-of-flight measurement is performed between the LOS detector in front of
the target and a large-area-plastic-scintillator wall (TOF13) located at the end of
the fragment arm (typically 20 m downstream from the target). At this position the
fragment’s spatial distribution is very broad and a size of 120×80 cm2 is required to
obtain a satisfactory acceptance. As mentioned above the time-of-flight resolution
requirement can be as demanding as 20 ps, which defines the design specifications for
this detector. The purpose of this wall is, however, twofold as it also acts as a precise
nuclear-charge measuring device for the heavy fragments at the end of their flight
path. Despite the strong position dependencies and non linearities observed in the
energy measurement of large plastic scintillators, detailed investigations have shown
that an excellent charge identification is achieved once these position dependencies
are corrected for. In addition to several improvements and a careful design of the detector (discussed in Chapters 3.3 and 4.3), this detector is also coupled to a position
sensitive detector built with 200 µm width square fibers, which covers its full surface
and deliver a very good position resolution measurement. This position-sensitive detector provides crucial position information for the tracking after the dipole magnet
19
and it can also be used for correcting remaining position dependencies of the TOF13
detector.
Large area gas detectors of up to 2.6x1 m2 , based on straw-tube technology, are used
to measure precisely the position of the evaporated protons that fly into the proton
arm. Their position resolution can be on the order of 100-200 µm and they are
designed to be vacuum compatible, while introducing minimal material budget.
20
2 Physics scenarios: Requirements and
design goals for the tracking detectors
In this section we outline the requirements for the tracking detectors as defined
by the physics scenarios. In general, the tracking detectors are placed before and
after the magnet for obtaining nuclear-charge identification, angles and positions
on target and behind the GLAD magnet for tracking the high-rigidity beam and
the protons. The position and timing tracking allows for an identification in terms
of A/Z. The energy-loss measurement corrected by the velocity of the fragment
provides a nuclear-charge identification.
The scenarios defining the tracking system configurations are:
• the tracking of heavy ions (in this context Z >50, A >100), requiring the setup
in a high-resolution mode.
• tracking of multi-particle (fragment-breakup) decay and in-flight proton evaporation, requiring the setup in a high-acceptance and multi-hit mode.
• high-rate experiments (∼1 MHz), requiring the exclusive use of plastic scintillator detectors.
2.1 High resolution mode: Tracking of heavy ions
The R3 B setup, located at the end of the high-energy branch of the Super-FRS,
will typically perform studies with beam energies of 1 AGeV provided by the FAIR
facility. This gives it a distinct advantage with respect to similar apparatuses around
the world, such as the SAMURAI setup at the RIKEN facility and the future HighRigidity Spectrometer at the FRIB facility, which operate at significantly lower
energies (a factor of three lower). One of the advantages is that at these higher
beam energies it is possible to obtain the heavier ions (around Sn with Z=50 and
above) in a fully-stripped state. When the incoming beam and/or the outgoing
fragments have a charge state which is different from their atomic number, then the
charge and mass identification becomes more challenging. Typically such studies,
without fully stripped ions, suffer from large background and ambiguities. Thus,
21
probing heavier nuclei constitutes one of the advantages aand thus main priorities
of the R3 B collaboration.
We mention here, as an example, some of these physics scenarios that drive the
design of the high-resolution mode.
As new radioactive-ion beam facilities are coming online worldwide, the significantly
improved beam intensities for the rarest of the isotopes give us access to study
these exotic species for the first time. One of the most fundamental measurements
systematically performed in all accessible even-even nuclei is the spectroscopy of
the first 2+ excited state. This state is known to provide a very sensitive probe of
nuclear structure and shape evolution. The highest energies for these states along an
isotopic or isotonic chain are found for doubly magic nuclei, while moving away from
closed shells large deformations and correlations drive the excitation energy of the
state to lower energies. Commonly this measurement is one of the first spectroscopic
information obtained in a nucleus and in many cases it has revealed the dramatic
change of shell structure as one moves away from the valley of β stability. This type
of measurements can be performed even when beam intensities are as low as few
particles per second as long as an efficient γ detector and relatively thick targets are
used. The excited 2+ states can be populated either by inelastic excitation on light
targets or by nucleon removal reactions (for example via (p,pN) reactions). The
commonly performed Coulomb excitation reactions using heavy targets to populate
this state can also be used but, it has a decreasing cross section with increasing
beam energy. Example of cases where these first 2+ states are unknown are e.g. the
isotopes heavier than
214 Pb.
In such experiments, the acceptance requirement of the tracking system is rather
moderate since the outgoing fragments of interest are all located around a similar
A/Z, with typical changes of less than a percent. On the other hand the nuclear
charge and mass resolution of the system has to be as good as possible in order to
allow for clean separation of incoming and outgoing particles that can minimise the
background in the final γ-ray spectrum. The best configuration for such studies is to
position the time-of-flight wall detector as far as possible (20 m) from the target, in
order to maximise the flight path and thus improve the velocity measurement. The
width of this detector (120 cm) defines then the momentum acceptance of the system
to be about ± 10% which covers well the region of interest for these experiments.
Systematic studies of heavy ions are essential for investigating other collective degrees of freedom in nuclei and their evolution towards more exotic systems. For
instance, the low-lying strength below the Giant Dipole Resonance (GDR) identified as Pygmy Dipole Resonance (PDR), carries information about the neutron
skin thickness of nuclei, which in turn is used in calculations of the symmetry term
coefficient in the equation of state. The PDR has been probed in the past within
22
the LAND/ALADIN collaboration as systematic studies of this mode in Sn and
Ni isotopes ([Adr05, Ros13]). The analysis, however, has suffered from the limited
tracking resolution of the setup particularly at the Sn mass region. A more recent
run to probe these excitations in the Sn isotopes has been performed in 2012 with
the use of some of the prototype detectors discussed in this document. The much
thinner detectors and improved position resolution has indeed shown a significant
improvement in the tracking resolution. In these experiments the requirement for the
tracking system is to cleanly identify the mass and nuclear charge of the fragments
and also measure the four-momentum vector of the fragments, which in combination
with the measured evaporated neutron is used to reconstruct the excitation energy
spectrum of the system.
The R3 B setup will also be well suited to study total reaction cross section, as well
as total charge-changing and total neutron-removal cross sections. This information
can be associated to the nuclear and charge radii and the thickness of the neutron
skin. Such studies have already been performed with the R3 B setup. With the
upgraded R3 B tracking detectors it will become possible to extend such studies to
heavier nuclei. In these experiments, the requirements for the tracking detection
system are the same as the ones discussed above.
The acceptance requirement is more challenging in the case of knockout reactions
from deeply bound states. In such reactions the excited nucleus can decay via multiparticle evaporation. In the region of neutron-rich Sn isotopes (with A∼130), for
example, reaction channels where more than ten neutrons or more than six protons
are removed from the projectile require an acceptance larger than ± 10%. This
acceptance limitation becomes much more severe in lighter ions (e.g. one-proton
removal from
12 C),
which, however, require a less demanding mass resolution. For
such experiments one can choose between the high-resolution mode, where the timeof-flight wall is at 20 m from the target, or higher-acceptance mode, where the
time-of-flight wall is moved to about half of that distance and the acceptance of the
heavy-ion tracking system increases to about ± 20% (see also Chapter 5).
Studies with heavy ions constitute a large part of the experimental physics program at R3 B and this is a region where R3 B can excel utilising the high-energy
fully-stripped heavy-ion beams delivered by the FAIR facility and Super-FRS. On
the other hand, as discussed earlier, the tracking of heavy ions poses the greatest
challenge for the tracking detectors. The charge separation can come close to the
percent level requiring a resolution of about 0.4% in charge or equivalently 0.8% in
energy loss measurement. Similarly, the large mass on the order of A=200 or more
sets a requirement for the tracking precision at the level of 10−3 .
23
2.2 High-acceptance mode: Tracking of light ions multi-particle decays
The main challenges set by the light-ion detection are the small energy loss (and
thus worse signal-to-noise ratio), the larger spatial distribution of the fragments and
their decay via multi-particle emission. As in the case of the heavy-ion experiments
discussed above, the experiments with light ions also benefits from the higher beam
energies available at the R3 B setup as compared to the other setups. The main
advantage is the forward focusing of the beam-like reaction fragments. In particular, even in a multi-particle decay with large relative energy between the decaying
particles, the high beam energy will still boost the decaying fragments to forward
angles increasing effectively the acceptance of the setup. In addition to the inherent
advantages offered by the high-energy beams, we also perform the following two adjustments of the setup to tailor the tracking detectors for this mode of operation.
The first adjustment is the replacement of the stripped position sensitive Si detector
(with pitch 3 mm), which is placed after the target, with a micro-strip Si detector
(with a finer strip pitch). With this modification the high granularity of the detector
allows the simultaneous detection of multiple charged fragments that originate from
the in-flight break up of the beam particles even when their relative energy (and thus
spatial separation) is small. As an example it is mentioned that at a distance less
than 1 m away from the target (where this detector is placed) the spatial separation
of emitted charged particles, with energies in the centre of mass on the order of
hundred of keV, is a few mm (depending also on the emitted angles with respect
to the beam direction). The necessary strip pitch to safely separate such hits, e.g.
2-3 mm apart, is about 700 µm.
An example of such studies is the two-proton decay of proton-rich Ne ions which has
been studied in the past in the LAND/ALADIN setup [Wam14]. In Section 5.1.5
we discuss the performance of the new setup with respect to this physics case. The
in-flight evaporated protons are bent by the magnetic field to large angles of about
40 degrees and are detected by the proton arm. The dimensions of the proton-arm
detectors are defined by the acceptance of the dipole magnet (± 80 mrad), and in
the dispersive plane are larger to accommodate the desired momentum acceptance.
We mention here the case of the first proton detector, which is placed at about
5 m downstream from the target. To cover the full acceptance, its dimension in the
non-dispersive plane should be about 1 m. For a 20% momentum acceptance, in
addition to the angular acceptance, its dimension in the dispersive plane should be
2 m.
Another example of experiments studied with the R3 B setup is the excitation of
the incoming light ion and its subsequent decay into more than one α particles.
For example, studies of three-α decays from
24
12 C
ions excited beyond the α-particle
evaporation threshold. The α particles (A/Z = 2) would then fly through the magnet
and can be detected in the fragment arm. For such cases, in addition to the multihit capability of the setup, it is also important to have a larger effective coverage
by the detectors after the magnet. Similarly, for light fragments with A ∼ 10 the
one-proton and one-neutron removal channels already change the magnetic rigidity
by more than ± 10% and thus the bending angle through the magnet also changes
by the same amount.
The second adjustment in this mode, in order to meet these requirements, is to move
the TOF13 detector to half of the nominal distance (i.e. to 10 m from the target),
which increases the momentum acceptance by a factor of two to ± 20%.
An extreme case of this type of experiment is, for example, the decay of an excited
6 He nucleus into all possible fragments such as protons, deuterons, tritons and α
particles. These fragments have an A/Z ratio which ranges from 1 up to 3. In
such cases and considering the momentum spread for each of these particles, the full
momentum acceptance of the magnet is filled and the angles of the particles range
from about 10 degrees up to 50 degrees. Even when the position measurement is
performed right in front of the exit of the dipole magnet, this large angular range
corresponds to a 2 m spread in the dispersion plane. To serve these physics cases,
the proton arm detectors (which can detect Z=1 and 2 with the same gas pressure
and voltage settings) are tilted such that they are perpendicular to the 18 degrees
bending axis and cover the largest possible acceptance in front of the magnet.
2.3 High-rate mode
This mode is dedicated to serve experiments where the beam production rate is not
the limiting factor and beam rates could reach ∼1 MHz. This can be particularly
useful for studies of the less exotic nuclei with high precision (thin targets) and high
selectivity (low cross sections).
As an example we mention here the quasi-free scattering of nucleons at large momentum transfer, which aims to probe the short-range correlations in nucleon-nucleon
pairs as has been reported in (e,e’p) experiments by the JLAB collaboration (see
Ref. [Sub08]). These experiments will first be performed with isotopes at or close to
the valley of stability, where some experimental information already exists, in order
to demonstrate the feasibility of such experiments in inverse kinematics and cross
check the reported observations. Such ions are available at high beam rates. It is
important to utilise the maximum possible rate in order to extract with sufficient
statistics the differential cross section as a function of the momentum transfer.
In many experiments, in which the resolution is of paramount importance, the straggling in the relatively thick targets that are commonly used (hundreds of mg/cm2 )
25
is the limiting factor (e.g. in reconstructing the excitation energy spectrum). The
use of thinner targets will significantly improve the resolution, but will yield lower
luminosity, unless high beam intensities are used.
Finally, secondary cocktail beams including several neighboring isotopes and isotones
give an opportunity for systematic studies along a chain of nuclei, such as for example
evolution of nuclear structure with isospin. Due to the large acceptance dipole
magnet of the R3 B setup, such studies can be performed simultaneously for all
nuclei in the secondary beam. The not so exotic species in the cocktail beam are
produced with high intensity, such as the total beam rate on the secondary target
can be high.
Although the proposed tracking system discussed in this report can accept already
at least a factor of ten more beam rate compared to the predecesor LAND/ALADIN
setup, when the beam rate exceeds 0.2 MHz certain adjustments are necessary. In
particular, for these studies the performance of the proposed silicon detectors will
deteriorate in terms of energy loss (charge identification). There are multiple reasons that set this limit such as pile-up effects becoming more severe, total charge
deposition in case of heavy-ions generating a current of few µA and total integrated
radiation hardness of the detector limiting the lifetime of the detector to less than
one experiment. In such cases, we plan the replacement of the Si detectors with
scintillators. For the charge identification before the target a relatively thick rectangular scintillator (1 - 5 mm thick depending on the required charge resolution) with
a simple four-side photomultiplier readout will be used very similar to the much
thinner LOS detector used for timing purposes. To minimise the background that
such thick material introduces it is planned to be placed at the entrance of the cave
before the first quadrupole. The position and angular information before and after
the target is performed with a set of crossed fiber detectors (for x and y identification) as described in Section 4.2.1. These detectors can deliver very accurate 2D
position information and can operate at high rates but with inadequate energy-loss
resolution for charge identification. As a drawback of this high-rate mode of operation the charge identification after the target will be compromised and one has to
rely on the single energy-loss measurement performed at the end of the setup with
the use of the TOF13 detector. This has the disadvantage that charge changing
reactions that happen in the materials after the target cannot be separated from
the charge changing reactions in the target. On the other hand, cases where one
is interested in neutron-removal cross sections, the single energy-loss measurement
can suffice.
26
3 Summary of Prototype Results
This chapter presents results from in-beam and on-the-bench tests of prototype
tracking detectors and electronic equipment. It also summarises the relevant experience obtained from experiments performed with the LAND/ALADIN setup over
the past 20 years. In particular, we present results and calculations with respect to
Si detectors, plastic fiber detectors, plastic scintillator paddle detectors.
3.1 Si detector
Over the past years we have performed detailed investigations on the use of Si detectors for the detection of relativistic heavy fragments and protons. The type of Si
detectors that we consider in this section are the continuous-area two-dimensional
position sensitive detectors (2D PSD) (Hamamatsu S5378-2 and Micron Semiconductor TL-63), strip position sensitive Si detectors (variants of the Micron Semiconductor X1 type) and Si micro strip detectors (Alpha magnetic spectrometer-AMS
sensor [Alp00]). Fig. 3.1 shows the types of detectors that have been tested. The
results presented are obtained from both α-source and in-beam measurements.
The Si micro strip detector technology is a widely used method for obtaining a very
precise position information. The main idea is that identical components (strips)
are laid along one side (single sided Si strip detectors) or along both sides (double
sided Si strip detectors). The strip width can be as small as few tenths of µm.
By reading out the strips individually one can determine the incident position of a
charged particle impinging on the detector with sub-strip resolution, typically few
tenths of µm, even when the signal generated is just few times above the noise
level. In addition, the very fine segmentation of the detectors results in a very small
capacitance per element which reduces the noise and makes the signal rise time
fast. The total charge generated by a heavy fragment can spread in more than one
neighbouring strips. The total energy-loss measurement is performed by summing
all energies from neighbouring strips.
The operation principle of the position sensitive detector is summarized in Fig. 3.2.
The incident particle looses energy and generates electron-hole pairs which drift
to the corresponding electrode. If a resistive layer is placed on one or both sides
27
Figure 3.1: Photos of the continuous area 2D Si PSD in their mounting frames.
Counting from left to right: First is the Hamamatsu S5378-02 45 ×
45 mm2 detector with a thickness of 300 µm and an anode in each corner
of the surface. Second is the Micron TL63-200 63 × 63 mm2 with a
thickness of 100 and 200 µm. Third is the the position sensitive strip
detector Micron X1-140 50×50 mm2 with a thickness of 140 and 300 µm.
It is separated in 16 resistive strips at its front layer which are read out
at the two ends and has 33 output channels including the total energy
channel. Fourth is the Si micro strip detector shown together with micro
wire bonded cable. The detector has a thickness of 300 µm and an active
area of 40 × 70 mm2 .
of the detector then the charge is not instantaneously collected by the preamplifier electronics but rather follows a diffusion process, similar to heat propagation,
and the collected signal depends strongly on the incident position, as discussed in
Ref. [Lae79]. Proper treatment of the position sensitive signals allows for a precise
reconstruction of the incident position, particularly when the energy deposition is
high enough. The most common way to reconstruct the position in a one-dimensional
detector is to use the following formulas:
Qanode = Qtotal
x
L
(3.1)
where L is the length of the strip and Qanode the charge measured at one of the
position channels. The total deposited energy or total charge Qtotal can be evaluated
via the total energy signal or by the sum of the two opposite position signals. The
distance from one of the anodes is then simply given by:
x=L
Qanode
.
Qtotal
(3.2)
Since the same estimation can be performed for the opposite channel as well, each
position can be estimated as:
x=
L Qanode2 − Qanode1
,
2
Qcathode
(3.3)
where x in this case starts from the middle of the strip. For a 2D PSD the x and y
28
Figure 3.2: Sketch of a one-dimensional position sensitive detector [Kno10]. The
lateral view shows the resistive layer on one side of the strip and the
normal contact on the opposite side. The collected charge at the anode
P depends on the position x where the strip is hit while the signal at
the non-resistive side is proportional to the deposited energy E.
positions are obtained using the following formulas:
L (Q1 + Q4 ) − (Q3 + Q2 )
,
2 Q1 + Q2 + Q3 + Q4
L (Q1 + Q2 ) − (Q3 + Q4 )
.
y=
2 Q1 + Q2 + Q3 + Q4
x=
(3.4)
(3.5)
as discussed in Refs. [Yan89] and [Bru92] which describe a detector with four anodes
in the corners of the front layer, which can be seen in Fig. 3.3(left). The anodes are
labeled Q1 , Q2 , Q3 , Q4 and the origin of the coordinate system lies in the middle of
P
the quadratic surface with length L. The sum 4i=1 Qi should be the same as the
charge Q collected at the cathode on the back side.
The resistive charge division method had been a popular method for obtaining a
sub mm position information from a Si detector before the advent of micro strip Si
detectors and the development of the associated compact electronics to read them
out. Nowadays this method is only used when the number of read-out channels has
to be kept to a minimum but also, as we will discuss in this section, when detecting
heavy ions which generate a large charge cloud of hundreds of µm.
The two technologies still hold distinct advantages and disadvantages that are briefly
introduced in the following paragraphs and presented in more detail while discussing
the prototype results. As mentioned in Chapter 2, for the tracking detectors of R3 B
we will use a hybrid solution which benefits from the advantages of both technologies
and depending on the physics cases and requirements.
The Si micro strip detectors offer:
• unprecedented position resolution
29
Q2
Q
Q3
Q1
Q4
Figure 3.3: (left) Schematic drawing of the Hamamatsu S5378-2 detector showing
the four electrodes at the corners of the detector, which serve for reconstructing the 2D incident position. (right) The position sensitive Si strip
detector with 16 strips read out from anodes at both ends of each strip,
which serve for reconstructing the 1D incident position along the strip.
• low-noise performance
• high-rate capability
• multi-hit capability
but these advantages come in the expense of:
• high number of readout channels
• non-trivial energy-loss measurement of heavy fragments with charge shared
between few neighbouring strips
The continuous position sensitive Si detectors have
• good position resolution when the deposited energy is large enough
• small number of readout channels
• large continuous electrodes collect the full charge minimizing incomplete charge
collection regions, charge sharing and coupling between neighbouring elements
(as in a micro strip detector)
but these advantages come in the expense of:
• higher capacitance and thus higher noise level
30
• slower pulses due to the diffusion process (depending on detector capacitance
and interanode resistance) limit the rate
• less granularity compared to micro strip detectors limits the overall detector
rate
• minimum or no multi-hit capability
Before discussing the results obtained for the detectors from the in-beam and αsource measurements it is in order to introduce the main factors that determine
their performance. As mentioned before, the strict requirements for minimum material budget in the beam line define that these detectors must serve simultaneously
as precise position detectors and charge-identification (energy loss) detectors. In the
case of the micro strip detectors the intrinsic energy resolution measurement (obtained e.g. with an α source) is of the order of 10-20 keV (σ). The energy loss for
heavy ions ranges from tenths of MeV to 1 GeV, meaning that the intrinsic energy
resolution of the detector is of the order of 10−3 or better and thus negligible in our
in-beam measurements. In the case of the position sensitive detectors the obtained
resolution (using an
241 Am
source) varied between 40-100 keV (σ) depending on
the detector thickness and the dynamic range (sense) of the preamplifier. Even in
the worse case the energy resolution corresponds to less than a percent of the total
energy loss for ions with charge number greater than eight (O) and a 300 µm thick
detector. The resolution of the total energy loss for in-beam measurements and
for all detector types is governed by the energy straggling which percentage-wise
becomes more prominent as the detector gets thinner as illustrated in Fig. 3.4.
Another consideration for the energy-loss resolution of the Si detectors is the uniformity of the Si wafer thickness. This is typically guaranteed to ± 5 µm and can
get as low as 2-3 µm. In the case of thin detectors this is a value that causes an
energy-loss difference comparable to the energy-loss resolution of the detector.
On the other hand the position resolution of a micro strip segmented detector is at
least
√w
12
with pitch w or better if a weighted position is reconstructed using charges
at neighbouring strips. For the position sensitive detector, however, the position
resolution is more directly related to the intrinsic energy resolution of the detector. Event-by-event variation of the energy loss is canceled out when one examines
the normalised position channels. Fig. 3.5 shows calculated position resolution as
expected from the energy resolution measurement (see also Ref. [Has88]). To first
order approximation we assume that dx/x resolution follows generally the dE/E
resolution. These values can be considered as an upper limit for the position since
typically certain noise contributions cancel themselves when subtracting the position
signals. For ions with charge greater than ten (Ne) it is expected that the obtained
resolution should be at least 200 µm (σ) for a 300 µm detector and improve further
as the energy loss increases for ions with higher Z.
31
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Figure 3.4: (left) Calculated ratio for energy straggling divided by energy loss as a
function of the charge number of the impinging ion with 500 AMeV and
for a 300 µm (blue) and a 100 µm (green) Si detector. The red line shows
the minimum % separation required to resolve neighbouring charges, i.e.
at least 2.35 times the resolution. (right) The separation in energy loss
divided by the (energy straggling)/(energy loss) ratio. This plot shows
that sufficient separation is possible for all charges for a 300 µm thick
detector, while this is not the case for the 100 µm detector.
Fig. 3.6 shows the calculated angular resolution at the target position obtained
with and without target (200 mg/cm2 C). The angular straggling introduced by the
target is significant and hinders the benefits of using a thin intermediate Si detector.
Taking into account also the fact that a thinner detector of the technology discussed
here, gives also worse position resolution, it is possible following these curves that
one does not always get better with the use of a thinner detector.
Another consideration for the position sensitive detectors is the signal rise time. It
is shown in Refs. [Lae79, Has88] that the signal rise time of such detectors follows in
general an RC time constant where R is the inter anode resistance seen by the charge
and C is the detector capacitance. It is shown in the measurements presented below
that indeed both the signal height and the signal rise time are strongly position
dependent. In order to effectively operate the detector at high rates the signal rise
time must be as fast as possible. This means higher segmentation (lower capacitance)
and lower interanode resistance. These in turn are against the “low-number of
channels” argument for this detector type and the optimum position resolution that
could be obtained with a high interanode resistance. As is also shown later in this
section the decay time of the preamplified pulse also depends on the position, an
effect which is attributed to coupling between opposite readout electrodes where
charge can flow from one channel back through the resistive layer to the opposite
channel. We show an effective way to minimise this effect.
32
H!$
#!!$
%&'("$)*+"$,-+*'"$.!!$/0$12$
%&'("$)*+"$,-+*'"$.!!$/0$12$3243$4&25$67-&06$
8&9&$
#!$
#$
!"#$%&'#"($%)**+%
!"#$
!"!#$
!"!!#$
#$
#!$
#!!$
,%
Figure 3.5: Calculated position resolution of Si detector, for which the position is
reconstructed from the charge division along a resistive surface. The
calculation assumes that the position resolution is directly related to the
energy resolution with which the charge sharing is measured. The energy
resolution considered is 100 keV (FWHM) for the high-gain preamplifiers
and 200 keV (FWHM) for the low-gain preamplifier coupled to a large
capacitance detector and short integration time. The calculated position resolution is shown for different ions (i.e. different energy losses) for
500 MeV/nucleon beam energy and a 300 µm detector thicknesses together with three data points (Ca and Ni from in-beam data and 5.5 MeV
α-particle measurements, which deposit the same energy as a C beam
transversing the detector at 500 MeV/nucleon). In practice the resolution is expected to flat out for high Z, limited by the charge cloud that
is generated.
3.1.1 In-beam measurements from past LAND/ALADIN experiments
(before 2013)
In the past LAND/ALADIN experiments the 2D PSD and the microstrip Si detectors
have been used.
3.1.1.1 2D position sensitive detectors
The 2D PSD continuous detectors (Hamamatsu S5378-2 and the Micron Semiconductor TL-63 shown in the first two photographs of Fig. 3.1) were read out by
preamplifiers (CSTA2 type from TU Darmstadt and MMPR1 type housed in an
MSI-8p box from mesytec with 1.5 and 1 GeV energy range, respectively) followed
by single-channel shaping amplifier (for example TENLEC-244) or the multi-channel
33
("%$
("#$
!"#$%!&'&()*%$+*"',-&!./'
("($
!"'$
#!!$+,$-.$
)!!$+,$-.$
(!!$+,$-.$
!"&$
!"%$
!"#$
!"#$%!&'&()*%$+*"'*"'2!&#(2',-&!./'
("%$
("#$
("($
!"'$
#!!$+,$-.$
)!!$+,$-.$
(!!$+,$-.$
!"&$
!"%$
!"#$
!$
!"!%$
!"($
!"(%$
!")$
!")%$
!"#$
!"#%$
!"*$
0*)1+*"'&()*%$+*"',--/'
!"*%$
!$
!"!%$
!"($
!"(%$
!")$
!")%$
!"#$
!"#%$
!"*$
!"*%$
0*)1+*"'&()*%$+*"',--/'
Figure 3.6: Calculated angular resolution as a function of detector resolution before
(left) and after a relatively thin (200 mg/cm2 ) C target (right) for a
136 Xe beam at ∼500 MeV/A and different detector thicknesses.
shaping amplifier MSCF-16 from mesytec. The analog to digital conversion was performed with Philips ADC CAMAC-type module or the MADC VME-type module.
The obtained position resolution has been in the order of 100 µm in experiments
with heavy ions such as Ni and Sn. The energy resolution turned out to be close
to the limit set by the energy straggling of the beam (Fig. 3.4). Despite the good
performance in terms of position and energy resolution and the small number of
channels (5 channels), this type of detector is rate limited due to the large RC time
constant mentioned above and discussed in more detail in Section 3.1.2.4.
The typical setup for evaluating and calibrating in beam the position of the 2D
PSD is to insert an active square-pixel scintillator mask close to the detector and
reconstruct the position on the Si detector using the four corner signals and gating
on the signal of the active pixel mask as shown in Fig. 3.7.
The Table 3.1 summarises the in-beam performance obtained for the different 2D
PSDs. The energy resolution is limited for all detector thicknesses by the ratio of the
energy straggling divided by the energy loss. The position resolution for the 100 µm
thick detector turned out to be significantly worse not only due to the expected
higher noise and lower energy loss but, also due to malfunction of the particular
detector.
3.1.1.2 Microstrip detector
The Si microstrip detectors have also been used extensively for in-beam measurements in the LAND/ALADIN setup for beam tracking and charge identification of
light ions and protons. The readout of the AMS type microstrip detector was performed with an integrated ASIC card which shapes and multiplexes the signals which
are then digitized by the SIDEREM NIM-type module. As discussed earlier in this
section, the position resolution obtained from these detectors for heavy fragments is
34
0.4
y (cm)
0.2
0
-0.2
-0.4
-0.6
0
2mm
0.5mm
0.2
0.4
0.6
x (cm)
0.8
1
1.2
Figure 3.7: Geometry of the plastic scintillator active pixel mask that is inserted in
front of the 2D PSD for calibration (left). 2D position reconstruction
from in-beam measurements with Ni ions at 500 AMeV using the four
signals of the Hamamatsu detector and after gating on the signals from
the scintillating mask with 0.5x0.5 mm2 square pixels placed in front
of the Si detector. The sharp reconstruction of the square-pixel edges
indicates a position resolution of the order of 100 µm.
Table 3.1: The main characteristics (active area (a.a.), thickness (d)) and the performance (charge resolution (δZ/Z) and position resolution (δx) in σ values)
are shown for the different types of position-sensitive Si detectors. The
results were obtained using a 136 Xe beam at ∼500 MeV/A. The last lines
present results calculated with ATIMA for the angular straggling (a.s.)
and energy straggling (e.s.) divided by energy loss (e.l.). The measured
charge resolution is limited by the energy straggling. Note also that for
a negligible velocity variation the following approximation is valid δZ/Z
[%] = 0.5 δE/E [%].
Hamamatsu
S5378
4.5 x 4.5
∼300
Micron
TL63-200
6.3 x 6.3
∼200
Micron
TL63-100
6.3 x 6.3
∼100
δZ/Z [%]
δx [µm]
0.6
100
0.8
150
1.1
350
a.s. [mrad]
e.s./e.l.[%]
0.4
1.3
0.33
1.6
0.23
2.3
a.a. [cm2 ]
d [µm]
particularly good. Values of the order of σ ∼15 µm have been reported in the anal-
ysis of the corresponding experiments [Alp00], which is better than that expected
√
from the strip pitch (i.e. ∼100/ 12 µm) since the generated charge is shared be-
tween neighbouring strips and the position can be reconstructed as a weighted mean
using the energies measured by these strips.
35
The energy-loss measurement suffers from a strong position dependency across the
strip, as shown for example in Figs. 3.8 and 3.9 and after correction in Fig. 3.10.
The effect becomes stronger for higher Z as is already evident when comparing the
position dependency between C isotopes in Fig. 3.8 and Ne isotopes in Fig. 3.9. This
effect has been one of the main concerns in using this detector technology for heavy
∆E
ions where the charge separation is more challenging.
4000
250
3000
200
150
2000
100
1000
50
0
0
0.2
0.4
0.6
0.8
1
0
x
∆E
Counts
Figure 3.8: Energy-loss measurement of C isotopes versus the position across the
width of a strip. The energy is obtained by the total energy of the cluster
associated with each hit, i.e. by summing all energies from neighbouring
channels.
x
∆E
Figure 3.9: (left) Same as Fig. 3.8 but for Ne isotopes. (right) Projection on the y
axis.
3.1.2 Measurements with an α-particle source and calculations
Based on the experience obtained with the continuous 2D PSDs and the challenges
that were faced, particularly with respect to increasing capacitance and rate performance, we investigated the advantages of using a resistive strip detector where
the position is reconstructed along the strip. In this way the per strip capacitance
is significantly reduced and the beam rate naturally distributes over more detector
elements and electronic channels. A dedicated apparatus has been setup at TU
36
Counts
∆E
x
∆E
Figure 3.10: (left) Corrected energy-loss measurement and (right) its projection on
the y axis.
Darmstadt to perform the α-source measurements with the position sensitive Si detectors and has been the work of a Master and a Bachelor thesis, see Refs [Syn14]
and [Pat13]. Part of their work is repeated in this report.
3.1.2.1 Setup
The setup for the α-source measurement consisted of a stand for the detector and
source and a set of collimators tailored for each of the measurements.
For example, for the Micron X1 strip detector two different collimators has been used
with 15 holes in the x-direction, a diameter of 0.5 mm and a distance of 3.34 mm and
16 holes in the y-direction. The one illuminated the middle of each strip (distance of
3.125 mm) and the second illuminated the region between the strips (see Fig. 3.19).
Electronics
Two different electronic setups has been used. The first one used
analog electronics to shape the signals and an analog-to-digital converter (ADC) to
determine the pulse height of those signals while the second one used a flash ADC
(FADC or digitizer) to do both at the same time. Both setups will be introduced in
this section and are shown in Fig. 3.11.
In both cases the detector signals were connected to a mesytec preamplifier. For the
Hamamatsu S5378 and most of the measurements with the Micron X1 the preamplifier box MSI-8p, which houses eight MMPR-1 35 MeV type preamplifier cards,
tailored to α-particle measurements with an energy E ≈ 5.5 MeV, was used.
In the setup with the analog electronics after the preamplifier stage the signals
were integrated using a mesytec shaping amplifier MSCF-16. In this discussion, we
investigated the performance of the detector for different shaping times (0.25 µs,
0.5 µs, 1 µs or 2 µs). After shaping the signals were digitized using the mesytec
analog to digital conversion module MADC-32.
37
VME Crate
(a)
Preamp
A1
A2
Shaping
Amp
Monitor Signal
C
A4
ADC
A3
Preamp
+ Invert
Detector
Invert
CFD
Trigger
TRIVA7
VME Crate
(b)
Preamp
A1
A2
C
A4
A3
Detector
Preamp
+ Invert
✏ Digitizer
✏✏
✏
✏✏
✏✏
✏
✏✏
✏✏
✏
✏
Trigger
✏✏
TRIVA7
Figure 3.11: Sketch of the electronic setup with (a) analog and (b) digital electronics
for the Hamamatsu S5378 detector. (a) The four anode signals A1, A2,
A3 and A4 and the cathode signal C are connected to preamplifiers.
Then, all five signals go to the shaping amplifier and from the amplifier
to the ADC where they are digitized. (b) Again the anode and cathode
signals are connected to the preamplifiers, but after that the output
signals go directly to the digitizer.
In the setup with the digital electronics the shaping amplifier and the ADC was
replaced with a flash ADC module (CAEN V1724) which digitizes the preamplifier
pulse at regular intervals of 10 ns and has an on board processing unit (Field Programable Gate Array - FPGA) for further on-line processing of the signal. However,
in this section we show results for which we have digitized the signal and performed
the pulse shaping analysis off-line where one has better control over the parameters
and the algorithms used.
3.1.2.2 Analysis
Trapezoidal Filter Algorithm
In order to obtain the position and total energy de-
position of a hit in the detectors, it is necessary to measure the collected charge
in each channel. Typically, in analog electronics, the preamplifier pulse would be
shaped into a Gaussian-shape pulse by the shaping amplifier and its height determined by an ADC. For digital electronics an algorithm is required to determine the
height of the pulses recorded with the digitizer.
38
Figure 3.12: Functional principle of a simple trapezoidal filter: The discrete values
of the pulse shape in (a) are summed up over two windows with length l
separately. One window starts at i and the second one at i + d, where d
is the delay between the two windows. The sums of the two windows are
subtracted from each other and plotted as the i-th point of the trapeze
in (b). This is done for all values of i. The height of the resulting
trapezoid corresponds to the integral of the tail-pulse and thus to the
deposited energy.
For this purpose a simple trapezoidal filter algorithm was implemented, which is
illustrated in Fig. 3.12 and described in the following. The data points of the pulse
were summed up over a time interval tl = l · ∆t, where l is the length of the window
shown in Fig. 3.12(a). This was done for two different starting points t1 and t2 =
t1 + d · ∆t with the delay d · ∆t. The first value was then subtracted from the second
one. The same was done for all t1 and the results were stored as a function of t1 .
This function looked more or less like a trapezoid and its height corresponded to the
collected charge. As one can see in Fig. 3.12(b) the plateau of the trapezoid is not
horizontal. In principle, this is due to the decay constant of the preamplifier and
can be corrected via a pole-zero correction.
The trapezoidal filter discussed in Ref. [Jor94] and illustrated in Fig. 3.12 (without
pole-zero correction) can be expressed (with pole-zero correction) as
s(i) = s(i − 1) + p(i) + dkl (i) · M
(3.6)
for n ≥ 0 and with the decay time of the original signal M . p(i) is again a recursive
function for n ≥ 0, while dkl (i) is defined by the data points ν(i) of the original
signal. m = l − k is the length of the flat-top, which results in a triangular shape
for l = k.
p(i) = p(i − 1) + dkl (i),
dkl (i) = ν(i) + ν(i − k) − ν(i − l) + ν(i − k − l).
(3.7)
(3.8)
39
3.1.2.3 Results
The main goal is to characterize the position and energy resolution of the different
detectors, compare them and understand their performance in detail. This is done
in detail in the first two sections below. The detailed analysis of the pulse shape is
presented separately.
Position Resolution
To determine the position resolution a collimator mask was
placed in front of the detector. The position was reconstructed using the pulse height
of the anode signals and the Eqs. 3.4 and 3.5.
For the Hamamatsu S5378 detector the measurements were done with both the
analog and the digital electronics. In both cases a collimator with 5 × 5 holes with
a diameter of 1 mm and a distance in-between of 1 cm was used.
With the analog setup the shaping time of the shaping amplifier was varied and the
position (and energy) resolution was determined for the different shaping times τs .
In Fig. 3.13 one can see the resulting differences for the shaping times. On the one
hand the position resolution is best for the smallest shaping time τs = 0.25 µs. On
the other hand the pattern of the holes is more linear for the highest shaping time
τs = 2 µs. In addition, for the higher shaping times all the holes have the same
diameter and the same position resolution while for the smaller shaping times the
inner holes have a worse resolution than the outer ones. The reported resolutions
are therefore an average resolution between the very good outer region and the inner
region. The mean values of the FWHM can be seen in Table 3.2.
τs /µs
FWHMx / mm
FWHMy / mm
0.25
1.264(7)
1.545(6)
0.5
1.588(7)
1.810(7)
1
2.235(8)
2.321(8)
2
2.812(8)
2.856(9)
Table 3.2: Mean position resolution of the Hamamatsu S5378 detector measured
with analog electronics. The FWHM was measured for both x- and ycoordinate and for different shaping times τs of the amplifier.
Additionally the position resolution was determined with the digital electronics. We
studied the influence of different values for the delay d and the length l1 of the
algorithm. Furthermore, the results were compared to the equivalent results for the
analog electronics.
As with the analog electronics the images of the collimator in Fig. 3.13 show some
irregularities. The inner nine holes have a worse position resolution than the outer
ones and their position is displaced, i.e. there is a large non-linearity in the position
1
An example of equivalent shaping for digital trapezoidal filter processing (l and d = l + gap) and
for analog shaping amplifier (with shaping constant τs ) is, for example, a length = 2×τs and
gap = τs .
40
reconstruction. This effect gets worse for small values d. Nevertheless the FWHM
for each hole was evaluated and the mean value for both x- and y-direction and for
different values d can be found in Table 3.3. It can be seen that the overall position
resolution gets better for smaller delays and becomes the smallest for d = l, which
corresponds to a triangular filter with no flat-top, i.e. gap = 0. Therefore, d = l is
used in the following.
d/µs
FWHMx / mm
FWHMy / mm
0.25
1.117(6)
1.100(6)
0.50
1.187(6)
1.179(6)
1.25
1.546(6)
1.514(6)
2.25
2.034(8)
1.939(8)
with digital electronics. The FWHM was measured for both x- and ycoordinate for l = 0.25 µs and varying values d as trapezoidal filter settings.
To compare the performance of the analog and digital electronics, the mean position
resolution was calculated for a triangular filter for different l. The results can be
found in Table 3.4 and compared to the values in Table 3.2 for the analog measurement. As one can see the FWHM for both the x- and y-position is smaller for
the measurements with the digitizer. This is attributed to much higher freedom in
adjusting the parameters in the digital system, where for example a triangular filter
(gap = 0) was found to give the best results. For both systems short shaping times
improve the resolution which will be discussed in more detail later in this section.
l/µs
FWHMx / mm
FWHMy / mm
0.05
1.195(6)
1.180(6)
0.10
1.104(6)
1.090(6)
0.25
1.117(6)
1.100(6)
0.50
1.183(6)
1.171(6)
1.00
1.399(6)
1.364(6)
2.00
1.837(7)
1.750(7)
with digital electronics. The FWHM was measured for both x- and ycoordinate for l = d as filter settings (triangular filter).
The Micron X1 detector was tested in a similar way as the Hamamatsu S5378
detector described above. The Micron X1 is a detector with strips in x-direction.
While the x-position is again measured via the resistive charge division at each side
of the anode, the y-position is determined only by the struck strip. Therefore, the
position resolution was evaluated only for x. By definition the position resolution
in the y-direction is simply σy =
√w
12
= 0.902 mm with the width w = 3.125 mm of
one strip.
For this detector the measurements were performed using only the digital electronics.
The first step was to investigate the optimum position resolution via the variation
of the delay d with a given length l = 0.25 µs. The results in Table 3.5 show the
same characteristic as the results for the Hamamatsu S5378 in Table 3.3. Namely,
the best position resolution is found for d = l = 0.25 µs, i.e., for a triangular filter
41
Figure 3.13: (top) x- and y-position where the α particles have hit the Hamamatsu S5378 detector covered by a collimator for different shaping times
τs = 0.25 µs, τs = 0.5 µs, τs = 1 µs and τs = 2 µs (from left to right)
of the shaping amplifier. The position resolution gets worse while the
linearity of the pattern gets better for higher shaping times. Furthermore, the position resolution depends on the position on the detector for
small shaping times. (bottom) The same plot obtained using the digital electronics and different lengths of the trapezoidal filter l = 0.05 µs,
l = 0.10 µs, l = 0.25 µs and l = 2.00 µs (from left to right). The position resolution depends on the position of the collimator hole. For the
inner holes the position resolution is almost the same for all l, while
for the outer holes it is best for l = 0.10 µs. Furthermore the pattern
of the holes is distorted, whereas the linearity of the pattern is best for
l = 2.00 µs.
(gap = 0) with short integration time.
d/µs
FWHMx / mm
0.25
0.362(33)
0.50
0.513(44)
0.75
0.631(46)
1.25
0.746(42)
2.25
1.081(62)
5.25
2.07(14)
Table 3.5: Position resolution of the Micron X1 detector measured with digital electronics. The FWHM was measured for the middle hole for one strip for
l = 0.25 µs and varying values d as trapezoidal filter settings.
The next step was the variation of l = d. In Fig. 3.14 the x-position of the hits
can be seen for both the best and the worst resolution. The results for all chosen
values are listed in Table 3.6. In contrast to the Hamamatsu S5378 the position
resolution is independent of the position of the hole. The best position resolution
is FWHM = 0.362 mm for l = 0.25 µs in Fig. 3.14(a), while for higher l the FWHM
increases. For l = 0.10 µs the resolution is worse than for l = 0.25 µs, too. A detailed
discussion about the best shaping time τs and length l respectively can be found in
section 3.1.2.4.
In comparison to the Hamamatsu S5378 detector the position resolution is much
better particularly for smaller l. The linearity of the strip detector is also much less
sensitive to the integration time when compared to the 2D PSD.
42
l/µs
FWHMx / mm
0.10
0.436(44)
0.25
0.362(33)
0.50
0.469(35)
1.00
0.638(42)
2.00
1.127(85)
hi
100
Entries
Mean
RMS
(a)
5294
0.1913
1.335
80
counts
counts
Table 3.6: Position resolution of the Micron X1 detector measured with digital electronics. The FWHM was measured for the middle hole for one strip for
l = d as trapezoidal filter settings.
hi
60
Entries
Mean
RMS
(b)
5294
0.1741
1.23
50
40
60
30
40
20
20
10
0
-2
-1
0
1
0
2
-2
-1
0
x/cm
1
2
x/cm
Figure 3.14: x-position where the α particles have hit one strip of the Micron X1
detector covered by a collimator for different lengths (a) l = 0.25 µs and
(b) l = 2.00 µs as trapezoidal filter settings. The FWHM is the same
for all holes in one setting and smaller for the smaller value of l.
Energy Resolution
In this section we present the results with respect to the total
energy resolution of the detectors. To get the deposited energy, the pulse height of
the cathode signal was evaluated and compared to the sum of the position signals,
which contain the same information. The energy of the
241 Am
α particles for the
highest intensity peak is E = 5.5 MeV and is low enough such that the whole energy
was deposited in the detector.
Fig. 3.15 shows the effect of the different shaping times τs of the shaping amplifier
on the measured total energy for the Hamamatsu S5378 detector. For small shaping
times the collected charge at the cathode, which is interpreted as the deposited
energy, depends on the position where the α particles hit the detector. For the inner
holes less charge is collected, while for the holes near the corners the maximum
amount is measured. This effect and the difference between the inner and outer
holes gets smaller with higher shaping times, but is still visible with the highest
possible τs = 2 µs allowed by our shaping amplifier. For even larger shaping times
the effect eventually disappears. This was confirmed using digital electronics where
this dependency is more straight forward since the gap between the two integration
windows has to be larger than the rise time of the pulse in order to obtain the
full pulse height. This effect is commonly referred to in the literature as ballistic
deficit.
For the measurements with the Micron X1 detector only the digital electronics were
tested. Again, the effect of the variables d and l to the total energy were studied. For
43
this detector, the effect of the position dependency is much smaller (c.f. Fig. 3.16).
This is expected since the effective capacitance per strip is reduced significantly
with respect to the continuous detector and thus the rise time is much faster and
the position-dependent variation is minimised.
Pulse Shapes To understand the correlation of the charge collection process with
the settings of the shaping amplifier and trapezoidal filter, the pulse shapes after
the preamplifier were recorded by the digitizer. The dependency of the pulse shape
on the hit position for both detectors is discussed first. This dependency influences
the choice of the shaping times and trapezoidal filter setting.
At first the pulse shape of one anode of the Hamamatsu S5378 was inspected.
Fig. 3.17(a) shows the reconstructed x and y positions of the detector. Fig. 3.17(b)
Figure 3.15: Energy measured by the Hamamatsu S5378 detector depending on the
x-position for different shaping times τs = 0.25 µs, τs = 0.5 µs, τs = 1 µs
and τs = 2 µs (from left to right). With a shaping time of τs = 0.25 µs
the measured energy for the hits in the corners is 3.1 times higher than
for the hits in the middle of the detector. This effect gets small if
the shaping time increases but is still present for the highest possible
shaping time of τs = 2 µs.
6
6
×10
E/a.u.
E/a.u.
×10
120
140
100
120
80
100
60
80
40
60
20
-2
-1
0
1
2
-2
x/cm
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
x/cm
Figure 3.16: Energy measured by the Micron X1 detector depending on the xposition for different lengths d = 0.25 µs (left) and d = 5.25 µs (right).
Even for small values of d and l the position dependence of the energy is
relatively small. For d = 5.25 µs the energy dependence on the position
is no longer visible.
44
shows the traces as recorded for the upper left anode for the holes along the highlighted diagonal. The signal with the highest amplitude corresponds to the hits
closer to this anode. From the traces it is also evident that the rise time of the
pulses depends on the position of the hits. The pulses corresponding to hits close
to the anode are rising much faster. On the other hand, for hits further away from
this anode the rise time is large with τr > 7 µs. To fully measure the energy, a
large shaping time would be necessary. A large shaping time has in turn two disadvantages. As shown in section 3.1.2.3 the position resolution is better for smaller
shaping times and a small shaping time should be chosen. Furthermore, using a
long shaping time results in problems with high rates leading to pile-up effects.
In contrast the rise time of the hole closest to the anode is very small with τr < 100 ns.
For this rise time a small shaping time can be used. However, a hole close to one
anode is far away from all the other anodes and the same problem occurs with the
corresponding signals there.
Figure 3.17: Anode rise times of the Hamamatsu S5378 detector for selected holes:
The pulse shapes of the calculated positions that correspond to the
highlighted region in (a) are shown in (b) for the upper uper left anode.
As one can see the rise time is much higher for the hits away from the
anode (τr > 7 µs) compared to those close to the anode (τr < 100 ns).
When looking at the averaged pulses of the Micron X1 detector in Fig. 3.18(a) a
similar but much smaller effect is observed. The signals of the 15 holes can be easily
differentiated in Fig. 3.18 and the rise time for all of them, although still position
dependent, is in all cases very short (τr ≈ 100 ns). However, the decay time of
the signals show a large variation despite the fact that they discharge via the same
preamplifier RC circuit. For the hole next to the anode the signal decays with
τd ≈ 21 µs, while for holes further away from the anode the decay time τd increases
until instead a second rise time for signals near the opposite anode occurs.
The decay constant of the preamplifier is defined by the feedback resistance Rf
and capacitance Cf , which in this case gives τd = Rf · Cf = 75 µs. Additionally the
decay is influenced by the coupling capacitor Cc and the resistance of the detector
surface. The resulting time constant depends on the position of the event and the
45
resistance between the hit point and the anode and therefore influences the pulse
shape depending on the position. For example, in this case for a 1.5 kΩ resistance
(from middle to edge of the strip) and a coupling capacitor of 10 nF, the resulting
time constant is τ = 15 µs, i.e. faster than the decay constant of the preamplifier.
Although the varying effective resistance, which is the basic principle of operation
for these detectors, can not be avoided, the impact of the second decay time can
be reduced by replacing the coupling capacitor Cc = 10 nF with a larger one with
Cc = 100 nF. The pulse shapes of the 15 holes with this new coupling capacitor
in Fig. 3.18(b) show a much smaller dependency of the decay time on the position
allowing for a better pole-zero correction.
Interstrip region
As discussed earlier, the strip PSD (Micron X1) has a good posi-
tion resolution along the strip, however, there are areas between the strips (interstrip
regions) which have a worse position and energy resolution due to incomplete charge
collection.
In order to examine this behaviour in more detail, a collimator mask which illuminates the region between the strips was used. The holes had a larger diameter
than the width of the interstrip region (0.1 mm inters trip gap is the typical value
claimed by the company). Therefore, the measurement with this collimator resulted
in a mixture of intrastrip and interstrip hits.
The collimator was placed in front of the Micron X1 detector and the signals of two
neighboring strips and the backside were read out. The pulse heights of the signals,
which represent the collected charge at the anode, is called Ei and their arrangement
is shown in Fig. 3.19.
Figure 3.18: Averaged pulses for different hit points on the Micron X1 with a coupling capacitor with (a) C = 10nF and (b) C = 100nF in the preamplifier. In both cases the pulse shapes of the 15 collimator holes are nicely
separated and have a rise time of τr ≈ 100 ns. In (a) the decay of the
hole next to the anode is much faster than for holes further away. For
events in the second half of the strip the pulse shapes show a second
rise time. In (b) this effect is still visible but much smaller.
46
E2
E1
❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜
❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜
❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜ ❜
E3
E4
Strip 2
Strip 1
Figure 3.19: Sketch of the experimental setup: A collimator with 15 holes in a row
is places such that middle area between the strips of the Micron X1 is
illuminated.
2.46*(SI1Emax-SI4Emax)/(SI1Emax+SI4Emax) {CUTGuse&&(CUTGstrip9 || CUTGbetween)}
Counts
E2 /a.u.
SI2Emax:SI3Emax
30000
(a)
25000
hi
700
Entries
18673
Mean
-0.1875
RMS
1.298
(b)
600
20000
500
400
15000
300
10000
200
5000
100
0
0
5000
10000
15000
20000
25000
30000
0
E3 /a.u.
-2
-1
0
1
2
x/cm
Figure 3.20: (a) Energy deposition for one strip of Micron X1. The energy collected
on one side is plotted over the energy collected on the other side. The
diagonal shows 15 holes where the whole energy of the α particles is
deposited in the strip. Events which hit the interstrip region deposit
only a fraction of energy in the strip. (b) x-position of events taken
with a collimator with holes between the strips and l = d = 0.25 µs as
trapezoidal filter settings. The position resolution is best for events in
the intrastrip region (red) and worse for those in the interstrip region
(blue). Without any differentiation between the regions (green) the
FWHM is slightly higher than for the best case.
In Fig. 3.20(a) the energy deposited on both sides of one strip can be seen. Along
the diagonal 15 holes are cleanly separated. The points close to the origin can be
viewed as pedestal values. The 15 holes on the diagonal correspond to events where
the full deposited energy is collected, while the lower energy “tails” correspond to
events where the charge is shared between neighbouring strips or where the charge is
not fully collected. A comparison of the position resolution in Fig. 3.20(b) indicates
that the interstrip region can still be used reliably for position measurement, but
with a sacrifice on the resolution by a factor of 2.
These interstrip events were also examined in terms of energy resolution. In Fig. 3.21
the deposited energy is shown for the intrastrip events, the interstrip events and for
all events without any distinction. The distribution which corresponds to events
from the middle of the strips has a mean value that is higher than the one which
corresponds to events from the interstrip region. The shift might be caused by the
smaller electric field in the area between the strips and the resulting slower charge
collection process. For higher integration times this effect decreases but it is still
47
present, which indicates that part of the charge is lost in areas where the electric
field is low and the charges move slow enough to get trapped before collected.
To determine the amount of these interstrip hits we used the following considerations.
The interstrip separation provided by the manufacturer for the Micron X1 detectors
is winter = 0.1 mm. Along the direction of the strip this results in a rectangular area
with a surface of 5 mm2 for each of the 15 interstrip regions. i.e. a total area of
75 mm2 . This area amounts to 2-3 % of the total active detector area. To confirm
this estimation we have used the collimator mask shown in Fig. 3.19 which has holes
with a diameter of d = 0.5 mm and an area of A = 0.196 mm2 . The overlap of the
collimator hole with the rectangular interstrip area is 25 %, which is consistent with
our measurements. As a conclusion we find that around 3 % of all the events hit the
area between the strips. Detectors with smaller interstrip region are preferable.
3.1.2.4 Discussion
In the previous section the results of our measurements with the two detectors and
the different electronic settings were shown. The dependence of the position and
energy resolution on the shaping time needs further investigation and explanation,
though. For this purpose the influence of the noise particularly on the position
resolution will be investigated in the first part of this section. Afterwards the ballistic
deficit effect is examined in more detail.
Influence of Noise on the Position Resolution
As seen in section 3.1.2.3 the posi-
tion resolution for the PSD minimises for rather small shaping times τs and lengths
l and d for analog and digital systems, respectively. Banu et al. in Ref. [Ban08]
attribute the positive effect of small shaping times to the noise which is generated
in the detector and the electronics. They address three major contributions to the
energy resolution of the anode signals. The first one is the thermal noise of the
resistance of the resistive strips on the front of the detector. It depends on the temperature T and the resistance Rstrip . The second contribution is the noise produced
in the preamplifier, which depends on the equivalent resistance of the field-effect
Figure 3.21: Energy resolution for (a) intrastrip or (b) interstrip events and (c) without a differentiation between the two regions determined with the simple
trapezoidal filter algorithm with d = 15 µs, l = 5 µs.
48
transistor (FET) Req of the preamplifier and again on the temperature. The last
part is produced by the leakage current I. Alltogether Banu et al give the energy
resolution as
FWHME,anode =
s
α2
τs T
TReq
+ (βC )2
+ γ 2 τs Ileak
Rstrip
τs
(3.9)
for one of the position signals. FWHME is in keV, τs being the shaping time is in
µs, resistances in kΩ, capacitance in nF, current in µA and temperature in K.
However, we have found that in our case this formula does not reproduce our experimental values. The reason might be in the poor knowledge of the Req of the FET
of the preamplifier and the fact that our measured leakage current is not exactly the
current which generates the shot noise.
In the general case we fitted the FWHME, anode of one anode with the function
F W HME,anode =
r
a · τs +
b
+ c.
τs
(3.10)
The coefficient a can be described as the influence of the current noise and b as the
influence of the voltage noise, while c describes the contribution to the noise that
is independent of the shaping time τs . In this case a, b and c are determined via
fitting:
a = 7.0064 ± 0.7123 b = 0.495696 ± 0.1029 c = 2.38591 ± 0.7539. (3.11)
In Fig. 3.22 the data together with the fit can be seen. The data points show
the expected behaviour and are in good agreement with the position measurements
discussed earlier, i.e. minimise for small shaping time.
A similar formula can be used to describe the noise of the energy signal and its contribution to the energy resolution. However, the total energy resolution is dominated
by the voltage noise and not by the current noise like the anode signals. Therefore,
the energy resolution minimises for larger shaping times.
Ballistic Deficit
As described in section 3.1.2.3, the measured total energy depends
on the position of the events for small shaping and integration times, while this effect
vanishes for higher shaping times and integration times respectively. The cause of
this effect is described by Lægsgaard in Ref. [Lae79] for a strip with a resistive
surface and we used their analysis to illustrate the ballistic deficit.
A strip detector with a resistive surface can be described as an RC circuit like in
Fig. 3.23. The charge collected at the anodes on the two ends of the strip can be
49
FWHME1 /%
FWHME1 /%
7
data
fit
theory
(a)
6
7
6
5
5
4
4
3
3
2
2
0
0.5
1
1.5
2
2.5
3
3.5
data
fit
theory
(b)
0
0.5
1
1.5
2
2.5
3
l/µs
3.5
l/µs
Figure 3.22: Energy resolution for one anode of the Micron X1 for the middle hole
of one strip depending on the length l. The red data points show
the measured energy resolution, while the green function is the fit on
Eq. 3.10, which describes the FWHMEi produced by different types
of noise. The blue function shows the theoretical prediction for (a)
Req = 0.25 kΩ and (b) Req = 1 kΩ.
Figure 3.23: Sketch of a one dimensional detector with a resistive surface. The
resistance RD and the capacitance CD of the detector determine its
time constant τD , on which the charge collection depends. The charges
collected at the anodes are called qEi and qEj , while the charge collected
at the cathode is called qE . [Lae79]
described as [Lae79]:
qEi (x, t) = q0
"
∞
x 2 X sin nπ 1 −
−
l
π
n
n=1
x
l
t
· exp −n2 π 2
τD
#
,
(3.12)
if a particle has hit a detector with length l at position x. On the total energy channel
the opposite charge of the summed anode charge is collected, which is described as:
qE (x, t) = −q0
"
∞
2 X sin nπ 1 −
1−
π
n
n=1
x
l
t
· (1 − cos nπ) · exp −n2 π 2
τD
#
. (3.13)
The behavior of both qEi and qE is shown in Fig. 3.24 for ten different hit points.
The anode signal for a hit next to an anode ( xl = 1) is maximal instantly, but the
signal rise time decreases with growing distance to the anode. To integrate over
50
the full collected charge independently of the position of the hit, it is necessary to
chose a shaping time τs > 0.3 τD . A similar argument is applicable for the cathode
channel. Because the cathode is symmetric, the positions
identically, as well as
x
l
= 0.9 and
x
l
x
l
= 1 and
x
l
= 0.0 behave
= 0.1 and so on. But again the signals at
the edges of the detectors are maximal instantly, while the signals in the middle at
x
l
= 0.5 need more than τ = 0.4 τD to grow to full height.
The resistive strip of the Micron X1 with its resistance R = 3 kΩ and Cstrip = 116 pF
has a time constant τD,X1 = 0.35 µs. Therefore a shaping time of τs > 0.3τD,anode =
0.10 µs is necessary to collect the full charge independently of the position of the
event.
This effect occurs for both the Hamamatsu S5378 and the Micron X1, the exact formulas, however, are different. In the case of the Hamamatsu S5378 detector, with the
much higher capacitance, the time constant has been found to be τD,S5378 = 7.8 µs.
The measured cathode energy depends strongly on the position of the hit for small
shaping times (c.f. section 3.1.3.2). With higher shaping times the dependence gets
smaller, because more of the charge gets integrated until the position dependence of
the energy signal vanishes with delays d > 2.25 µs ≈ 0.3τD , which is consistent with
our measurements.
3.1.3 In-beam measurements from April 2014 and September 2014 runs
Following our findings from the past LAND/ALADIN in-beam experiments and
particularly through the α-particle measurements we have requested certain modifications to the X1-type detectors. Three custom made detectors were tested in the
recent in-beam tests. Of paramount importance is to obtain a faster signal. The
faster signal not only improves the rate performance but also it can improve the
position resolution since as indicated in the previous section the optimum position
resolution is obtained for rather small integration times.
For this purpose we have requested the following detector customisations:
• detectors with lower inter anode resistance of 1.5 kΩ instead of the typical
3.0-3.5 kΩ used in this type of detectors. As results from Eq. 3.12 the signal
rise time, determined by the detector RC, becomes a factor of two faster. This
choice is made based on the fact that in most cases the deposited energy in
our experiments is high enough to deliver a good position resolution even with
lower inter anode resistances.
• a P-type position sensitive detector, where the position signals collect the
electrons. In this way, the first part of the signal (due to electrons) is collected
51
as fast as possible. This is a pioneering detector tested for the first time in our
experiments.
These detectors have been tested in the following in-beam R3 B test experiments:
• s438 in April 2014, using a
58 Ni
• s438b in October 2014, using a
primary beam
48 Ca
primary beam
Fig. 3.25 shows the arrangement of the detectors for the 1st in-beam test. Two of the
micron X1 detectors, one P-type and one N-type, with 300 µm thickness each and
1.5 kΩ inter-anode resistance are placed vertical to each other, such that combined
they can deliver a 2D position information. A dedicated fiber detector with 200 µm
1.2
1.0
1
0.8
Ei
q /q
0
0.9
0.6
0.8
0.4
0.7
0.6 0.5
0.2
0
0
0.1
0.2
0.3
0.4
0.5
0.6
0.3
0.4
0.5
0.6
τ/ τD
1.2
1
1.0
0.9
0.8
Ei
q /q
0
0.8
0.7
0.6
0.6
0.5
0.4
0.3
0.2
0.1
0.4
0.2
0
0
0.1
0.2
τ/ τD
Figure 3.24: Behavior of the collected charge for (a) the anode qEi and (b) the cathode qE depending on the position xl of the hit. (a) The further away
from the anode the hit is, the longer the anode signal needs to reach
its maximum. A shaping time of τs > 0.3 τD is necessary to determine
the full signal height, which is equivalent to the position xl . (b) For
the cathode signal the hits in the middle have a slower rise time then
the ones at the edges. A shaping time of τs > 0.4 τD is necessary to
determine the full signal height, which is equivalent to the whole energy
deposited in the detector. [Pat13]
52
fibers was constructed and placed in front of the Si detectors, to serve as a benchmark
for the position resolution reconstructed from the Si detectors. The fiber detector
has two read-out channels, one that corresponds to the horizontal fibbers and one
that corresponds to the vertical fibers. Requiring a trigger in either channel enables
to select hits along each direction and when both channels are required then the
selected events correspond to a 200×200 µm2 square.
Figure 3.25: The Si setup for the in-beam tests performed in April 2014. The beam
transverses through the fiber pixel detector and then through a set of
3 detectors closely mounted with a 2 cm distance between them. The
first and third of these detectors are Si resistive strip detector of the X1
type. The second detector in between is the continuous area 2D PSD.
The electronic read-out used in these in-beam tests were MPR-16 preamplifier from
mesytec and direct digitisation of the preamplifier pulse with the Struck digitisers
(1st beam test) and the GSI FEBEX digitisers [Feb] (2nd beam test). The settings
for the FPGA trapezoidal filter were set for the Struck system to be about 150 ns
integration window and 32 ns gap window for the position channels and about 500 ns
window both for integration and the gap parameter for the energy channels. For the
FEBEX system the parameters of the a similar filter were set to be 250 ns integration
and gap for all channels (position and total energy).
3.1.3.1 Position resolution
First we present the results of the in-beam position resolution obtained with these
detectors. Fig. 3.26 shows the energy measured by the fiber mask versus the reconstructed position obtained from the P-type detector in the first in-beam experiment.
The position resolution after subtracting the contribution of the width of the fiber,
is found to be about 60 µm (σ). A value of about 100 µm was found for the N-type
detector of the same thickness. The better performance of the P-type can be understood by looking at the signals reordered from the two detectors for the same event
in which an ion transverses the detectors in a central area shown in Fig. 3.27. The
53
fast collection of the electrons on the strips of the P-type combined with the short
integration times can qualitatively explain this difference in performance.
Figure 3.26: Position reconstructed along the strip of the P-type detector in mm.
The fibers are sharply reconstructed and the position resolution obtained is of the order of 60 µm (σ).
Figure 3.27: Pulses recorded for the P and N type for the same event obtained during
the in-beam test with 58 Ni beam. The red curve shows the sum of the
position signals of the P-type detector which collect the electrons. The
blue curve shows the total energy signal obtained from the back side of
the same detector. The black doded and solid curves show the signal
measured for the N-type detector from the position and the total energy
channel, respectively.
3.1.3.2 Energy resolution
The energy-loss resolution was also evaluated in these in-beam tests. The results
are consistent with all our previous in-beam data, namely the resolution is limited
by the energy straggling of the beam divided by the total energy loss as discussed
54
earlier and shown in Fig. 3.4. Of primarily interest is also the energy-loss resolution
obtained from the sum of the position channels, which is what one would use in high
rate measurements. Fig. 3.28 shows the sum energy of two opposite position channels (of the same strip) versus the reconstructed position from the second detector
placed with its strips vertically to the one examined here. In this way it is possible
to examine, in addition to the energy-loss resolution, the position dependency of
the measured energy across the short dimension of the strip and particularly the
behaviour in the interstrip region. In Section 3.1.2 we discussed the effects observed
in the interstrip region as obtained from the α-source measurements. Here we have
a more illustrative way of the same effects as the ions punch through both vertically
placed detectors. Indeed, the charge collection at the interstrip region is shared between the strips when the hit is close to the interstrip region. In Fig. 3.28 the energy
of two neighbouring strips is summed (i.e. four position signals are summed). The
summed energy, however, is still significantly less than in the intrastrip region. This
shows that the charge is not only shared between the strips but it is also not fully
collected. These plots are generated with rather short shaping times for the trapezoidal filter (150 ns integration and 32 ns gap) and the situation improves as one
integrates longer. For example, the total energy channel was integrated for about
E/a.u.
E/a.u.
0.5 µs, the result is shown in Fig. 3.30.
x/mm
x/mm
Figure 3.28: (left)The energy loss obtained from summing of the two opposite position channels versus the reconstructed position from the second perpendicularly placed detector. (right) The same plot as left but after
summing all four position channels of two neighbouring strips. The
peaking time in the FPGA is set to about 0.15 µs for both plots.
3.1.4 Conclusions
Following our investigations described int his Section we conclude the following:
• the use of digital electronics, which allow for a much finer tuning of the shaping parameters, is necessary in order to obtain the optimum resolution from
Si PSD. In addition, with the use of digital electronics different shaping parameters can be used in parallel for a single electronic channel.
55
EP−type /a.u.
180
×103
160
102
140
120
100
10
80
60
40
1
20
10
20
30
40
50
60
70
80
×103
100
EN−type /a.u.
90
EN−type /a.u.
Figure 3.29: Energy-loss measurement for two detectors placed next to each other.
The energy-loss resolution for the P-type (y-axis) is 1.8%. Thus, the
charge resolution obtained for 58 Ni is about 0.9% or 0.25 charge units,
consistent with previous experiments and with the limit that the energy
straggling sets, as shown in Fig. 3.4. The origin of the worse resolution
for the N-type detector (x-axis) is explained in Fig. 3.30
200 ×10
3
180
160
10
EP−type /a.u.
140
×10
3
160
22
20
120
155
18
150
16
145
14
100
1
80
12
140
10
135
6
130
4
125
-1
60
8
40
20
10-1
2
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0
x/mm
0
-0.1
0
0.1
0.2
0.3
0.4
0.5
0.6
x/mm
Figure 3.30: (left)Energy-loss measurement obtained from the total energy channel
of the P-type detector, using an integration time of about 0.5 µs, is
plotted versus the position reconstructed from the other placed detector
placed perpendicular to this one. The incomplete charge collection
is reduced, but it still remains comparable to the detector resolution.
(right) The same plot for the N-type detector. This particular detector
suffers from strong position dependent energy non linearities. Such
detector, for example, does not meet our performance requirements and
is returned to the company for replacement. This position dependency
explains the worse energy resolution in x-axis of Fig. 3.29.
• the strip PSD detector performs better in terms of resolution, rate and linearity
due its lower capacitance
• a position resolution of the order of 150 µm can be achieved even with than α
source which has a relatively small energy deposition (5.5 MeV)
56
• energy resolution is limited by the energy straggling of the beam in the thin
detector material. A charge separation of more than 3 σ can be achieved with
a 300 µm thick detector, while for thinner detectors the separation is less.
Based on these conclusions we discuss in Section 4.1 the final design for these detectors.
57
3.2 Fiber detectors
Fiber detectors have been used for years by the LAND/ALADIN collaboration. They
are used to determine the position of charged particles. A fiber detector consists
of a set of thin fibers made from scintillating plastic. The fibers are placed along
side each other, so that together they form an active detector area. The position
of a particle hitting the detector is given by the position of the fiber which is hit,
and the position resolution is given by the fiber width. So far two fiber detectors,
with 512 fibers each, have been used at the LAND/ALADIN setup. The fibers
have a square profile of 1000×1000 µm2 giving a resolution of 288 µm. One end of
the fibers is connected to a position-sensitive PMT. The fibers are placed in a grid
on the PMT, and the fiber number can be identified from the 2D-position on the
PMT. The position on the PMT can be determined from the signal heights of the
34 PMT channels [Cub98]. The ability to identify one out of 512 fibers with only 34
readout channels is the advantage of this system. The uncertainty in the position
determination leads on the other hand to misidentification and imposes a limit on
the number of fibers on one PMT.
A new fiber detector had been build and tested for the first time in 2012. This
detector had 1024 fibers with a profile of 250×250 µm2 . Multianode PMTs were
used to determine the fiber number for this detector. Four multianode PMTs from
Hamamatsu each one with 256 anodes [Ham2] were used. One end of each of the
1024 fibers was placed onto one anode. Hence the fiber number was determined
by the anode which gave a signal. A new GSI-developed ASIC called GEMEX was
used to read out the 1024 channels. The cornerstone of the GEMEX board is two
NXYTERs. The NXYTERs are designed to handle small signals at a high rates
and with low cost per channel. Each NXYTER has 128 channels, hence a GEMEX
board has 256 channels, which corresponds to the number of channels of a PMT.
The demand for higher spatial resolution (thinner fibers) and larger active areas
leads to a rapid increase in the number of fibers per detector. The largest fiber
detector in the proposed setup will have four layers of 6084 fibers each. Even with
the GEMEX system this would be a lot of channels to read out and a scheme
to reduce the number of channels has been tested. Instead of reading each fiber
individually at one end, we will read out the fibers at both ends. This will enable us
to attach a set of fibers to one channel. By combining the fibers differently in the
two ends we will be able to make a matrix, that will uniquely determine the fiber
number. A small example of the bundling and the corresponding matrix can be seen
in picture 3.31. This will lead to a reduction in the number of read-out channels
√
from N to 2 N , where N is the number of fibers. For the largest fiber detector,
this will reduce the number of channels from 24336 to 624 enabling the use of the
multianode PMTs with the GEMEX boards.
58
MPPC
1
2
3
4
1
1
5
9
13
2
2
6
10
14
3
3
7
11
15
4
4
8
12
16
Figure 3.31: A drawing of the bundling scheme and the corresponding matrix.
The tracking system will also contain smaller fiber detectors; the bundling will for
these detectors reduce the number of channels to 128. The minimum of 256 channels
of a GEMEX card would then be overscaled and another readout system has been
tested. The light will be detected by a special type of photodiodes called MultiPixel
Photon Counters (MPPC). A MPPC consists of a set of avalanche photo diodes
(the pixels) all working in Geiger-mode and all parallelly coupled to each other.
The signal height is given by the number of pixels firing, and the dynamic range
of the MPPCs are given by the number of pixels. We have used the S12572-025P
from Hamamatsu, which has 14400 pixels, each with a dimension of 25×25 µm2 .
The advantage of the MPPC is the possibility of detecting light and heavy particles
with the same detector. The number of photons, that are produced and reach the
MPPCs, ranges from 10 for protons to more than 50000 for heavy fragments. To
detect all photons from both types of particles would require a large dynamic range,
but since a 200×200 µm2 fiber only covers 8×8 pixels the photons will never hit
more than 100 pixels even though more than 50000 reach the MPPC. This will
automatically reduce the dynamic range to 0-100 pixels. This will also destroy all
information regarding the deposited energy, but since the detectors will not be used
for energy measurements this will not matter. The signals from the MPPCs will be
readout by a digitizer called FEBEX, which is built at GSI. The digitizers are very
fast and will be able to cope with the expected high beam rate at FAIR. The FEBEX
system has a low sampling rate (50 MHz), and the pure signal from the MPPCs are
too fast to be detected at this sampling rate. Preamplifiers that also stretch the
signals will be needed. Such preamplifier boards have been already developed at TU
Darmstadt.
59
Two detectors with the two new read-out schemes have been tested in three experiments at GSI in 2014. The detectors were tested with two medium heavy nuclei
(58 Ni and
48 Ca)
and heavy nuclei (238 U and fission fragments). The detector with
multianode PMTs was tested with an upgraded version of the GEMEX boards, and
a prototype using the bundling scheme and MPPCs was tested. The results of these
tests will be presented in the next sections.
3.2.1 Fiber detector with MPPC
Two prototypes for the small fibers have been built. The first one consists of 256
fibers with a profile of 200×200 µm2 placed on a 290×240 mm2 large frame. It has
been tested using a
58 Ni
beam. The second one has 1024 fibers and has been used
in the last two experiments. The fibers have been attached to a frame at the GSI
detector-lab. The fibers were bought on a spool and winded onto a large frame using
a winding machine at the GSI detector-lab. The fibers were afterwards glued to the
detector frame. The winding machine had previously been used for wire chambers,
but with some slight modifications it also works for plastic fibers. The machine can
place the fibers side by side. Since this was the first time it was used for plastic
fibers it also served as a test of that machine. The first frame with the 256 fibers
mounted on it can be seen in Fig. 3.32 along with a zoom on a part of the detector
to show the quality of the mounting. The bundling can also be seen in the figure.
The overall quality of the fiber alignment is very good, but from the in-beam test
of the detector it is clear that there was gaps between some of the fibers. That has
been improved for the second version by applying more tension to the fibers when
winding them. The result was a more homogeneous efficiency across the detector,
which will be shown later in this section.
(a)
(b)
Figure 3.32: A picture of the frame with fibers for the prototype. The bundling of
the fibers can be seen on the left. The right side is a close up of the
fibers.
The fibers were sorted in 16 (32 for the second version) bundles each containing 16
60
(32) fibers at each end. The sorting had to be done by hand and was done one
fiber after the other. We plan to use a laser with a narrow spot size to improve this
process in the future. Pointing the laser light onto one fiber will make that fiber
light up, making it easily identifiable. Afterwards the laser light can also be used to
test the quality of the sorting. The laser light will be directed onto one bundle of
fibers in one end, which should lead to one fiber in each of the bundles on the other
end shining. If this is not the case the bundling has been done wrongly and has to
be redone.
The fibers were read-out by 16 (32) MPPCs at each end. A board containing 16
MPPCs can be seen in Fig. 3.33a. The board also contains the 16 preamplifiers seen
as the small boards on top of the main board. The design allow for an easy change
in the amplification by simply exchanging the preamplifier boards. The fibers are
placed on top of the MPPCs by placing the metal bar containing the fibers (fig
3.33b) on six screws placed such that each of the 16 bundles are placed above one
MPPC.
(a)
(b)
Figure 3.33: (a) A picture of a board containing 16 MPPCs and preamplifier boards.
(b) A picture of the structure, that will ensure each fiber is placed on
top of a MPPC.
The signals were read out with the SIS3316 digitizers from Struck in the first experiment and by FEBEX digitizers in the last two. The Struck digitizers were used at
sampling rates of 250 MHz and 25 MHz, well above and below the 50 MHz sampling
rate of the FEBEX digitizers.
3.2.1.1 Results of the in-beam tests
First step was to check if the signals from the MPPCs were stretched enough for
a digitizer with the low sampling rate to reproduce the shape and reproduce the
energy. This was done by taking traces of the signals. Fig.
3.34 shows traces
taken at all three sample rates. The peaks have a width close to 800 ns, which is
the width the preamplifiers were designed to provide. The signal shapes are very
well reproduced even at a sample rate of 25 MHz and it is clear that the signals
can be identified on top of the background also at the low sampling rate of the
61
FEBEX digitizers, Fig. 3.34c. The traces also show that the signals always come
at a given time after the trigger. This information is used to provide an additional
software gate on the time to separate energy signals from random coincidences. This
will be especially important for high-rate experiments, where the probability of two
8000
Signal [au]
Signal [au]
Signal [au]
particles hitting within the same time window is very large.
8200
8000
8300
8200
8100
7000
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7800
6000
7900
7600
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t [au]
(a)
80
100
0
t [au]
(b)
20
40
60
80
100 120 140 160 180 200 220
t [au]
(c)
Figure 3.34: Traces taken at three sample rates: (a) 250 MHz, (b) 25 MHz and (c)
50 MHz (FEBEX).
The energy spectrum from one MPPC can be seen in Fig. 3.35. Energy cuts are
used software-wise, in order to identify the MPPCs, which sees light from a fiber.
The fiber which was hit can then afterwards be determined from the identified
MPPCs . The spectrum can be divided into four sections. The first one is the
background from noise in electronics and the MPPCs. The noise is the sharp peak
at low energy, ranging only from 0 to 500 ch. in the spectrum. The second part is a
long tail, which appears above the noise. This tail stems from neighboring MPPCs
giving small signals when one MPPC sees a large signal. These events only occur
in coincidence with a larger signal in another MPPCs (cross-talk). The true signal
is always significantly larger than these cross-talk signals, and it is easy to make a
software gate to get rid of these. The third part is a flat distribution over a large
energy range stemming from particles that only deposit a fraction of the energy in
the active part of the fibers. This is also seen in fig 3.28, where it is clear that
particles hitting the edge of a fiber do deposit less energy. The last part stems from
particles depositing the maximum possible energy in the fibers. The last two parts
are used in the further analysis and a software gate is made above 5000 au.
Ideally, in order to make a clear identification only one MPPC at each end should
provide a signal, hence a very important value is the multiplicity of MPPCs per
event. The multiplicity of the two boards is shown in Fig.
is clear, from Fig.
3.36 for the
58 Ni.
It
3.36, that only a fraction (42.6%) of the events have a 1x1
multiplicity (one MPPC with a signal at one end and one in the other end), and the
next step is to understand the events with higher multiplicity. Fig. 3.36 also shows
that 38.3% of the events have multiplicity 0x0. A significant amount of those are
due to the fact that this prototype detector has a very narrow active area and did
not cover the entire beam, as shown in Fig.
3.38. The detector had an external
trigger, hence events where the particles did not hit the fiber detector, but others
62
N
104
103
102
10
1
0
5000 10000 15000 20000 25000 30000 35000 40000 45000
E
Figure 3.35: An energy spectrum from one MPPC taken with a sampling rate of
25MHz. The MPPC is the same as used in Fig. 3.34.
in the setup, did lead to 0x0 event. The final detector will be eight times wider and
cover all the outgoing angles of the beam and beam-like fragments. A small part of
the 0x0 events stems from particles hitting the cladding around the fibers. This is
a very small fraction and as it will shown in section 3.2.3 that the efficiency of the
N2
detector is above 90% within the area it covers.
16
14
104
12
103
10
8
102
6
4
10
2
0
0
2
4
6
8
10
12
14
16
N1
1
Figure 3.36: The number of MPPCs giving a signal above the energy cut for board
1 vs. board 2.
To identify the events where a fiber is hit we considered events in which at least one
MPPC gve a signal. While the events with 1x1 are trivial to understand, the rest
of the events requires a more detailed analysis of the energies.
We considered the events for which two MPPCs on one or two boards give a signal.
Fig. 3.37 shows the energies of the two MPPCs on one board against each other
if two MPPCs on one board give a signal and only one MPPC at the other end.
The energy at the other end is not shown. The two plots in Fig.
3.37 show two
different types of behaviour. For board 1 both of the two MPPCs give signals far
63
above the background, and this corresponds to events, where a particle hits two
neighboring fibers which are connected to two different MPPCs on board 1 and the
same on board 2. This type of events is not seen in Fig.
3.37b, due to the way
the fibers are bundled. At this end almost all neighboring fibers are connected to
the same MPPC and if two neighboring fibers share the energy, the coresponding
MPPC does see the light from both fibers and gives a signal of twice the height. The
signals in Fig. 3.37b indicate a pure signal in one MPPC and noise slightly above
the threshold in another MPPC. The true signal stems in this case from the MPPC
with the highest signal. From similar plots for multiplicity 2x2 (two MPPCs in both
ends give a signal) events shows, that these events stem from particles hitting two
50000
E2
E2
neighboring fibers which goes to two different MPPC in both ends.
50000
18
16
25
40000
40000
14
20
12
30000
30000
10
15
8
20000
20000
10
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6
4
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5
2
0
0
10000
20000
30000
40000
50000
0
0
0
10000
20000
30000
40000
E1
50000
0
E1
(a)
(b)
Figure 3.37: The energies of the two MPPCs on one board against each other, for
events where two MPPC in board 1 (a) or board 2 (b) gives a signal
and one MPPC on the other board.
From the energy signals of the 2x1 and 2x2 events it is clear that if more than one
MPPC gives a signal at one end, the one with the largest signal is the true one.
Using this knowledge it is possible to assign a MPPC number for both ends and
then using the matrix to find the corresponding fiber. Fig. 3.38 shows the MPPC
numbers and the identified fibers from the
58 Ni.
The beam profile is clearly seen.
16
N
I2
Similar plots have been achived for the two most recent experiments.
103
14
1800
1600
12
1400
10
102
800
6
10
4
600
400
2
0
0
1200
1000
8
200
2
4
6
8
(a)
10
12
14
16
I1
1
0
0
50
100
150
200
250
Fiber
(b)
Figure 3.38: (a) The MPPC on each board against each other for each event. (b)
The fiber number for each event.
64
3.2.2 Fiber detector with multianode PMT
The fiber detector with multianode PMTs was placed 50 cm after the prototype
with the MPPCs in all three experiments. The fibers have a square profile with a
dimension of 250×250 µm2 plus two layers of cladding to avoid cross talk resulting
in an area of 24×29 cm2 . Each fiber is connected to an anode on one of four 256channel multianode PMTs. The 256 channels from one PMT were connected to one
GEMEX board via an attenuator board to reduce the signal height. In total four
PMTs, four attenuator boards, and four GEMEX boards were used. A picture of
the detector can be seen in Fig. 3.39. The white part is the active area, and the
color stems from the extra layer of white cladding that was added to avoid cross
talk. The boxes containing the four PMTs can be seen in the bottom. Three of
the attenuator boards reduced the signal by a factor of 390 and the last one by a
factor of 3300. This was to test which attenuation is required to fit the signal to
the dynamic range of the GEMEX boards. The factor of 3300 turned out to be too
much, and an attenuation factor of 390 will be used in the final design.
Figure 3.39: A picture of the detector using multianode PMTs.
For this detector each channel corresponds to a fiber and the important part is to
identify the channel that gets the signal from a fiber. The GEMEX boards provide
an energy and a time signal, and both can be used in the identification. Also for this
readout the energy signal provides the important cut while the time provides an extra
cut in cases where it is needed. Fig. 3.40 shows energy signals from two GEMEX
boards, the energy scale is in arbitrary units and inverted, hence the baseline is at
about 2500 and values below 2500 correspond to signals above 0. Fig. 3.40a and b
show energy spectra from a PMT with an attenuation of 390. Fig. 3.40b shows the
energy from one channel. Energy signals above the noise level can clearly be seen
proving that an attenuation factor of 390 provides a signal that fits very well to the
dynamic range of the GEMEX board. Fig. 3.40a shows the energy of all channels
for one NXYTER (128 channels), from this is it clear to see that the baseline and
noise level is channel dependent.
65
N
E
3000
100
50
2500
80
40
2000
60
30
1500
40
20
1000
20
500
0
0
20
40
60
80
100
120
Channel
0
10
0
0
500
1000
1500
2000
2500
3000
E
(a)
(b)
Figure 3.40: (a) The energy signal for each channel on one GEMEX board. The real
signals are attenuated by a factor of 390. (b) The energy signal from
one channel in (a).
The channel dependency on the noise level leads to the requirement of individual
energy cuts for each channel. The energy cuts are placed right above the noise
peak for instance at 1500 ch. for the spectrum in 3.40b. A spectrum from the
58 Ni
experiment displaying the fiber number for each event is shown in Fig. 3.41. The
spectrum is made by identifying the fiber number from the GEMEX channel if the
signal is above background. Also for this read out system the beam profile is clearly
N
visible.
1200
1000
800
600
400
200
0
0
200
400
600
800
1000
Fiber
Figure 3.41: Fiber numbers identified after making a software gate on the energy
signals from the GEMEX boards. The depicted distribution shows the
58 Ni beam profile in the x-direction.
66
3.2.3 Comparison between the two fiber detectors
The results from the two detectors can be compared to check the efficiency of the
detectors. Fig.
3.42a shows fiber numbers from the two fiber detectors against
each other. A correlation is clearly seen between the two detectors. The position
resolution can be calculated by subtracting the positions measured with the two
detectors. This difference can be seen in Fig. 3.42b along with a Gaussian fit. The
resolution is determined to σx = 0.39 mm, which mainly stems from straggling in
N
PMT
the air between the two detectors.
1000
300
800
3000
2500
250
2000
600
200
1500
150
400
1000
100
200
50
0
0
50
100
150
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250
MPPC
0
500
0
-10
-8
-6
-4
-2
(a)
0
2
4
6
8
10
dx
(b)
Figure 3.42: (a) Fiber numbers from the two fiber detectors against each other. (b)
the position resolution determined by subtracting the positions from
the two detectors.
To obtain a measure of the efficiency of the two detectors, the spectra with the
identified fibers from 3.38b and 3.41 are compared to plots with the extra condition,
that the other fiber detector also detect the particle. This is shown in Fig. 3.43a
and b for the
58 Ni
is shown in Fig.
experiment. The ratio between the gated and non-gated spectra
3.43c and d. These spectra show the efficiency as a function of
the position for the detectors. The over all efficiency is higher for the detector with
the MPPCs, Fig. 3.43c, but at the same time the efficiency is also more positiondependent. This is due to gaps in between fibers and overlaps in the first detector.
This has been improved in the second fiber-detector version, where the winding was
done with more tension. An efficiency plot from the
238 U
experiment is shown in
Fig. 3.44. Here the efficiency is almost constant at around 93% across the entire
detector.
The lower overall efficiency for the fiber detector with the PMTs is expected to stem
from the extra layers of cladding on the fibers, which will lead to extra deadlayers.
Since cross-talk hasn’t been a problem even without the cladding, only fibers with
one layer of cladding will be used. From these results of the prototype a 90 %
detection efficiency is expected.
67
N
N
1800
1200
1600
1000
1400
1200
800
1000
600
800
600
400
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0
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Fiber
50
1
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0.2
400
450
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250
Fiber
150
200
250
Fiber
1
0.8
0
150
(b)
Efficiency
Efficiency
(a)
100
550
600
650
Fiber
0
0
50
(c)
100
(d)
Figure 3.43: (a) All identified fiber numbers for the fiber detector with PMTs (blue)
and the identified fibers in coincidence with a fiber in the other detector
(red). (b) The same as (a), but for the detector with MPPC. (c) The
ratio between the two spectra in (a). (d) The ratio between the two
spectra in (b).
3.3 Time-of-flight plastic scintillator wall
One detector of the R3 B setup is a time-of-flight wall TOF13 made of plastic scintillators. This detector will be situated about 20 m down-stream of the reaction target,
behind the large dipole magnet GLAD, and will serve to determine the nuclear charge
(by energy loss measurements) and the velocity (by time-of-flight measurements) of
the fragments produced by reactions in the target. Although the current version of
the ToF wall was successfully used in many experiments in the past, we suggest to
build a new ToF detector with superior time and energy resolution at beam rates
up to 1 MHz. It is not planned to use a completely new detector concept but rather
take advantage of the experience collected over the last years and advance the current design. Therefore, we plan to build a new ToF detector based on fast plastic
scintillator material with a relative energy resolution of σ∆E /∆E<1% and a relative
time resolution of σt /t<0.02%. This will enable separating A from A-1 as well as
Z from Z-1 also in the region of the heaviest nuclei which will be delivered by the
Super-FRS.
The existing version of the ToF wall consists of 8 horizontal and 8 vertical paddles,
each 60 mm broad, 480 mm long and 5 mm thick. A light guide at both ends
68
efficiency
1
0.8
0.6
0.4
0.2
0
100
200
300
400
500
600
700
Fiber
Figure 3.44: The efficiency of the second version of the GSI winded detector measured with a 238 U beam.
reduces the rectangular cross section of the paddles to a circle with a diameter of
15 mm which is coupled to PMs from Hamamatsu (H6533). The signals are split
in an energy and time branch. The signals of the time branch are processed by a
constant-fraction discriminator (CFD) and recorded by a time-to-digital converter
(TDC). The time signals are also used to produce, together with time signals of
other detectors, a high level trigger signal for the read-out.
From previous experiments it became obvious that the existing versions of ToF walls
used in the R3 B/LAND setup have several disadvantages: Since the signals of the
PMs have to be split in a time and an energy branch, the analog signals for the
energy branch were delayed by about 600 ns until the trigger decision was made.
This long delay caused a damping of the signal by a factor 10, and made it necessary
to work with rather large PM’s output signals of the order of several volts. Moreover,
for the existing version of the ToF walls very fast photomultipliers from Hamamatsu
H6533 without active voltage divider have been used. Thus, for large currents, the
voltages at the last dynodes of the PMs could not be kept constant and the gain
of the PM varied. This resulted in a very strong dependence of the peak position
in the energy spectrum on the counting rate, leading to a bad energy resolution for
varying beam rates and effectively no resolution at higher rates (above ∼ 10kHz).
The new design will overcome the shortcomings in the following manner:
• We will completely omit the light guides and couple the scintillator directly to
the PMs in order to obtain the maximum photon numbers.
• We will use PMs with active voltage dividers.
69
• We will use electronics which allows to draw much smaller currents/charges
from the PMs.
The new detector will have four planes of scintillators and the active part will cover
an area of 1200x800 mm2 . Each plane will contain 44 vertical scintillator paddles
with the dimensions 800x27x3 mm3 (first two planes) or 800x27x5 mm3 (third and
fourth plane). Each scintillator paddle will be read out by photomultipliers (PMs)
at each end.
In this way, we will reach our design goals of 20 ps time resolution, energy resolution
better than 1% and high counting-rate capabilities up to 1 MHz.
3.3.1 Prototype developments and results
3.3.1.1 Energy resolution at high rates
The energy resolution of a prototype of the new ToF wall detector has been tested
using a test stand with a fast LED and also during the GSI beam time in April 2014
(experiment S438). With the LED test stand the conditions of previous experiments
could be simulated. The goal was to improve the setup in order to reach an energy
resolution of better than 1%, even at counting rates in the range 1kHz-1MHz.
3.3.1.2 LED tests
In the first step, the influence of the photomultiplier on the energy resolution has
been studied using a test stand with a fast LED, which was developed for the master
thesis of Julian Gerbig [Ger13]. The LED emits light in the same wavelength region
as the EJ200 or BC408 plastic scintillator, and the intensity as well as the pulse rate
can be varied. To obtain realistic conditions, the LED was pulsed in random mode
using a Le Croy pulser with external trigger. Two different PMs (see Table 3.7 for
their properties) from Hamamatsu were tested:
• PM1: Model R8619, a cost effective PM with an active voltage divider. This
PM is also used for the NeuLAND detector.
• PM2: Model H6533, which is a very fast PM but without active voltage divider.
This model was used in the past for the previously used time-of-flight wall
NTF.
At the beginning, the conditions of the previous experiments were simulated. The
intensity of the LED was set to simulate medium heavy nuclei, such as e.g. Ni
and the voltages of both PMs were set to extract large charges from PMs as it was
done in previous experiments. The frequency of the LED was varied between 5
70
Parameter
Diameter (inch)
Spectral response (nm)
Quantum efficiency
Dynode stages
Anode pulse rise-time (ns)
Electron transit time (ns)
Transit-time spread TTS (ns)
R8619-20
1
300-650
27%
10
2.5
28
1.2
H6533
1
300-650
22%
10
0.7
10
0.16
Table 3.7: Characteristics of tested photomultipliers.
Figure 3.45: The left side shows the relative nuclear-charge resolution and the right
side the shift in the peak position in the nuclear-charge spectra for
different rates.
and 800 kHz 2 . The analog outputs of the PMs were sent directly to a charge-totime converter (QTC IWATSU CLC101). These signals were then recorded by the
multi-event FPGA TDC VFTX2 [Kur96] developed at GSI.
Although the nuclear-charge resolution for both PMs was good for each individual
rate, it was observed that the peak position in the nuclear-charge spectrum shifted
with frequency (see Fig. 3.45). Especially for H6533 the shift was dramatic. For
large currents, the voltages at the last dynodes can not be kept constant and the
2
800 kHz is the maximum frequency that can be reached with Le Croy pulser using external
trigger.
71
gain of the PM is changing, see e.g. Fig. 12.15 in Ref.
[Ham06], leading to the
observed strong shift in the peak position. Thus, although H6533 has excellent
timing properties, it is not suited at all for nuclear-charge measurements at high or
varying counting rates.
The situation can be improved by taking PMs with active voltage dividers as in case
of R8619. For R8619 the shift is much smaller (see Fig. 3.45), but still visible and
too large for resolving nuclear-charges of heavy fragments. Further improvement in
the high counting-rate capabilities can be made by reducing the high voltage of the
PMs and, thus, also the anode current. This is shown in Fig. 3.46, where in case of
R8619 the shift in the peak position as a function of the counting rate is shown for
three different values of extracted PM’s charges. One can clearly see, that for lower
values of extracted charge the variation in the peak position with the counting rate
stay below 1 %.
Figure 3.46: Shift in the peak position in the nuclear-charge spectra for different
rates measured using R8619 at three different values of extracted PM’s
charges.
However, for too low high-voltage values, the relative resolution of nuclear-charge
measurements gets worse as illustrated in Fig.
3.47. Therefore, for an excellent
performance of the new ToF wall it is mandatory to use PMs with active voltage
dividers and to reduce the signal amplitudes via the HV of the PMs to a region were
the PMs work best with regard to rate stability and nuclear-charge resolution.
3.3.1.3 Tests with beam
In April 2014 during the S438 experiment a prototype detector consisting of 4 layers
could be tested with 58 Ni beams at various counting rates. Each layer was composed
of 4 paddles, each with a length of 800 mm and a width of 27 mm, but those in the
72
Figure 3.47: Relative nuclear-charge resolution for different rates measured using
H6533 (left) and R8619 (right) at three different values of extracted
PM charges.
first two layers had a thickness of 3 mm and those in the last two layers a thickness
of 5 mm, see Fig. 3.48. The paddles have been made out of scintillation material
EJ200 and are read out at both ends with the R8619 PMs from Hamamatsu.
Three different types of read-out electronics were tested; for more details see section
3.3.1.5. Half of the paddles was read out with a TAMEX prototype consisting of
LANDFEE2, a PECL-LVDS converter, and a VFTX2 (in a later stage the three
components will be combined on one PCB board). The other half was read out by
PADI and some paddles in addition with the QTC IWATSU CLC101. With the
TAMEX and PADI cards the time-over-threshold (ToT) was measured. Only the
QTC measured the charge of the signals. Due to problems with the TAMEX readout during this test beam time only results with the PADI will be presented here.
Please note, that in the October run of the S438 experiment also TAMEX readout channels have been successfully used. The preliminary results of the October
run analysis shows that using TAMEX read-out one can achieve the same time and
energy resolution as with PADI read-out. The energy loss of each fragment passing
the scintillators leads to production of light which was read out by two PMs for each
paddle. Fig. 3.49 shows the response of the PMs for a run where the
58 Ni
beam
was swept along a paddle by varying the field strength of ALADIN. The geometrical
73
Figure 3.48: Schematic view of the TOF13 prototype detector used in the test experiment. The labeling of different paddles is also shown. Paddles 1-8
had a thickness of 3 mm thick, while paddles 9-16 were 5 mm thick.
mean of the two ToT-measurements is not independent of the position along the
paddle and needs to be calibrated. This is done by fitting an appropriate function
to the response of each single PM. The result is then an energy loss measurement for
the fragments which is independent of the position as it can be seen in Fig. 3.49.
The nuclear charge of the reaction products can be determined after calibrating the
measured ToT. Figs. 3.50-3.53 show nuclear charges determined using the prototype
detector. The high voltage of the PMs was set to 500 V, corresponding to a PM
signal height of about 200 mV. By fitting the main peak one obtains at the lowest
rate of 5 kHz a resolution of σZ =0.19 charge units (for Z=28) which corresponds to
σZ /Z=0.68%. Even at the highest rate of 1000 kHz an excellent relative nuclearcharge resolution of 0.83% has been obtained, for a summary see Table 3.8. From
these values we can estimate the achievable nuclear-charge resolution in case of 208 Pb
at 1 AGeV. The main contributions to the energy-loss and, thus, nuclear-charge
resolution are coming from the number of detected photoelectrons and energy-loss
straggling. Considering that the 208 Pb beam at 1 AGeV passing through the TOF13
detector produces in average about 7 times more photoelectrons than the 58 Ni beam
at 0.5 AGeV, and, that the energy-loss straggling in case of lead is about a factor 5
larger as compared to nickel, we can estimate that the energy-loss resolution in case
of 208 Pb would be by a factor ∼3.8 worse as compared to the 58 Ni case. Therefore, the
nuclear-charge resolution σZ of 0.2 charge units as measured in the
would translate into σZ =0.4 charge units in case of
208 Pb
58 Ni
experiment
beam. This is below the
value of 0.5% charge resolution which is necessary to separate Z from Z − 1 in the
Pb-region.
74
ToT2 vs. position of paddle 14
ToT / ns
ToT / ns
ToT1 vs. position of paddle 14
32
30
32
30
102
102
28
28
26
26
10
24
22
20
22
-40 -30 -20 -10
0
10
20 30 40
Position / cm
20
1
ToT vs. position of paddle 14
-40 -30 -20 -10
0
10
20 30 40
Position / cm
30
32
30
102
102
28
28
26
26
10
24
22
20
1
Calibrated ToT vs. position of paddle 14
32
ToT / ns
ToT / ns
10
24
10
24
22
-40 -30 -20 -10
0
10
20 30 40
Position / cm
1
20
-40 -30 -20 -10
0
10
20 30 40
Position / cm
Figure 3.49: Measured Time-over-Threshold ToT for a run with 58 Ni beam at 500
AMeV, where the primary beam was swept over the full paddle length.
The top row shows the response ToT1 and ToT2 of the, respectively,
left and right PMs of the same paddle. The geometric mean of the two
measurements (bottom left) depends on the position along the paddle.
After a calibration one obtains a position independent ToT measurement (bottom right).
As already mentioned, not only the nuclear-charge resolution but also the stability
of the nuclear-charge measurements at different counting rates is important. In
Table 3.8 we show a shift in the position of the main Z=28 peak relative to the 5 kHz
measurement. We see that up to about 300 kHz counting rate the shift is about 1 %
or less. Only at higher rates the shift becomes larger than 1% which is not acceptable.
The situation can be considerably improved if one performs measurements with
smaller PM signal heights. By decreasing the HV values to 400 V, corresponding
to about 60 mV PM signal height, we have reached during the experiment very
stable nuclear-charge measurements, where relative shift in Z remained below 1%
75
1
5kHz
104
Counts
Counts
5kHz
Atomic number
10
3
Entries
92965
χ 2 / ndf
22.88 / 9
Constant
5832 ± 35.8
Mean
28.08 ± 0.00
Atomic number
104
295725
χ2 / ndf
120.1 / 13
Constant
2.421e+04 ± 6.680e+01
28.01 ± 0.00
Mean
0.1915 ± 0.0014
Sigma
Entries
10
3
0.1952 ± 0.0004
Sigma
102
102
10
10
5
10
15
20
25
30
5
10
15
Atomic number
20
25
30
Atomic number
Figure 3.50: Nuclear charge of the reaction products for a 58 Ni beam at 500 AMeV
measured with the prototype of the TOF13 detector at 5 kHz counting
rate. The spectrum is obtained from the scintillator paddles read out
with PADI. Left: Nuclear charge measured using paddles 3,8,12 and
15; right: Nuclear charge measured using paddles 3,7,11 and 15. The
high voltage of the PMs was set to 500 V, corresponding to a PM signal
height of ∼ 200 mV. An excellent relative nuclear-charge resolution of
σZ /Z=0.68% can be achieved.
Rate / kHz
5
59
375
1000
σZ
0.19
0.19
0.23
0.23
σZ /Z / %
0.68
0.68
0.82
0.82
Shift in Z / %
0.0
0.7
1.4
2.5
Table 3.8: Nuclear-charge resolution measured with the TOF13 prototype at different counting rates for the PM signal height of ∼ 200 mV. Also shown
(last column) is a shift in the position of the main peak relative to the
5 kHz measurement.
also for the highest rates of 1000 kHz while still keeping the excellent nuclear-charge
resolution, see Fig. 3.54.
Another good aspect of the TOF13 detector is that for each event the charge of the
passing ion is measured four times. Making correlation plots of charges measured
by different paddles one can remove all those events in which the passing ion has
suffered changes in its charge. Such events, see Fig.
3.55, are situated in the
correlation plots off-diagonal and can, thus, be easily removed from the analysis.
76
59kHz
Counts
Counts
59kHz
Atomic number
104
Entries
182575
χ 2 / ndf
71.46 / 12
104
Constant1.085e+04 ± 4.535e+01
27.99 ± 0.00
Mean
10
Atomic number
3
566015
χ2 / ndf
84.91 / 12
Constant 4.616e+04 ± 9.430e+01
27.8 ± 0.0
Mean
0.1964 ± 0.0007
Sigma
Entries
0.1907 ± 0.0003
Sigma
10
3
102
102
10
10
5
10
15
20
25
30
5
10
Atomic number
15
20
25
30
Atomic number
Figure 3.51: The same as Fig. 3.50 but at 59 kHz counting rate. The same relative
charge resolution of σZ /Z=0.68% has been achieved.
Rate / kHz
5
59
375
1000
σt / ps
41
41
45
64
σtdet / ps
14
14
16
23
Table 3.9: Time resolution measured with the TOF13 prototype between paddles 3
and 15 at different counting rates for the PM signal height of ∼ 200 mV.
Also shown (last column) is a time resolution of the whole detector.
3.3.1.4 Time resolution
In the same S438 experiment also the time resolution of the prototype detector has
been studied. Figure 3.56 shows the time-of-flight measured between the paddles
3 and 15 for different counting rates, see also Table 3.9. The measured sigma
between two paddles at 5 kHz amounts to 41 ps, which would correspond to 14 ps
time resolution for the whole detector. This value is well within the design goal of
20 ps. Even at 1000 kHz one could reach a time resolution for the whole detector
of 23 ps, which is rather close to the design goal. Please note, that the contribution
from the electronics based on PADI amounted to 25 ps per read-out channel in the
April run. In the October run, for the both read-out systems (PADI and TAMEX)
contribution of the electronics was about 20 ps.
77
104
10
375kHz
Counts
Counts
375kHz
Atomic number
3
Entries
140220
χ2 / ndf
81.62 / 13
Constant
7553 ± 36.5
Mean
27.73 ± 0.00
Sigma
Atomic number
10
4
Entries
291945
χ2 / ndf
687.4 / 17
Constant
0.2203 ± 0.0010
10
102
3
1.879e+04 ± 5.433e+01
27.61 ± 0.00
Mean
0.2309 ± 0.0005
Sigma
102
10
10
5
10
15
20
25
30
Atomic number
5
10
15
20
25
Atomic number
Figure 3.52: The same as Fig. 3.50 but at 375 kHz counting rate. The relative
3.3.1.5 Readout electronics
In the test experiment with
58 Ni
beams three different read-out electronics were
tested: A prototype of the TAMEX card, PADI, and a QTC module IWATSU
CLC101. For the final readout electronics a new multichannel electronic card (TAMEX)
has been designed by the GSI CSEE group. The card is a combination of the existing
TacQuila [Koc05] and a FPGA based TDC from the VFTX module [Frü12]. A time
resolution of about 15 ps is expected for this card. The energy loss of the fragment
can be measured either by time-over-threshold (ToT) or by an add-on QTC card.
At the time of the experiment only a prototype of the TAMEX card was available
and tested (see section 3.3.1.3). This version did not have the QTC. The TAMEX
card will also be used and produced in large quantities for the NeuLAND detector
(6000 channels). Therefore, it will be very cost effective to use the same electronics
for the read-out of the ToF wall.
Another possibility of read-out electronics is also the general purpose Pre-AmplifierDiscriminator (PADI, [Cio14]) together with VFTX modules. PADI was developed
for the CBM time-of-flight wall and should be also well suited for our purpose.
For PADI a time resolution of about 15 ps can be expected and the charge can be
measured by a ToT-measurement only.
The third read-out for the test experiment was the QTC IWATSU CLC101. This
78
1000kHz
10
Counts
Counts
1000kHz
Atomic number
3
19560
Entries
χ2
/ ndf
Constant
Mean
Sigma
Atomic number
53.49 / 12
10
997.7 ± 13.2
3
27.43 ± 0.00
Entries
41885
χ2 / ndf
58.67 / 10
Constant
2668 ± 23.2
Mean
27.34 ± 0.00
Sigma
0.2301 ± 0.0028
0.2297 ± 0.0033
102
102
10
5
10
10
15
20
25
5
10
15
20
Atomic number
25
Atomic number
Figure 3.53: The same as Fig. 3.50 but at 1000 kHz counting rate. The relative
module integrates the signal and applies afterwards a ToT measurement. In this
way, the response of the charge measurement is linear. As the other read-outs it
can handle high count rates up to 1 MHz but for this QTC the time resolution is
only 100 ps at best. However, it was used for the test experiment to have linear
charge measurements which can be compared to the ToF measurements of the other
read-outs.
3.3.2 Time-resolution simulations
The objective of the simulations and calculations was to model the timing response
of the detector and to evaluate the contribution of the various processes which
contribute to the timing resolution. For these studies GEANT4 simulations were
performed. However, simulations with photon tracking are time consuming and,
therefore, also calculations using the statistical model were performed.
3.3.2.1 Time resolution using statistical model
In order to study different effects influencing the time resolution and search for the
compromise between best-performance capabilities and costs we have used the statistical model as originally described in Ref. [Pos50]. There are several statistical
processes that are limiting the attainable time resolution of scintillation detectors:
79
1000kHz
104
Atomic number
Entries
107770
χ2 / ndf
48.37 / 14
Constant
10
3
Counts
Counts
1000kHz
Atomic number
10
4
153.3 / 15
2.74e+04 ± 6.65e+01
28.08 ± 0.00
Mean
28.1 ± 0.0
Sigma
376205
χ 2 / ndf
Constant
7260 ± 34.5
Mean
Entries
0.2307 ± 0.0005
Sigma
0.2383 ± 0.0011
10
3
102
102
10
18
10
20
22
24
26
28
30
18
20
Atomic number
22
24
26
28
Atomic number
Figure 3.54: Nuclear charge of the reaction products for a 58 Ni beam at 500 AMeV
measured with the prototype of TOF13 detector at 1000kHz counting
rate. High voltage of the PMs was set to 400 V, corresponding to a PM
signal height of ∼ 60 mV. The spectrum is obtained from the scintillator paddles read out with PADI. Left: Nuclear charge measured using
paddles 3,8,12 and 15; right: Nuclear charge measured using paddles
3,7,11 and 15. Achieved relative nuclear-charge resolution amount to
σZ /Z=0.82%.
Time spread in the energy transfer to the optical levels of the scintillation, decay
time of the excited states, fluctuations in the propagation time of photons through
the scintillator, creation of photo-electrons within the photo cathode of the photomultiplier, as well as the associated electronics. First studies on the statistical
limitations on the achievable time resolution using scintillation detectors have been
done in early 50-ies by Post and Schiff [Pos50]. The basic idea of this model was
that the probability PM (t) that M photo-multiplier pulses occur between the time
zero - defined as the time of the initial excitation of the scintillator, and the time t
is given by the Poisson distribution:
PM (t) =
1
· [N (t)]M · exp[−N (t)],
M!
(3.14)
where N (t) is the average expected number of photomultiplier pulses between [0, t],
R∞
with Rtot = N (t)dt being the average total number of created photoelectrons.
0
Starting from this probability distribution, one can calculate the probability that
80
Atomic number from paddle 15
5kHz
102
28
26
24
10
22
20
18
16
16
18
20
22
24
26
28
Atomic number from paddle 12
1
Figure 3.55: Correlation between charge measured by paddles 12 and 15. Contributions from events where passing ions suffer changes in their charge are
clearly seen and can be removed from the analysis.
the Qth photoelectron is detected in the time interval [t, t + dt] as [Pos50]:
WQ (t) = PQ−1 (t) ·
1
dN (t)
dN (t)
=
· [N (t)]Q−1 · exp[−N (t)] ·
.
dt
(Q − 1)!
dt
(3.15)
The time resolution σt can then be calculated from the variance of the time signal:
σt2
1
= ht i − hti = tot ·
WQ
2
2
Zt
0
2
t · WQ (t)dt −
Zt
0
t · WQ (t)dt
2
,
(3.16)
where WQtot is the total probability that the Qth photoelectron occurs between [0, ∞]
PQ Ri−1
tot
tot
and is given as [Pos50]: WQ = 1 − exp − Rtot · i=1 (i−1)! .
Usually, the function dN (t)/dt is given as a convolution of different above-mentioned
contributions influencing the time resolution. On the other hand, as these processes
- photon creation and photon transport, photoelectron conversion, and signal processing, are independent processes, one can calculate the timing resolution for each
of these processes (σi ) using above equations, and then obtain the total time resoluP
tion as a quadratic sum of individual components, i.e. σt2 = σi2 . The advantage is
that in this case one can study and optimize the influence of different contributions
81
16000
14000
12000
10000
59kHz
Counts
Counts
5kHz
ToF
Entries
311850
χ2 / ndf
1410 / 41
30000
25000
0.01539 ± 0.00008
Sigma
0.04112 ± 0.00006
552485
χ 2 / ndf
1983 / 41
Constant2.701e+04 ± 4.618e+01
Constant1.507e+04 ± 3.408e+01
Mean
ToF
Entries
20000
Mean
0.01882 ± 0.00006
Sigma
0.04065 ± 0.00005
15000
8000
6000
10000
4000
5000
2000
0
-2
-1.5
-1
-0.5
0
0.5
1
0
-2
1.5
2
Time / ns
-1.5
-1
ToF
14000
12000
Entries
325280
χ 2 / ndf
2055 / 65
Mean
Sigma
0.5
1
1.5
2
Time / ns
700
0.005784 ± 0.000080
0.5
1
1.5
2
Time / ns
ToF
900
800
Constant 1.424e+04 ± 3.163e+01
10000
0
1000kHz
Counts
Counts
375kHz
-0.5
600
Entries
24200
χ 2 / ndf
1111 / 81
717 ± 6.5
Constant
Mean
0.03498 ± 0.00044
Sigma
0.06354 ± 0.00040
0.04518 ± 0.00006
8000
500
6000
400
300
4000
200
2000
0
-2
100
-1.5
-1
-0.5
0
0.5
1
1.5
2
Time / ns
0
-2
-1.5
-1
-0.5
0
Figure 3.56: Time of flight measured between paddles 3 and 15 at different counting
rates.
- scintillator, photomultiplier, and electronics, to the timing resolution separately.
One of the important ingredients of the statistical model is the shape of the measured light pulse dN (t)/dt. This shape is of course influenced by different processes
mentioned above. The primary shape is given by the light-production mechanism,
and in case of plastic scintillator it has been shown [Mos77] that the best-suited
shape is given by a convolution of an exponential and a Gaussian function, so-called
ExpGaussian [Haw13]:
2
dN (t)
σ
1
t−3·σ
t−3·σ
σ
=
· exp
− √
−
· 1 − erf √
,
dt
2·τ
2 · τ2
τ
2·τ
2·σ
(3.17)
where σ = τrise /ln(9) and τ = τdecay , with τrise and τdecay being, respectively, rise
and decay time of a given scintillator material as given by the supplier.
In case of small-size scintillator detectors the light-production mechanism has a
dominant role. For timing properties of larger-size detectors light transport becomes very important, and to consider this effect we have followed the work of
Ref.
[Ach07]. Knowing the light-pulse shape seen by a PM, using the statistical
model we can calculate the contribution of the scintillator σsci . The contribution
82
Parameter
Rise time (ns)
Decay time (ns)
Attenuation length (cm)
Light output (% Anthracene)
Scintillation efficiency (photons/1 MeV e−1 )
Wavelength of max emission (nm)
Ratio H:C atoms
Density (g/cm3 )
Refractive index
EJ200
0.9
2.1
210
64
10000
425
1.102
1.023
1.58
EJ204
0.7
1.8
140
68
10400
408
1.100
1.023
1.58
EJ232
0.35
1.6
17
55
8400
370
1.101
1.02
1.58
Table 3.10: Characteristics of considered scintillation materials from the Eljen Technology [Elj].
from the PM is determined by its transient-time-spread (tts) and can be calculated
√
as: σP M q
= tts/(2.35 · Rtot ). Then, the total time resolution σt can be calculated
as: σt =
2 + σ2
2
σsci
P M + σel . For simplicity, we assume that the traversing particle
is passing through the middle of a paddle. We have assumed numerical values of
Rtot which correspond to the number of photoelectrons created due to a passage of
relativistic 1 AGeV ions ranging from 12 C to
238 U
through given thickness and type
of scintillator material. To do this, we have calculated the energy loss of the passing
ions using the ATIMA code [Ati], taken the number of electrons per MeV deposited
energy as given by the scintillator supplier, calculated the quenching factor suited for
heavy-ion beams at relativistic energies according to Ref. [Sal82], taken the transport efficiency from GEANT simulations and considered the quantum efficiency of
a given photomultiplier as given by the supplier of photomultiplier.
For calculations presented here, we have assumed three different scintillation materials: EJ200, EJ204 and EJ232 from Eljen Technolgy, see Table 3.10; very similar
properties have also BC408, BC404 and BC422, respectively, from Bicron. We have
considered only the R8619 photomultiplier. The contribution of electronics σel was
assumed to amount to 15 ps.
In Table 3.11 we present the time resolution needed to separate A from A − 1 in
different mass regions. In figure 3.57 we present the achievable time resolution of the
new TOF wall detector calculated using the above-described statistical model for
the same nuclei as shown in Table 3.11. Due to the higher deposited energy, and,
thus, higher light production, the calculated time resolutions improve with increasing
nuclear-charge of the considered fragments. For all three scintillation materials, the
calculated time resolution fulfill fully the design goals. The best resolution, especially
in case of light nuclei, is achieved in case of EJ232 material. Nevertheless, due to
porosity of the material EJ232, it is not possible to make paddles of the size needed
for the ToF wall. Thus, the EJ232 material cannot be consider as a material for the
planned detector.
83
Fragment
12 C
58 Ni
132 Sn
208 Pb
238 U
σtneeded (ps)
106
28
15
15
15
Table 3.11: The table shows, for each considered fragment, the time resolution
σtneeded needed to separate A and A − 1. We assumed 1 AGeV kinetic
energy and 20 m long flight path.
Figure 3.57: Calculated time resolution for several fragments: 12 C (full thick line),
58 Ni (dashed thick line), 132 Sn (dashed-dotted line), 208 Pb (full thin
line) and 238 U (dashed thin line) at 1AGeV kinetic energy. The contribution of electronics σel has been assumed to amount to 15 ps.
In case of EJ200 and EJ204 scintillation material we show in Fig. 3.58 the different
contributions to the total time resolution of the new ToF wall detector assuming
208 Pb
beam at 1 AGeV.
First of all, it can be seen that the contribution of the photomultiplier to the time
resolution is negligible. This is due to the high amount of produced photons after
the passage of relativistic heavy ions through the detector. Thus, there is no need
to use fast and expensive photomultipliers in order to obtain good time resolution.
Much more important for our purpose is, as already explained in section 3.3.1.1, the
active base which increases the capability of sustaining high counting rates.
84
The contribution of the electronics for the whole detector is about 5 ps. Such a good
electronic resolution can be achieved using a 16 channel leading edge VME-FPGATDC (VFTX2) [Kur96] developed in the CSEE departement at GSI, see section
3.3.1.5. Please note, that with four layers of paddles, we measure four times the
ToF, and thus we get for the whole detector a better electronic resolution than for
one readout channel.
The contribution from the light production and light transport in the scintillator
material to the total time resolution is of the same order as the contribution from
the electronics.
Figure 3.58: Different contributions to the total time resolution of the new ToF
wall detector calculated for 208 Pb at 1 AGeV assuming EJ200 (left)
or EJ204 (right) scintillation material: photomultiplier (dash-dotted
line), electronics (thin full line), scintillator (dashed line), total (thick
full line).
3.3.2.2 GEANT4 simulations
The geometry of the individual scintillators as well as the full detector assembly
was studied using the GEANT4 simulation toolkit including the package for optical
photon tracking. The simulations included light production by fragments traversing
the scintillators, tracking of the photons to the PMs at the far ends of the scintillators, quantum efficiency of the photo-cathode, convolution with the single-electron
response of the PM and leading edge timing at a given threshold.
For the production of the scintillation light in GEANT4 the bi-exponential function
It =
I0
(exp[−t/τd ] − exp[−t/τr ]),
τd − τr
(3.18)
is used where I0 is the photon yield, τr and τd are the raise and decay time of the
scintillator material.
85
One of the most important ingredients to calculate the expected timing resolution
is the number of photons produced by the impinging ion. The energy loss can be
calculated rather accurate but the light output is quenched and depends on the
ionisation density. To calculate the quenching factor is one of the largest systematic
uncertainties in the prediction of the time resolution.
In GEANT4 the Birk’s formula is implemented in order to calculate the photon yield
dL/dx as a function of the energy loss per path length dE/dx:
dL
dE/dx
= L0 ·
.
dx
1 + kB · dE/dx
(3.19)
kB is Birk’s constant and the proposed value for polystyrene-based scintillators is
0.126 mm/MeV (see also [Lev11]). With this value GEANT4 calculated that 400000
photons are produced for each incoming Ni fragment. However, if one compares this
with the results of the statistical model calculation where Ref.
[Sal82] was used
to calculate the quenching factor GEANT4 obtains a factor of 4 more photons. (It
should be mentioned here that the quenching factor calculated with Birk’s law in
Ref. [Lev11] is used for minimum ionising particles and Ref. [Sal82] is more suited
for heavy ions.) Therefore, we adapted a value of 0.55 mm/MeV for Birk’s constant
which produces for the Ni beam the same amount of photons.
The photons were then tracked until they reached the PMs at both ends of the scintillator. The quantum efficiency of 28 % of the photo-cathode was taken into account
and the TTS of the PM was simulated by adding a transit time which followed a
truncated gaussian distribution with the corresponding FWHM. The photo-electron
number and their arrival time was then recorded. The scintillator bar was read out
by two PMs with a diameter of 25 mm. Fig. 3.59 shows the arrival time distribution
of the electrons.
Figure 3.59: Arrival time distribution of the photons (left) or accordingly photoelectrons at the PM and their resulting electric pulse (right).
By plotting the arrival time distributions of the nth electron for many events, see
Fig. 3.60 one can obtain the expected time resolution as the spread (sigma)of the
distribution.
86
Figure 3.60: Arrival time distribution of the 1st (left) and the 20th (right) photoelectron. The spread of the distribution yields the achievable time
resolution.
In addition, the single electron response (SER) of a PM was recorded and digitized.
In this way, an electronic pulse (see Fig.
3.59) could be reconstructed by adding
the SER at each time a photo-electron was registered in the simulations. By looking at the time when the signal was larger then a given threshold a leading edge
discriminator could be simulated.
In this way, simulations for a 500 AMeV Ni beam impinging on a scintillator with
the dimensions 800x27x5 mm3 were performed. These simulations can be directly
compared to the S438 beam time in April 2014 where a prototype detector was
irradiated by a 500 AMeV
58 Ni
beam.
3.3.2.3 Comparison between GEANT4 simulations and statistical model
The predictions of the statistical model and GEANT4 simulations for the time resolution of the TOF13 detector are shown in Fig. 3.61. We have in both cases assumed the same experimental conditions that were met during the S438 beam time
in April 2014:
58 Ni
beam at 500 AMeV kinetic energy impinging on the TOF13
detector made out of EJ204 scintillation material. For the contribution from the
electronics we have assumed 25 ps as it was measured during the experiment with
the electronics based on PADI.
Agreement between these two sets of calculations is rather good. In both cases, the
strongest contribution comes from the electronics. As already mentioned, using the
TAMEX based read-out, we will be able to reduce the contribution from the electronics for the whole detector down to 5 ps. The contribution from the scintillation
material - light production and light transport is also rather similar in both cases.
Somewhat different are predictions on the contribution of the photomultiplier which
is in the statistical model smaller than in the GEANT4 simulation. This can be
understood, as in the GEANT4 details of the single-electron response of the photomultiplier have been considered, which was not the case in the statistical model, see
section 3.3.2.1.
87
Figure 3.61: Time resolution of the TOF13 detector calculated with GEANT4 (left)
and statistical model (right) assuming 58 Ni beam at 500 AMeV. Shown
are different contributions to the total time resolution (full thick line):
electronics (thin full line), photomultiplier (dashed-dotted line) and
light production and light transport ( dashed line).
3.3.2.4 Comparison to experimental data
As already mentioned above, during the April run a time resolution of 14 ps has been
obtained using the prototype of TOF13 detector. In the simulations, see Fig. 3.61
we obtain about 15 ps for low values of photoelectron threshold and about 10 ps
for the higher photoelectron thresholds. This good agreement between simulation
and experimental data confirms that our design goal of 15 ps time resolution for the
heaviest beams can be reached with the TOF13 detector, see Fig. 3.58.
3.4 Conclusions
The results presented in this chapter have provided us with a detailed understanding
of the performance advantages and limitations of the chosen detector technologies.
In this chapter we have also shown that critical components of the systems, such as
the electronic readout, has been successfully tested in beam. Based on these results
and the specific physics requirements, we have set the technical specifications and
the design details of each tracking detector system, which are discussed in the next
chapter. The performance of the full system is examined by means of Monte-Carlo
simulations in Chapter 5.
88
4 Technical Specifications and Design
Details of Tracking Detectors
This chapter presents the technical specifications and the design details of each of
the tracking detector systems, based on the prototype results and calculations. The
detectors are discussed in the following order, first Si detectors, then fiber detectors,
TOF13 detector and finally the proton arm detectors.
4.1 Si detectors
As described in Chapter 2, Si detectors are used for energy loss (charge identification)
and precise position measurement before and after the target. The energy loss
measurements do not only need to be precise enough to distinguish neighbouring
charges but must also be as straight forward as possible with minimum corrections
and calibration requirements such that they can be used for an unambiguous online
monitoring of the incoming beams.
The nominal configuration is placing two Si detectors before the target. From the
two 2D-position measurements the angle and position on target is determined. The
outgoing angle is determined by the position on target and the third position measurement between the target and the large acceptance dipole magnet. This holds
as long as the target thickness at most around 1 cm. For larger target thicknesses
(as in the case of a Liq. H2 target of the order of 10 cm long or a gas target of
the order of 50 cm long) using the target position as the first point for determining
the outgoing angle is no longer valid and two position measurements are required
after the target in order to obtain the outgoing angle and to reconstruct the reaction
vertex inside the long target.
Based on our experience from in-beam tests with both double-sided microstrip Si
detectors and position sensitive strip Si detectors we have designed the final system
as a hybrid that can make the most out of both technologies, depending on the
physics requirements. Furthermore, to meet specific requirements of the versatile
physics program the detector technology chosen can be adapted to the different
needs by straight forward modifications such as different segmentation, choice of
89
thickness or mounting. In the following we discuss first the common specifications
for the Si detectors and then present in more detail each type.
4.1.1 Detector size and thicknesses
The size of the detectors before the target are determined by the size of the incoming
beam taking into account that typically the focusing is not at the detector position
but somewhere closer to the target. Typical sizes of secondary beam spots at about
two meters before the target can require a detector size of up to 10x10 cm2 . The
size of the detector after the target is also required to be large due to the scattering
and reactions that take place at the target area in addition to the large spread in
the incoming angles. The maximum size of 10x10 cm2 active area is planned for this
detector. At a distance of ∼0.70 m from the target its acceptance is ±70 mrad close
to the maximum acceptance of the dipole magnet of ± 80 mrad. Its maximum size
is basically limited by the available detector technology of 6-inch wafers.
The thickness of the detectors is chosen as the optimal solution between position resolution, energy loss measurement, energy straggling and angular straggling but also
the detector rise time, which defines the rate performance. In addition, since these
detectors are placed in the beam line they must introduce a minimum background
reaction rate and should withstand radiation damage. The first detector is chosen
to have a thickness of 300 µm, which allows for a clean identification of neighbouring
charges. The second detector closer to the target has a thickness of 100 µm thickness
to minimise the contribution of the angular straggling in the determination of the
incoming angle and position on target and also to generate less background in the
target recoil detectors. Its energy-loss measurement although a factor of two worse
than the first detector is particularly useful as a redundant measurement and clean
out charge changing reactions that occurred in the first detector. The thickness of
this detector is crucial since the angular straggling introduced by its material is the
dominant factor in the resolution of the fragment’s trajectory and thus in its mass
and momentum measurement. On the other hand, as we have shown, limiting the
thickness compromise the charge identification. Its optimum thickness is determined
for each type of experiments and can vary from 100 - 300 µm.
4.1.2 Detector technology
For the Si detector after the target whose size is very large the current detector
technology sets a limit of ∼150 µm thickness below which the detector becomes too
fragile to process. We foresee that this will improve in the near future to 100 µm.
Thinner detectors are also more radiation hard since they deplete at much lower
90
voltage. As discussed in Section 3.1.2.3, special care should be taken to have interstrip separation as small as possible. The detectors planned will have interstrip
separation of 30 µm. This is a factor of three less compared to the prototype strip
detectors discussed earlier.
Another requirement for the Si detector technology is the uniformity of the Si wafer
thickness. We request the lowest guaranteed uniformity of about 2 µm. A detailed
scanning of the detectors with a narrow primary beam can be used to correct for
the energy-loss variation due to the non uniformity of the detector thickness.
4.1.2.1 Microstrip detector
For the experiments where multi-hit capability is required, the PSD Si strip detector
after the target is replaced by a microstrip detector. Considering the number of
electronic channels and the requirement for minimum material in the beam line we
chose as an optimum design for the Si microstrip detector a double-sided detector
with a size of 10x10 cm2 and a pitch of about 700 µm on both sides. Such a detector,
for example, is the TTT series provided by Micron Semiconductor. Based on our
measurements we expect that it can operate at rates of up to 50 kHz per strip.
4.1.2.2 Position sensitive detectors
As discussed in Section 3.1 the position sensitive Si detectors can provide an excellent
position resolution with large continuous active areas where charge collection is
complete, as long as the deposited energy is large enough. This detector is also
foreseen to be cooled for reduction of leakage current, improvement in resolution and
charge collection time. As discussed earlier, the resistive layer that is introduced for
this detector coupled with the detector capacitance is introducing a time constant
which limits the detector rise time. It was shown, for example, that a continuous
surface of the size and of thickness that we are discussing here would cause detector
rise times of the order of several µs, which is too slow given the beam-rates that we
expect. The technology of the detector will be a double sided resistive strip detector
with a pitch of about 3 mm. The separation of the detector into strips shares the
main beam rate between two or more strips but also reduces the capacitance such
that the charge diffusion along the resistive layer becomes much faster as does the
rise time as well. Furthermore, in this way the resistive charge division takes place
along one dimension (in contrast to a continuous 2D detector), which simplifies the
position reconstruction.
In Section 3.1.3 was shown that the P-type single-sided detector, in which the strips
are collecting the electrons, has a much faster signal rise time and a better position
resolution. For the future double-sided detector discussed here, this side will be
91
used for reconstructing the x-position, which being along the dispersion axis it is
more demanding for the reconstruction of the charged particle trajectory through
the magnetic field. For the total energy-loss measurement the sum of the p-side
strip signals will be used. The electron collection (and thus half of the maximum
signal height) occurs very fast and allows for short shaping times to be used. The
electron collection is also less prone to charge collection problems caused by radiation
damage.
4.1.3 Operation and radiation hardness
The detectors will be in vacuum, operated in an over biased mode and cooled to
-200 C to improve the charge collection time and reduce the leakage current which
will inevitably be built after heavy irradiation. The detectors are foreseen to be
changed after each experimental campaign.
4.1.4 Electronics
• The large position sensitive Si detector (10x10 cm2 ) has 32 strips per side with
two readout channels at each end of the strip, summing up into a total of 128
electronic channels.
• The microstrip detector of the same size and a strip pitch of about 700 µm
results in a total of 256 electronic channels.
• The two position sensitive detectors before the target require a total of 192
channels.
We plan to use the same electronics for both type of detectors, i.e. a total of 448
channels.
The signals will be first shaped using preamplifiers inside the vacuum chamber and
then digitised using the FEBEX system [Feb]. Cooling of the preamplifier electronics
and the detector is foreseen for reducing the noise and the leakage current.
4.1.5 Sumarry
Table 4.1 summarises the design for the Si detectors, as presented in the previous
sections.
92
Table 4.1: This table shows the dimensions for each detector and the required electronic channels. Detector position 1 and 2 refer to the two position measurements before the target, while 3 and 4 refer to those after the target.
All detectors are double sided strip detectors.
Detector position
1
2
3
4
(3)
type
resistive strip
resistive strip
resistive strip
resistive strip
microstrip
size (cm2 )
10x10
5x5
10x10
10x10
10x10
electr. chan.
128
64
128
128
256
thickness µm
300
100
100-300
100-300
100-300
4.2 Fiber detectors
The tracking system will contain a total of five fiber detectors. This section contains the specifications for these detectors. The fiber detectors will be designed to
provide position measurements with very high precision, at various places along the
trajectories of the beam and of the beam-like fragments. Two fiber detectors will
be placed in front of the target, one directly after the target, one directly after the
GLAD magnet and the last one at the end of the track, just in front of the Time
of Flight wall. This will enable an accuarate determination of the fragment’s flight
path. The placements can be seen in Fig. 1.2. The fiber detectors will on the other
hand not provide any time or energy information.
Besides being crucial for fragment tracking, the accurate position provided by the
fiber detectors is of great importance in the energy-loss and time calibration of both
the silicon detectors and the Time-of-Flight wall.
The active area of the detectors will be made of one, two, or four layers of thin
fibers depending on the requirements on the measurement. The fibers are made of
scintillating plastic called SCSF-78 from the company Kuraray [Kur]. The plastic has
a peak emission of light with a wavelength of 450 nm (blue) and a long attenuation
length (>4.0 m). The profile of the fibers will be squared with a size of 200×200
µm2 . The square profile will enable a tighter packing of the fibers compared to
round fibers, which will lead to total detection efficiency of 90% for one detector,
see Fig.
4.1. The 200 µm will give a position resolution of σ = 57.7µm across
the fiber. All detectors will be wound and attached to their frames at GSI using
the same machine as tested with the prototype, section 3.2.1. The idea of bundling
the fibers will also be applied to all the detectors to reduce the number of read-out
channels. The sorting and bundling of the fibers will be done by hand using a laser
to help identifying the fibers individually as described in section 3.2.1.
93
Figure 4.1: A drawing showing the difference between fibers with square and round
profiles. The thickness of the detector with square fibers is always constant, while the round fibers will have areas with very little material,
leading to too small signals and dead regions.
4.2.1 The structure of the first four detectors
The dimensions of the active areas of the detectors are chosen to first of all fit the
requirements named in chapter 2 and are further optimized to fit the electronics used
for the detector. An overview of the dimensions and requirements of the detectors is
presented in Table 4.2. The three first fiber detectors are meant to be alternatives to
the silicon detectors in experiments with high rate and without the need for energy
information, section 2.3. Furthermore they will be used in the position calibration
of the silicon detectors. Therefore the detectors need to cover at least the active
area of the silicon detectors, while still being able to fit within the same vacuum
chamber. Hence the silicon detectors around the target area will be 5×5 cm2 and
10×10 cm2 , this corresponds to 250 and 500 fibers respectively. To optimize the
number of readout channels 512 fibers will be used, which will lead to an active
area of 10.24×10.24 cm2 , only slightly larger than for the silicon detectors. The 512
fibers will be read out by two times 32 channels. All three detectors will provide xand y-position, which is achived by having two layers of fibers rotated 90o against
each other.
The fourth detector will be placed right after the GLAD dipole magnet in the large
vacuum chamber. It only needs to provide the x-position, and the detector will only
consist of one layer of fibers to reduce the angular straggling. The detector can be
placed at any position between 2.5 and 4.7 m from the center of the magnet and any
fragment hitting the ToF-wall should go through this fiber detector as well. The
width of the detector will be 40.96 cm corresponding to 2048 fibers. This width is
larger than the spread in the heavy fragments at 4.7 m from the magnet and any
particle within the range of the TOF-wall will go through the fourth fiber detector.
The light produced in the fibers will be detected by MPPCs like for the prototype
described in section 3.2. GSI build digitizers, FEBEX, will be used to read the
signals from the MPPCs. The FEBEX digitizer is build around a flash-ADC and
runs at a sampling rate of 50 MHz. The results from the prototype show that this
rate will be sufficient to distinguish true signals from background.
94
Detector
Number
1
2
3
4
5
total
xy
xy
xy
xy
x
xy
Size
[cm2 ]
10.24×10.24
10.24×10.24
10.24×10.24
40.96×24.0
121.68×90
Number of
Fibers
512+512
512+512
512+512
2048
4*6084
Lfibers
[m]
510
510
510
2050
60840
65000
Number of
MPPCs
4x32
4x32
4x32
2x64
8x78 (PMT anodes)
512 MPPCs
4 Multianode PMTs
Single/Multihit
Requirement
S
S
M
M
M
Table 4.2: An overview of the five fiber detectors.
4.2.2 The fifth fiber detector
The large fiber detector will serve two purposes. The main purpose of this detector
is to provide the final x-position to be used in the tracking. Secondly, it shall provide
both x- and y-positions of the particle on the TOF-wall. The energy measurement of
the TOF-wall has a position dependency. This can be corrected for by determining
the energy loss within small areas of the TOF-wall. The fiber detector can be used
to dermine in which area a particle hits and then the position dependency of the
energy can be corrected for event by event. The active area of the fiber detector and
the TOF-wall need to be the same and will be 120×80 cm2 . The tracking requires
both a high resolution in x and high detection efficiency (>90%). The detector will
consist of four layers of 200×200 µm2 fibers to account for the two requirements.
The detector will be placed directly in front of the TOF-wall. This will be achieved
by designing one large construction for both the fiber detector and the TOF-wall.
The frames with the fibers will be placed on the holding structure and behind those
frames the TOF-wall paddles will be placed on a separate frame, see Fig. 4.3. In this
way the position of the fibers relative to the TOF-paddles will be constant, while at
the same time it will be possible to access each part individually for maintenance.
The size of the detector will lead to extra concerns when designing it compared
to the small ones. A 100 cm long fiber will be more likely to bend than a 5 cm
long, especially if it is placed horizontally, where gravity will pull the fiber down.
Two measures will be taken to account for this problem. First of all a thin foil of
mylar will be placed on one side of each layer of fibers to provide an extra support.
Secondly the fibers will not be placed vertically as they will be in detectors 1, 2
and 3. Instead four identical frames with an active area of 122x90 cm will be made.
Each frame will have 6084 fibers each 90 cm long. The frames will be placed such
that each fiber has an angle to the y-axis of 7o , two of the frames tilted clockwise
and two anticlockwise, see Fig.
4.4. This will lead to a position resolution of
σx = 58.3µm for x and σy = 462µm for y measurements, which is sufficient to fulfill
the requirement in position resolution for the detector.
95
√
Four layers of 6084 fibers will lead to 624 readout channels (4 · 2 · 6084) if the fibers
are bundled together using the same scheme presented previously. The readout
electronics and the light detectors will be different to the ones for the small fiber
detectors due to the larger amount of channels. Instead of MPPCs and FEBEX
cards, the multianode PMTs and the GEMEX cards will be used. This combination
has already been used by our collaboration, the latest results being presented in
section 3.2. The development of the GEMEX boards has been ongoing the last
years, and a new improved version is now available.
4.3 Time-of-flight plastic scintillator wall
4.3.1 Technical specifications and design details
In this chapter the choice of the scintillator and the mechanical design will be discussed.
4.3.1.1 Choice of the scintillator
The detector will be used at the end of an evacuated beam line with a diameter
of 50 cm. (For the R3 B cave a continuation of the evacuated beam line with a diameter of 80 cm is foreseen.) The ToF wall should fully cover this area but should
not be unnecessary large to avoid loss of light by attenuation in the scintillators.
In order to avoid light reflections at the ends of the scintillator it should match the
diameter of the PM. This also avoids the use of any light guides and improves the
timing properties of the detector. The thickness of the scintillators should be large
enough to produce sufficient light for good energy and timing resolution. On the
other hand, reactions of the heavy nuclei in the scintillator lead to products with
lower charge and therefore to a wrong nuclear-charge determination. For a 1 AGeV
Sn beam we have a reaction probability of 11 % in 5 mm plastic scintillator and for
a 1 AGeV Pb beam 14 %. This is a rather high value. In 3 mm plastic the reaction
probability reduces to 6 % and 8 % for Sn and Pb, respectively. Therefore, it would
be beneficial to build a variable detector with the option to change the scintillators
easily. This would be also advantageous for a quick and more frequent exchange of
the scintillators in the middle of the detector. These scintillators see the high rate
of the un-reacted beam and experience radiation damages.
All this considerations led to the choice of scintillators with the dimensions of
800x27x5 mm3 for lighter beams and 800x27x3 mm3 for the heavier ions.
In principle, two layers of plastic scintillators behind each other, shifted by half the
width would be sufficient for a full geometrical coverage of the fragment trajectories.
96
However, considering how precious beam time is, it is useful to have some redundancy in case one channels stops working during the experiment. Therefore, it is
planned to have 2x2 layers of scintillators. In cases where the fragments pass the
scintillators without reactions this leads also to an improvement of the energy and
√
time resolution by a factor 2.
As mentioned before, we aim for a detector with an energy resolution of σ∆E <1%.
The variation in scintillator thickness along one paddle contributes of course to the
energy resolution. The accuracy quoted by the companies is ±0.15 mm which corre√
sponds to a sigma value of 0.15 mm/ 12=0.043 mm. For a 3 mm thick paddle this
contributes already with more than 1.4% the energy resolution. A possible solution
are handpicked scintillators for which Eljen quotes an accuracy of ±0.127 mm which
√
corresponds to a sigma value of 0.127 mm/ 12=0.037 mm and a relative variation
in thickness of 1.2%. In this case a position dependent calibration of the energy loss
along the paddles will be necessary to meat the envisaged energy resolution.
The existing ToF wall has vertical and horizontal paddles. The vertical ones can be
calibrated easily by varying the field strength of the magnetic dipole and sweeping
the calibration beam over all paddles. The horizontal paddles however, especially
the ones at top and bottom can not be reached with the calibration beam. The beam
height is 2 m and with the beam steering it is not possible to move the beam as
much up and down as it is needed. Because of this the new ToF wall will have only
vertical paddles. The position information for the calibration of the effective speed
of light of the paddles will come from a fiber detector infront of the ToF wall (as
described in Section 4.2.2). This fiber detector will be hosted in the same housing
and can deliver the beam position to an accuracy of about 0.06 mm in x direction
and 1.6 mm in y direction, which is sufficient for the calibration of the ToF wall.
4.3.1.2 Mechanical design
It is foreseen to have a modular design with several frames, see Figs. 4.2,4.3. Each
TOF13 frame holds a double plane of scintillators, one with a thickness of 3 mm
and another one with thickness of 5 mm. The frames can be combined to form a
light-tight housing. Additional frames containing fiber detectors in front of the ToF
wall are also foreseen. These are needed for the beam tracking but can also serve
to obtain position dependent calibration parameters for the ToF wall. At the front
and rear side the detector is closed by two frames holding a thin black plastic foil in
order to make the assembly light-tight. The frames of the ToF wall, see Fig. 4.5,
are quadratical and can be mounted with paddle in vertical or horizontal allignment
if necessary for calibration runs. The active area should have dimensions of 120 cm
width and 80 cm height. Each plane will contain 44 vertical scintillator paddles with
the dimensions 800x27x3 mm3 (first two planes) or 800x27x5 mm3 (third and fourth
97
plane). Each scintillator paddle will be read out by PMs on both far ends. The
mounting of the PMs and the scintillators is done in a way to ensure fast exchange,
see Fig. 4.6. Especially for high beam rates one expects radiation damage for the
paddles which are hit by the unreacted beam and therefore a more frequent exchange
of scintillators.
Figure 4.2: Mechanical design of the setup containing fibre and TOF13 detectors.
The high voltage distribution is realized by CAEN Multi-pin HV modules. Each
module has 28 channels and 13 multi-pin connectors are distributed along the frames.
The signals cables are connected to 8-fold MMX connectors, matching the 8-fold
geometry of the electronic cards.
98
Figure 4.3: View of dingle frames: First and last plane are light-tight housing, second
and third frame are forseen for fibre detectors, fourth and fifth frames
represent two TOF13 planes.
99
H-H
Figure 4.4: Front and top view of the large fiber detector (fiber5) detector. Fibers
are not shown, but will be mounted on the rotated frames
100
Figure 4.5: Front, top and side view of the TOF13 detector. Cut-out parts F and G
are shown in Fig. 4.6.
Figure 4.6: Details of the PMs and the scintillator paddles mounting.
101
4.4 Proton-arm detectors
The straw-tube detectors have been successfully operated for the precise position
measurement of minimum ionising particles by many collaborations (see Ref. [Byc06]
and Refs. therein). For the proton tracking behind GLAD we will use the protonarm spectrometer (PAS) based on straw-tube detectors for tracking the evaporated
protons. A thick plastic scintillator wall for time-of-flight measurement and triggering purposes is placed at the end of the PAS. The PAS consists of four straw-tube
walls (STW), two for each coordinate (X1,Y1,X2,Y2). All four STW are placed inside the large vacuum chamber behind GLAD. To facilitate access and maintenance
the layout of the PAS detector is placed on a movable platform inside the large vacuum chamber behind GLAD. The electronic front-end readout cards, gas system,
high- and low- voltage power supply and other services of the PAS are placed at the
detector and mechanical frame structures.
The PAS will be built by the PNPI group in Russia, which has long experience and
expertise in developing gas detectors.
4.4.1 The straw layout in the PAS
The layout and position of the four STW of the proton arm spectrometer inside
the vacuum chamber is presented in Fig. 4.7 and their dimensions are shown in
Table 4.3. The dimensions of the first STW (X1 plane) of the PAS are presented
in Fig. 4.8. This plane consists of vertically placed one-meter long plastic (Kapton)
straws covering a two-meter wide area. This plane measures the X coordinate introducing minimal straggling. The dimensions of the other three STWs (Y1, X2
and Y2 planes) of the PAS are presented in Figs. 4.9 and 4.10. These three planes
consist of horizontal, vertical and horizontal aluminium straws for measuring the Y,
X, and Y coordinates, respectively.
The PAS covers an active area that matches fully the geometrical acceptance given
by the gap of the dipole magnet of ±80 mrad, thus providing a full coverage of the
available solid angle behind the magnet to detect the emitted protons. The geometry
and the operational parameters of the PAS are optimized to detect minimum-ionising
particles with an efficiency larger than 95% and a spatial resolution of σ ≤ 200 µm.
The angular resolution of a single STW is about 10 mrad. Evidently, the total
angular resolution is improved by using multiple planes separated by some distance,
as it is shown in Fig. 4.7. With this geometry, the angular resolution in the dispersive
direction is about 0.2 mrad, significantly less than the typical angular straggling that
a 500-1000 MeV energy proton suffers while transversing the first plane.
102
STW
X1
Y1
X2
Y2
Dimensions
[mm3 ]
2092x1112x92
2611x1092x104
2592x1112x104
2611x1092x104
Material
of straws
Ka
Al
Al
Al
Aperture
[mm2 ]
2000x1000
2500x1000
2500x1000
2500x1000
Total
Straws
(max)
610
310
760
310
1990
Preamp.
boards
39
20
48
20
127
HV
inputs
3×2=6
3×2=6
3×3=9
3×2=6
27
Table 4.3: An overview of the four STW of the PAS.
In the active detector part, the first STW (X1 plane) has a thickness of 0.372 mm
(mean) Kapton for orthogonal tracks. This high transparency allows less shadowing
on the following detectors and background-free tracking.
Each STW of the PAS consists of three layers of straw tubes filled with a gas mixture
at the overpressure of 1 bar. The tubes are glued together, each layer being shifted
by one tube radius with respect to the previous layer. Then, for an orthogonal
proton track, lower detection efficiency close to the tube wall is always combined
with high efficiency in the straw center that is placed in the adjacent staggered layer.
Also, the track’s left/right ambiguity from the wire can be disentangled in the next
layer.
Due to the overpressure the thin-wall Kapton tubes have a perfect and strong cylindrical shape and the modules become self-supporting. This approach shows a high
rigidity and mechanical precision. As a consequence, a light-weight support frame
is sufficient to hold the tube layers.
The granularity (tube diameter) of 10 mm and the straws’ precise alignment allows a
continuous tracking with a few hits per track, which is important in order to resolve
complex track patterns.
103
Figure 4.7: Layout of the drift chamber planes X1 (yellow), Y1 (light blue), X2 (blue)
and Y2 (red) inside the vacuum chamber behind the GLAD magnet.
104
Figure 4.8: Dimensions of the X1 plane used for the first measurement of the X
coordinate. The plane consists of vertical Ka tubes
Figure 4.9: Dimensions of the Y1 and Y2 planes used for the measurements of the
Y coordinate. The planes consist of horisontal Al tubes
105
Figure 4.10: Dimensions of the X2 plane used for the second measurement of the X
coordinate. The plane consists of vertical Al tubes
106
4.4.2 Assembly, positioning, alignment and commissioning of the
STW
The construction of the STW consists of several assembly steps, starting with the
production of the single straw tubes and ending with final self-supporting straw
modules, consisting of several straw layers. The mechanical frame structure has to
support and precisely position the STW at both ends. In addition the structure
has to support all the electronic readout and supply elements, which are connected
and placed at the detector front-end patch panels: the electronic readout cards, all
readout and supply cables, the gas manifolds and supply pipes, high voltage and
low voltage cables.
The position accuracy of the mounted STW relative to the precision alignment marks
in the end angles of the PAS mechanical frame should be better than 100 µm. Due
to the close packaging of the glued straws in a module with a precise tube-to-tube
distance, the deviations in the position of a single straw is less than 100 µm. The
overall mechanical precision in the X,Y plane will be below of 200 µm.
In the following, the main steps of the assembly procedure of the single straw tubes
and walls are described below:
• The PAS, including the readout electronics, high voltage supply, gas manifold
lines, and distribution of all cables and supply pipes, are mounted on the
mechanical frame structure that is placed on the movable platform, which is
located outside of GLAD vacuum chamber.
• Overall experimental investigations of the detector performance, the electron-
ics coupling and the noise suppression are carryied out first outside the vacuum
chamber.
• The PAS is moved inside the vacuum chamber using a special railing system.
Before sliding the PAS inside the vacuum chamber, it is necessary to take out
all the front-end electronic cards from STWs and install them back when they
are placed inside the vacuum chamber.
• The final tests, inspections and commissioning of the STWs have to be carried
out when detector is placed inside of vacuum chamber.
4.4.3 Straw Tube Description
Straw tube is proportional counter that is used as a drift chamber with corresponding
structure of electric field inside. The PAS straws will be operating in the proportional
mode. The drift time information can be converted to distance R from the particle
track position to the anode wire by drift velocity. The main goal is to make a
107
correct choice of the gas mixture that has to perform both the X-T relation as linear
as possible and keeps almost constant drift velocity for all particles that are passing
the straw on different distances from the anode wire. This not trivial task because
of strong changing of electric field tension along the straw radius and because of
the drift velocity is different for every gas. Therefore, this fundamental space/time
relation has to be calibrated using reference tracks with known space and drift time
information.
Straw detectors have the simplest geometry of highly symmetrical, cylindrical tubes
and have several advantages which are summarized in the following:
• small radiation length, X/X0 ∼0.05% per tube, if straws with very thin (∼60 µm)
film tubes are used. Kapton was chosen as basic material for straw cathode
because of its good mechanical properties, small amount of materials and resistance to radiation damage;
• high spatial resolution σ ≤200 µm depending on the tube diameter and gas
characteristics;
• high detection efficiency per straw for about 99,5% of the inner tube radius
and minimal dead zones.
The first STW (X1 plane) uses the high-pressure thin-wall straw tubes designed
at JINR (Joint Institute for Nuclear Research, Dubna) [Dav08]. These tubes with
diameter of 10 mm are wound with two 19 mm wide Kapton ribbons (see Fig. 4.11).
A 40 µm-thick XC 160 conducting film (produced by DUPONT) and a 12.5 µmthick film with an approximately 500 Å-thick aluminized layer deposited on its inner
side serve as the inner and outer ribbons, respectively. The total thickness of the
tube walls is about 62 µm. These tubes have been successfully operated at high
differential pressure (up to 4 bar). This is an important feature because the PAS
has to be placed within vacuum.
!
!Fig. 2
Figure 4.11: Schematic drawing of a straw tube wound with two Kapton ribbons
and an intermediate aluminized layer.
108
The tubes contain a minimum amount of material, and the gas loss is negligibly
small. The 25 µm-diameter gold-plated tungsten anode wire is stretched by a weight
of 70 g and placed in the copper pins (0.1 mm hole). At such wire tension, the
calculated gravitational sag is below 10 µm, which is below the projected spatial
resolution of 200 µm. Working parameters of tubes are presented in Figs. 4.12, 4.13,
4.14 and 4.15.
The other three STWs are made out of Aluminum tubes with a wall thickness of
about 300 µm. Their diameter is 10 mm. Although the angular straggling introduced by these tubes is significantly higher compared to the thin Kapton tubes their
influence in the dispersive angular measurement is negligible since they are placed
at the end of the track. Aluminum tubes are used in order to minimise the total
leakage of the gas into the vacuum and to constrain the total cost of the array.
The high-voltage connectors are placed at the bottom side of the plane, whereas the
signal connectors are located at the top side. The front-end and readout electronics
are located at the top side of the plane. Zoomed views of the bottom and top sides
are presented in Fig. 4.16.
The maximum drift time of the primary-ionisation electron cloud is of the order of
100 - 120 ns (exact numbers depend on the final choice for the gas mixture). This
means that a total count rate in a single tube is available at the level of more than
100 kHz.
Operation of the detectors in vacuum implies and demands a perfect sealing of the
straw tubes at their end-cap regions. Working-gas loss in high-differential-pressure
of straw tubes may be caused by both their improper sealing during assembly and
gas diffusion through the walls. Differences in the gas permeabilities for different
components of the gaseous mixture may help to determine the minimum required
mixture flow through the detector that ensures a constant ratio of the partial pressures of its components. The detectors based on straw tubes contain a minimum
amount of substance, and the gas loss is negligibly small [Dav08]. Nevertheless, additional pumping of the vacuum volume is foreseen, to prevent spoiling of the vacuum
by micro-leaks from the tubes. The thicknesses of the straw tubes are sufficient to
operate the detector in vacuum and to avoid deformations.
109
Fig..3
Figure 4.12: Time dependence of the pressure of the straw-tube-filling gaseous
mixture.
!
Fig.4
Figure 4.13: Time instability of the length of the straw tube at fixed pressures of (1)
4 and
(2) 1 bar.
!
Fig.5
Figure 4.14: Radius of a straw tube as a function of the differential pressure of the
tube-filling gaseous mixture: (1) ordinary straw tube and (2) a straw
tube strenghtened with carbon filaments.
110
Fig.6
Figure 4.15: Elongation of a 1.55-m-long straw tube as a function of the differential
pressure of the gaseous mixture: (1) without and (2) with strengthened
walls of the straw tube.
(a)
(b)
Figure 4.16: Zoomed view of the bottom (a) and top (b) side of the X1 plane with
HV supply scheme.
111
4.4.4 Detector material budget
Evaluation of radiation length for the first STW (6 layers of Kapton straws) were
performed using the SRIM2011 software. SRIM calculates the stopping and range
of ions (10 eV - 2 GeV/amu) into matter using a quantum mechanical treatment of
ion-atom collisions.
Ionization, which occurs along the particle track in the straw tube, under the influence of protons (energy of 700 MeV) is 120 e/cm, of α particle (energy of 700 MeV)
is 472 e/cm and of Li ions (energy of 700 MeV) is 1570e/cm.
Results of these calculations are presented in Table 4.4 for protons and for a few
different incident particles:
Element
Straw-tube
material
Anode wire
Gas mixture
Ar-CO2 -CF4
(at 1 Bar)
Particle type
Proton
α particle
Li ions
B ions
Proton
Proton
dE/dX0
[MeV/mm]
0.315
2.560
8.594
33.284
2.565
4.613 × 10−3
Total X/X0
of one straw [%]
Total X/X0
of one STW [%]
X0
[mm]
8.19 × 102
101
30.1
7.76
101
5.6 × 104
Proton
0.057
Proton
0.172
X/X0
1.46
1.19
3.99
1.55
2.48
1.79
×
×
×
×
×
×
10−4
10−3
10−3
10−2
10−4
10−4
Table 4.4: Mean thickness in radiation lengths of the different straw tube components for the protons, α particles, Li and B ions with the energy of
700 MeV for one straw tube.
4.4.5 Gas mixture
The optimal choice of the working gas mixture is yet to be determined. Drift velocities and diffusion coefficients as functions of the electric field tension for some gas
mixtures intended to use in the detector were calculated by the program Garfield
and are shown in Fig. 4.17. The main requirements, that should be taken into account for the choice of the most suited gas mixture, are: good spatial resolution;
rate capability; radiation hardness; radiation length; chemical inactivity; working
voltage; working pressure; accessibility on the market and price. The need of high
spatial resolution in the STW requires high amplitude anode signals even for the
single electron clusters, thus requiring high gas gain. Computed gas gain for the
10mm straws via applied high voltage was is presented in Fig. 4.18.
112
diffusion coefficients via
Figure 4.17: Drift velocities and diffusion coefficients via field tension the gas mix!
tures intended to use.
Figure 4.18: Simulated gas gain versus high voltage.
In many gas mixtures the drift velocity becomes saturated and does not depend
strongly on the electric field strength. That makes the reconstruction of the track
coordinates easier. In this case, the electric field inhomogeneities do not play a
significant role, which makes the calibration simpler. An overpressure can be used
in these cases to reduce the diffusion.
All simulations have been performed using the GARFIELD and the MAGBOLTZ
packages. It was shown, that dependence of the space/time relation on the temperature is not significant and therefore, it will not be necessary to control the
temperature variation very precisely.
113
Figure 4.19: Electric field distribution in straw tube.
4.4.6 Influence of magnetic field on the R3 B straw detector operation
Since the straw chambers will be located in the vacuum volume where residual
magnetic field from the GLAD magnet exists, it is desirable that X-T relation does
not suffer from modifications in a magnet field. The path lengths of the electrons
moving from its particle track to anode wire in crossed electric and magnetic fields
is increased in compared with the case of the lack of a magnetic field. This causes
changes in space/time (X-T) base relation and, consequently, can cause errors in
determining of the particle coordinates in the detectors. To evaluate the influence of
magnetic field (MF) on the straw detector parameters GARFIELD and MAGBOLTZ
packages have been used. The maximal values of MF components were presented
R3 B collaboration: BX =0.0003 T; BY =0,006 T; BZ =0.015 T. The following gas
mixture 60%Ar+30%CO2 +10%CF4 was taken in consideration. An applied high
voltage HV= 1.9 kV was chosen so to provide the gas gain value of about G=5×104 .
In calculating used protons with an energy of 700 MeV. The distribution of electric
field tension in straw is presented at Fig. 4.19.
As one can see in Fig. 4.20, an influence of the magnetic field is strongest in that
space area of straws, where the weak electric field (up to 1×104 V/cm) is present.
This zone is very large and occupies more than 95% of tube cross section. It is seen
that the average angle between the velocity vector and the vector of the electric
drift field is approximately 0.350 throughout the range of angles between the electric
and magnetic fields. This corresponds to an error in the determination of particle
coordinates does not exceed of a few microns. In other words, the influence of the
magnetic field on the straw operation parameters can be ignored completely.
114
Figure 4.20: Dependence of the angle between the electron velocity ~v and electric
~ versus electric field tension E in the straw for different angles
field E
0
~ and B.
~
(0 , 300 , 600 and 900 ) between E
4.4.7 The Gas System
The preferred gas mixture for our detector could be based on Ar/CO2 /CF4 gas components. These gas mixtures have very good capability to tolerate high irradiation
levels since no deposits on the straw tube electrodes from polymerisation reactions
occur, provided that there is a clean gas environment including all materials and
parts of the detector and gas supply system in contact with the gas. For all gas
components a high purity grade is required (for Ar - 5.0, for CO2 - 4.8 and for CF4
- 5.0).
The gas supply lines consist of polished stainless steel tubes. We are going to use
pre-mixed gas in the bottles. Since argon, CO2 and CF4 are non-flammable, not too
expensive, and no recirculation and containment of the gas mixture is needed, and
the gas supply of the detector is done in flushing mode.
The STWs will be operated at a gas pressure up to 2 bar (absolute) and preferably
at room temperature. The total PAS gas volume of about 300 liters is exchanged
typically every a few hours with a flow rate of about 3 liters per minute to refresh
the gas mixture and to prevent an accumulation of contaminants in the detector and
gas system.
115
The scheme of the gas distribution system of the PAS is shown in Fig. 4.21. The gas
system consists of supply gas bottles with pre-mixed gas mixture, cleaning filters in
the gas line, a few digital mass flow controllers, regulated by a pressure transducer
to set a constant absolute pressure of about 1 to 2 Bar in the detector with accuracy
of about 1% (absolute), the supply lines in and out of the detector, inlet valves and a
dedicated exhaust line at the R3 B experimental area. The mass flow controller and
meter devices are based on digital electronics. In these devices the analog sensor
signal is sent directly to a micro processor. By doing so, optimum signal stability and
accuracy is achieved. An integral alarm function continuously checks the difference
between the set point and the measured value. If the supply pressure drops the
instrument gives a warning. In addition, the instrument runs a self diagnostics
routine, and controller settings can be remotely adjusted with a hand terminal or
a computer using an RS-485 protocol. In order to control a possible gas leak level
from STW detectors in vacuum, a few mass flow meters are inserted behind of each
STW. The PAS gas supply system is equipped by the slow control system that
performs remote control of the gas flow parameters, with the possibility to switch
to manual/local operation.
4.4.8 Front-end electronics and read-out
The four STW of the PAS consist of about 2000 channels. The electronics are
placed in vacuum close to the straw-tube detectors for optimum performance in
terms of noise.
The collaboration has experience with the coordinate-readout-
system (CROS3) electronics family developed at PNPI, which are implemented for
a number of detectors such as the LAND (GSI), the SFB/TR-16/B1 Spectrometer (Bonn) [PNP06]. For the proton arm the development of a third-generation
(CROS3-S) of these electronics is proposed and discussed below. Other alternatives
of already developed electronics for straw tubes are still under investigation, such as
the WASA straw-tube electronics [WAS].
The chamber-mounted FEE cards of CROS3-S will be modified to incorporate the
ASD-8 amplifier-shaper discriminator ASIC, with a variable threshold, which is also
used to read out the HADES drift chambers at GSI.
The block-diagram of the CROS3-S is illustrated in Fig. 4.22 and depicts the following cards:
• AD16-S - 16-channel Amplifier/Discriminator (Digitizer). It is located on the
detector.
• CCB16 - 16-channel Concentrator. It is located either on the detector or very
close to the detector.
• CSB - System Buffer. It is a PCI card located in a remote PC.
116
Gas cabinet
Gas
mixture
Digital Mass
Flow meters
manifold
X1
plane
manifold
GLAD vacuum
chamber
Digital Mass
Flow
controllers
manifold
Y1
plane
Pressure Sensors
Alarm
manifold
Exhaust
manifold
X2
plane
manifold
Digital Mass
Flow meters
manifold
Y2
plane
Pressure Sensors
Alarm
manifold
Filters
GFM
Gas manifold
Figure 4.21: Schematic view of the gas system.
The dimensions (WxLxH) of these modules in mm are:
• AD16: 100×160×12,
• CCB16: 140×180×12,
• CSB: standard for PCI-32.
The AD16-S card is a modification of the 96-channel CDR96 [PNP06] Digitizer and
will be designed and developed especially for the proton arm of the R3 B experiment.
The analog part of the AD16-S performs input-signal amplification and shaping, as
well as pulse discrimination with a peaking time of 30 ns, minimum threshold of
7 fC, double pulse resolution of 80 ns, power dissipation of about 35 mW/channel,
and an operational threshold ≤15 fC. The digital part is implemented in a Xilinx
Spartan Family FPGA that performs both time digitization and the readout tasks.
The delay range compensates trigger latencies of up to 2.5 µs in 10 ns steps. The
finest time-bin resolution is 2.5 ns and the maximum number of time slices is 255.
117
The readout is performed via an STP CAT5 cable at a rate of 100 Mb/s. The
CCB16 card is a standard CROS3 card. The CCB16 cards are configured as a twolevel system. Low level CCB16 cards collect data from 16 AD16 cards and send the
readout data to the top level CCB16 cards via copper STP CAT5 cables at a rate
of 100 Mb/s. The top level CCB16 cards collect data from all 16 low-level CCB16
cards and send that data to the CSB card via an optical fibre at a rate of 2.0 Gb/s.
The CBS-B is implemented as an universal PCI card. It contains a system buffer
to collect readout data from the CCB16 cards. It also provides all necessary system
constants such as delay and gate values to be loaded onto the CCB16 and AD16-S
cards.
The PRS should have a Detector Control System (DCS) that should be as one branch
of the general R3 B Experiment control system. This system has to continuously
collect the actual parameters from the supply and electronic readout systems and
compare them with their set values and etc. For the high voltage, the straw tubes
will require boards which are able to provide up to 3 kV with a current per channel
of about 100 µA. The straws will be grounded to sectors, which will fed in parallel
by a signal system channel each. High voltage supplied, current ramp up and down
times, will be the parameters to be controlled by the detector control system (DCS)
using a dedicated bus or Ethernet connection.
59:9':)(!
!!!!4567*!
!!!!!4567*!
!!!!!4567*!
!!!!!4567*!
';330<!
!!!!!''867!
!!!!!!''867!
!!!!!!!>;?!>0@0A!
';330<!
!!!!''867!
!!!!!!:;3!!>0@0A!
"#=0<!
!!!!!!!'*8!
*!-.-/01!-0/23!
Figure 4.22: CROS3-S system setup.
!
4.4.9 The plastic scintillator wall of the proton arm
The triggering of protons will be performed with a plastic scintillator wall placed
and the end of the proton arm right after the vacuum exit window of the GLAD
magnet. Its size will cover 2.7x1.2 m2 and consist of vertically placed paddles read
out by photomultiplier tubes. The technology for this detector is the one developed
for NeuLAND and TOF13 detector and no further investigations are foreseen.
118
4.5 Vacuum chambers
4.5.1 All-in-one vacuum chamber for the detectors before the target
Before the target there are a few detection systems foreseen within a distance of less
than 1.5 m. Namely, the two Si detectors, the start timing detector, two small fiber
detectors and the anti-coincident detector. In many cases these detectors and/or
their electronics will share common requirements such as cooling infrastructure. In
addition, for calibration and beam-diagnostic purposes it is of great advantage if
detectors can be driven in and out of the beam line automatically. This will require
a motor system and drivers coupled to each detector support frame. In order to
accommodate these needs we intend to build a common large vacuum chamber for
the detector before the target. Such a chamber will also allow all the detectors to be
mounted and aligned to each other on the floor before mounting the hole chamber
into the beam line. A preliminary drawing showing how such chamber will look like
is shown in Fig. 4.23.
Figure 4.23: The vacuum chamber to accommodate all the tracking detectors before
the target.
4.5.2 Vacuum pipe for the fragment arm
A large vacuum chamber coupled to the GLAD magnet has already been designed
and ordered. This will accommodate the first fiber detector after the magnet, the
straw tubes and the plastic scintillators for proton triggering. This vacuum chamber
extends to about 2 m after the exit of the GLAD magnet. However, the TOF13 detector for measuring time, energy loss and position of the heavy fragments is located
119
at least another ten meters away. The presence of the air between the large vacuum
chamber of GLAD and the detector introduces significant angular straggling and
constitutes a rather thick target where reactions can take place and generate large
background. For this purpose, we plan to construct a vacuum pipe that connects to
the GLAD vacuum chamber and extend towards the TOF13 detector. This vacuum
pipe is foreseen to have a varying diameter (e.g. two different diameter pipes) that
will cover the full surface of TOF13 on one end and the fragment exit window on
the other end, such that the detector acceptance is not compromised.
120
5 Monte Carlo Simulations
5.1 Monte Carlo Simulations
In order to investigate the fragment-mass resolution and the fragment-proton(s)
relative-energy resolution achievable with the new detectors, as well as the influence
of effects such as detector resolution and straggling, Monte-Carlo simulations were
carried out using the R3BRoot framework [Ber11]. R3BRoot has been succesfully
used in the past to determine the experimental response of the current LAND/R3 B
setup, for example for the study of breakup reactions of the proton-dripline nucleus
17 Ne
[Wam14].
For the present simulations, the geometry and calculated/simulated magnetic field
map of the future GLAD magnet were implemented, instead of the present ALADIN
magnet. For the detectors, the geometries and software classes of the currently existing detectors were modified. In order to mimick the proton straw tubes, plastic
scintillator detectors with the same length and width and a material-equivalent
depth were used for the simulation. Figure 5.1 shows an overview of the setup. The
different detectors are indicated in the figure, and their characteristics used in the
simulations are summarised in Table 5.1.
Si
Fiber 4
Fiber 5
Target
GLAD
Strawtubes
TOF13
Figure 5.1: Overview of the detectors used in the simulations. Their characteristics
are summarised in Table 5.1.
121
Table 5.1: Summary of detector characteristics used in the Monte-Carlo simulations.
Detector
Material
Thickness
Size
Resolution
Si
Silicon
150 µm
(10 × 10) cm
x, y: 100 µm
Fiber 4
Plastic
200 µm
(41 × 30) cm
x: 200 µm uniform
Fiber 5
Plastic
800 µm
(120 × 80) cm x: 60 µm, y: 1 mm
ToF
Plastic
16 mm
(120 × 80) cm t: 20 ps
Straw Tube 1 Plastic Equiv.
750 µm
(200 × 100) cm x: 100 µm
Straw Tube 2 Plastic Equiv.
750 µm
(260 × 100) cm x, y: 100 µm
Figure 5.2: An example of a simulated event of
130 Sn
+ p going through GLAD.
5.1.1 Simulation
Three physics cases were simulated and discussed: First, the decay of 16 F to 15 O+p,
which was both measured and simulated using the existing setup and which can
therefore be used as a benchmark for the performance of the new tracking detectors;
second, the removal of several neutrons from 130 Sn mimicking Coulomb-dissociation
using a 0.5 mm thick lead target; and third, the system of
resolution of the relative energy measurement. Although
130 Sn+p
130 Sn
to assess the
is too neutron-rich
to evaporate a proton, it was used nevertheless, to keep the simulations simple and
the results for the fragment comparable.
These events were randomly generated assuming gaussian distributions for the position and angle in x- and y-direction of the incoming beam, and a uniform distribution
over the target thickness for the z-position of the interaction points, see Table 5.2.
They were then simulated using Geant3 as transport engine and then digitised. An
example of a simulated event, consisting of
130 Sn
and a proton, at a total kinetic
energy of 1 AGeV and a relative energy of 1 MeV, is shown in Fig. 5.2. In the
figure, a cross section of GLAD can be seen as well as the Si-detector in front of it
122
Table 5.2: Properties of the incoming
Quantity Mean
Sigma
β
0.876
2%
X0
0
3 mm
Y0
0
3 mm
Z0
0
target thickness
dX
0
3 mrad
dY
0
3 mrad
beam for the simulations of 130 Sn + p.
Comment
1 AGeV
X position on target
Y position on target
Uniform distribution
Incoming angle in X-Z-plane
Incoming angle in Y-Z-plane
and the fiber and straw-tube detectors directly behind it.
5.1.2 Tracking
The output of these simulations (positions and times of hits on various detectors)
was then fed into the “tracker” - a data analysis code used for tracking of charged
particles through the dipole magnet of the LAND/R3 B experiment. The goal of
the tracker is to reconstruct mass and momentum of each involved particle and to
calculate the relative (kinetic) energy of the particles.
The mass of the heavy fragment was then reconstructed by calculating the magnetic
rigidity Bρ from the deflection of the particle in the magnetic field of GLAD, the
measured time-of-flight and the charge, which is determined from the energy deposition on the Si detector. The tracker therefore varies mass and β of the particle
until the simulated hit times and hit positions match the ones calculated from the
assumed trajectory. The track of the particle inside GLAD is calculated using a
Runge-Kutta-4 stepping code and the calculated magnetic field map.
For the proton, mass and charge are known and only β needs to be varied until the
measured times and positions are matched.
5.1.3 Mass and relative energy
The non-zero resolutions of fragment mass and relative energy are caused by interactions with matter (energy- and angular straggling in target and detectors),
spatial- and time-resolutions of the detector systems and numerical uncertainties of
the computer codes used for the data anaylsis (tracking).
To investigate the resolution of mass and relative energy, the system of
130 Sn
+
proton with a relative energy of 1 MeV at a beam energy of 1 AGeV has been
123
Table 5.3: Resolutions for the reconstructed mass and relative energy of 130 Sn +
proton.
intertarget
detector
fragment mass relative energy
actions
resolutions
(sigma)
(sigma)
0.032
12 keV
xy
0.045
20 keV
time
0.153
24 keV
all
0.154
30 keV
on
0.117
43 keV
on
all
0.199
53 keV
on
0.25 mm Pb
0.117
101 keV
on
0.25 mm Pb
all
0.199
106 keV
on
0.5 mm Pb
0.116
139 keV
on
0.5 mm Pb
all
0.200
142 keV
simulated. The results of the simulation have been tracked, and the
130 Sn
mass and
relative energy have been reconstructed. Interactions with matter, resolutions of
detectors, and target thickness have been individually switched on or off in these
simulations in order to study their respective influence. The results are summarised
in Table 5.3 and Figures 5.3 and 5.4.
1400
2500
σ: 12 keV
1200
σ: 30 keV
1000
2000
800
1500
600
1000
400
500
0
0.5 0.6 0.7 0.8 0.9
200
1 1.1 1.2 1.3 1.4 1.5
Relative energy (MeV)
0
0.5 0.6 0.7 0.8 0.9
1 1.1 1.2 1.3 1.4 1.5
Figure 5.3: Reconstructed relative energy of 130 Sn + proton at 1 AGeV with interactions disabled. Left: All detector resolutions disabled. The remaining
peak width corresponds to numerical uncertainties. Right: All detector
resolutions enabled.
As the mass is determined from the measured time-of-flight and from the positions
measured by the detectors after the target, the straggling in the target has almost
124
800
700
700
σ: 43 keV
σ: 53 keV
600
600
500
500
400
400
300
300
200
200
100
100
0
0.5 0.6 0.7 0.8 0.9
0
0.5 0.6 0.7 0.8 0.9
1 1.1 1.2 1.3 1.4 1.5
400
350
1 1.1 1.2 1.3 1.4 1.5
300
σ: 142 keV
σ: 106 keV
250
300
250
200
200
150
150
100
100
50
50
0
0.5 0.6 0.7 0.8 0.9
1 1.1 1.2 1.3 1.4 1.5
0
0.5 0.6 0.7 0.8 0.9
1 1.1 1.2 1.3 1.4 1.5
Figure 5.4: Relative energy for 130 Sn + proton with interactions enabled. Top left:
Without target and without detector resolutions. Top right: Without
target but with detector resolutions. Bottom left: With 0.25 mm Pb
target and detector resolutions. Bottom right: With 0.5 mm Pb target
and detector resolutions.
no effect on the mass resolution. In other words, the mass resolution is essentially
determined by the resolution of the time-of-flight measurement and the angular
straggling in the materials.
The reconstructed momentum (angle and absolute value) at the place of the reaction
in the target, however, is largely influenced by straggling in the target, since angle
and velocity are significantly altered.
5.1.4 Acceptance for heavy fragments
If the incident particle loses one or more neutrons in the target, the remaining,
lighter, fragment is deflected in the GLAD magnet by a larger angle. If the charge-
125
over-mass ratio becomes too large, the fragment will not hit the foreseen detector(s)
anymore and the acceptance drops.
Figure 5.5 shows the acceptance of various Sn isotopes if
132 Sn
is centered on the
ToF wall. The shape of the curve is largely determined by the width of the beam
on the ToF wall and hence by the angular and velocity distribution of the incoming
beam as well as energy losses in target and detectors. To emphasise the effect, the
incoming beam velocity has been set to a constant value. All other incoming beam
properties have been varied as described in Table 5.2.
Under these conditions,
122 Sn
(10 neutrons removed from
132 Sn)
can still be mea-
sured without significant loss of acceptance. With 13 neutron removals, the center
of the
119 Sn
beam is close to the border but still within the active area, leading to
Acceptance (%)
an acceptance of well above 50%.
100
80
60
40
20
0
114
118
122
126
130
Mass number of Sn (Z=50)
Figure 5.5: Acceptance of various Sn isotopes if 132 Sn is centered on the 120 cm wide
tof wall.
5.1.5 Comparison to the existing LAND/ALADIN setup
For the s318 experiment at the LAND/ALADIN setup in Cave C (“Study of the
Borromean dripline nucleus 17 Ne”) extensive simulations [Wam14] have been carried
out in order to determine the experimental response. These simulations have now
been repeated using the GLAD magnet and the detectors as described above. To
account for the lighter fragment, however, larger detector resolutions have been used.
For the time-of-flight measurement, a resolution of 70 ps has been used and for the
126
position measurement with the Si detector between target and GLAD, a position
resolution of 200 µm has been used.
Most suited for a direct comparison is the decay of
stemming from one-proton knockout of an incoming
16 F
17 Ne
into
15 O
and proton,
beam at an energy of
500 AMeV. The incoming beam properties have been taken eventwise from the measurements during the experiment. The derived momenta of
15 O
and proton - which
are the input data for the simulation - are the same in both case (LAND/ALADIN
and R3 B/GLAD).
Figure 5.6 shows the results of this comparison: The resolutions of the simulated
and reconstructed mass and relative energies. While the relative energy could only
be determined with an uncertainty of ≈ 140 keV using the LAND/ALADIN setup,
the new setup will substantially reduce the uncertainty by more than a factor of
two down to 65 keV. Although the mass resolution for these light ions was already
sufficient for eventwise identification with the LAND/ALADIN setup, the new setup
leads to a further improvement by a factor of 3.
5.1.6 Efficiency and acceptance of protons
The detection of proton(s) is hampered at large and very small relative energies.
At large relative energies, the spatial distribution is larger than the active area of
the strawtubes, thus reducing the acceptance. At very small relative energies, the
protons are too close together be to registered as two individual hits, thus reducing
the efficiency. Although the presence of two protons might still be detected from
the energy of the measured signals, a precise position information is not obtained
and the relative energy can only be estimated.
5.1.6.1 Strawtubes
The position of the proton on the strawtubes has been simulated using the system of
15 O + p at beam energies of 1 AGeV, see Figure 5.7. Up to 10 MeV relative energy,
almost all protons hit the active areas. For larger relative energies, the acceptance
linearly drops to about 72% at 20 MeV and 50% at 30 MeV. The effect of dispersion
in the beam energy (2% in β) is included and leads to the small deviation from 100%
starting at 5 MeV.
To investigate the efficiency of the strawtubes for two protons, similar simulations
have been performed for 15 O + 2p with relative energies between 20 keV and 200 keV.
Figure 5.8 shows the distance of the protons on the first strawtube for various relative
energies.
127
3500
700
σ: 0.04
3000
600
2500
500
2000
400
1500
300
1000
200
500
100
0
13
13.5
14
14.5
15
15.5
16
16.5
17
σ: 65 keV
0
0.5 0.6 0.7 0.8 0.9
1
1.1 1.2 1.3 1.4 1.5
Mass A
1200
1000
300
σ: 0.13
σ: 139 keV
250
800
200
600
150
400
100
200
50
0
13
13.5
14
14.5
15
15.5
16
16.5
17
0
0.5 0.6 0.7 0.8 0.9
Mass A
1
1.1 1.2 1.3 1.4 1.5
Figure 5.6: Comparison of the future R3B/GLAD setup to the existing
15 O + proton at a relative energy of
LAND/ALADIN setup.
0.9598 MeV, from the decay of the J π = 2− state in 16 F, have been
simulated for both setups. The beam parameters have been taken from
experimental data taken with the LAND/ALADIN setup at a 17 Ne beam
energy of 500 AMeV [Wam14]. Top row: Results for the detector properties of the planned R3 B setup and the GLAD magnet, see Table 5.1.
Bottom row: Results for the detector properties used during the s318
experiment and the ALADIN magnet.
As Figure 5.9 shows, with the expected separation threshold of 0.5 cm no significant
loss of efficiency is expected (min. 96% at 20 keV relative energy). Even with a
comfortably larger threshold of 1 cm, only the lowest energies show a somewhat
reduced efficiency.
5.1.6.2 Si detector
In principle, the proton(s) don’t need to be detected in front of GLAD. To calculate
their momenta via tracking, it is enough to know the interaction point in the target
which can be extracted from the fit of the fragment track.
However, an additional position measurement on the Si detector would improve the
128
Acceptance (%)
Acceptance (%)
100
80
60
100
80
60
40
40
20
20
0
5
10
15
20
25
30
0
5
10
15
20
25
30
Distance on first StrawTube (cm)
Figure 5.7: Acceptance of the proton for large relative energies at the planned R3 B
setup (left) in comparison to the existing LAND/ALADIN setup (right).
5
4.5
180
4
160
3.5
140
3
120
2.5
100
2
80
1.5
60
1
40
0.5
20
0
0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
0
Figure 5.8: Distance of the protons of the 15 O + 2p system on the first strawtube.
The protons might be identified as one single hit if the distance falls
below 0.5 cm, thus lowering the 2p efficiency.
quality of the fits and can be used to check for correct and consistent positions of
detectors during the tracking procedure. For such cases, the alternative microstrip
Si detector can be used. Also here, the efficiency for the detection of the proton
depends on its distance to the fragment (since they should hit different strips in
order to be detected) and hence on the relative energy, see Figure 5.10.
129
Acceptance (%)
100
90
80
70
60
50
0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
Distance on PSP (cm)
Figure 5.9: Efficiency for the separate detection of both protons from the system 15 O
+ 2p vs. relative energy. The protons might be identified as one single
hit if their distance falls below the separation threshold, thus lowering
the 2p efficiency. Solid line: Separation threshold of 0.5 cm (as planned).
Dashed line: Separation threshold of 1 cm. Dotted line: For comparison
the existing LAND/ALADIN setup with a separation threshold of ≈
1.7 cm.
1
140
0.9
120
0.8
0.7
100
0.6
80
0.5
60
0.4
0.3
40
0.2
20
0.1
0
0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
0
Figure 5.10: Distance between proton and 15 O vs. relative energy on the Si detector.
130
5.2 Conclusion
From the Monte Carlo simulations presented in this chapter we conclude that the
design of the in-beam tracking detectors satisfies the requirements outlined in Chapters 1 and 2. The obtained mass resolution, as presented in Table 5.3, allows for a
five-σ separation in the mass region of neutron rich Sn isotopes. The resolution of
the relative energy measurement improves by more than a factor of two compared
to the current setup, as shown in Fig. 5.6. The acceptance of an evaporated proton
with high excitation energy is drastically improved, as shown in Fig. 5.7.
131
132
6 Radiation Environment and Safety
Issues
6.1 Radiation environment
The radiation dose that the tracking detectors will be exposed to is significantly
higher than any other detection system in the R3 B setup. The detectors will be used
in experiments with secondary radioactive beams and also primary stable beams with
maximum rates of about 106 . Furthermore, this intensity is not evenly distributed
over the full detector areas and small regions of the detectors, where the unreacted
beam is going through, see almost the full radiation dose. On the other hand the
production rates for the very exotic nuclei, which constitute a large fraction of the
R3 B physics program, is considerably lower (few hundred to few thousand ions/s).
Thus, for the estimation of the overall dose an average of 105 ions/s is adopted, for
the integrated dose estimates. Assuming two months of total operation per year and
neglecting the duty cycle, the average dose estimation is then about 1012 ions/year.
The actual radiation dose depends on the charge of the ions. All detectors are
foreseen to be partially or fully replaced when radiation damage occurs. Typical indications of radiation damage are the reduced charge collection in the semiconductor
detectors and the reduced light output for the scintillator detectors.
In particular, the Si detectors will be replaced after each run. The plastic scintillators
typically survive for a dose of 1-10 Mrad, For heavy ions this dose is deposited in
the central paddles (where the unreacted beam is hitting) within few experiments.
Thus, the corresponding paddles of the TOF13 detector will be replaced regularly.
This is important in order to maintain optimal time and energy resolution. On the
other hand, the fiber detectors survive longer since a signal above noise is adequate
for them to be fully operational. A lifetime of longer than a year is expected for the
fiber detectors.
6.2 Safety issues
The infrastructure for installation and operation of the detectors will be done according to the safety requirements of FAIR and the European and German safety
133
regulations. All electrical equipment and gas systems will comply with the legally
required safety code.
Low- (∼ 10-100 V) and high-voltage (∼ 1000-2000 V) power supplies are used for
powering the detectors, photomultipliers and associated electronics. Standard operation procedures according to VDE1 will be followed to guarantee the safety.
The Si detectors and the small fiber detectors are light enough to be installed manually without safety concerns. The larger TOF13 detector and the coupled large
fiber detector will be transported by crane operated by qualified personnel.
The proton arm detectors are particularly fragile. They will be installed by qualified
personnel and a protective foil will protect the straw tubes when not in use. The
detectors will be assembled on a platform outside the vacuum chamber and will be
slide in the vacuum chamber with the use of a rail system. The maximum pressure
of the gas will be regulated.
Most tracking detectors are operated in vacuum and ear-protection safety equipment
is foreseen when operating next to the evacuated beam line.
1
VDE: Association for Electrical, Electronic & Information Technologies (Verband der Elektrotechnik Elektronik Informationstechnik e.V.)
134
7 Cost estimate and Funding scheme
7.1 Cost Estimate
Not available in public version.
135
7.2 Funding Scheme
136
8 Time Schedule Table and Milestones
137
138
Bibliography
[Ach07] P. Achenbach et al, Nucl. Instr. Meth. A578 (2007) 253
[Alp00] B Alpat, et al. High-precision tracking and charge selection with silicon strip
detectors for relativistic ions. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 446 522, 2000.
[Adr05] P. Adrich et al., Phys. Rev. Lett. 95, (2005) 132501
[Ati] http://web-docs.gsi.de/ weick/atima/
[Ban08] A Banu, Y Li, M McCleskey, M Bullough, S Walsh, et al. Performance
evaluation of position-sensitive silicon detectors with four-corner readout. Nuclear Instruments and Methods in Physics Research Section A: Accelerators,
Spectrometers, Detectors and Associated Equipment, 593(3):399–406, 2008.
[Bru92] M Bruno, M D’Agostino, ML Fiandri, E Fuschini, PM Milazzo, et al.
Position determination and resolution of two-dimensional position-sensitive
solid-state detectors. Nuclear Instruments and Methods in Physics Research
Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,
311(1):189–194, 1992.
[Byc06] V.N. Bychkov Nucl. Instr. Meth. A556 (2006) 66
[Cal11] Technical Design Report of CALIFA Barrel: The R3B CALorimeter for In
Flight detection of ? rays and high energy charged pArticles (2011)
http://www.fair-center.eu/fileadmin/fair/publications exp/CALIFA BARREL TDR web.pdf
[Cio14] M. Ciobanu et al, IEE Trans. Nucl. Sci. 61 (2014) 1015.
[Ber11] D.Bertini, Journal of Physics: Conference Series, 331(3) (2011) 032036
[Cio14] M. Ciobanu et al, IEE Trans. Nucl. Sci. 61 (2014) 1015.
[Cub98] J. Cub et al., Nucl. Instr. Meth. A402 (1998) 67.
[Dav08] V.I. Davkov et al., Nuclear experimental technique, 2008, Vol.51, No.6,
hh.787-791.
[Elj] www.eljentechnology.com
139
[Feb] Febex Homepage. https://www.gsi.de/de/work/organisation/
wissenschaftlich technologische abteilungen/experiment elektronik/
digitalelektronik/digitalelektronik/module/font end module/
febex/febex3a.htm?nr=%2Fproc%2Fself%2Fen.
[Frü12] J. Frühauf, J. Hoffmann, E. Bayer and N. Kurz, GSI Scientific Report, 300
(2012).
[Gea69] C W Gear. Graphics in a time sharing environment. Proceedings for the Skytop Conference on Computer Systems in Experimental Nucelar Physics, pages
552–565, 1969.
[Ger13] J. Gerbig, “Kalibrierung eines Szintillationsdetektors mit Hilfe von LEDs”,
Masterarbeit 2013, Goethe-Universität Frankfurt
[Ham06] psec.uchicago.edu/links/pmt handbook complete.pdf
[Ham1] Hamamatsu S5378 Datasheet.
[Ham2] H9500 Manual.
http://www.hamamatsu.com/resources/pdf/etd/H9500 H9500-03 TPMH1309E01.pdf.
[Ham3] S12572-025P Manual.
http://www.hamamatsu.com/resources/pdf/ssd/s12572-025 etc kapd1043e03.pdf.
[Has88] Nobuyuki Hasebe, et al, Charge Waveform of a New Two-Dimensional
Position-Sensitive Silicon Detector. Jpn. J. Appl. Phys., 27 816, 1988.
[Haw13] N.P. Hawkes and G.C Taylor, Nucl. Instr. Meth. A 729 (2013) 522
[Syn14] I.J. Syndikus. Testing of Position Sensitive Silicon Detectors for the R3BSetup. Master-Thesis, TU Darmstadt, 2014.
[Jor94] Valentin T Jordanov and Glenn F Knoll. Digital synthesis of pulse shapes in
real time for high resolution radiation spectroscopy. Nuclear Instruments and
Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors
and Associated Equipment, 345(2):337–345, 1994.
[Kno10] Glenn F Knoll. Radiation detection and measurement. Wiley, 2010.
[Koc05] K. Koch, H. Hardel, R. Schulze, E. Badura, and J. Hoffmann, IEEE Transactions on Nuclear Science 52, 745-747 June (2005).
[Kur] SCSF 78-fiber Manual. http://kuraraypsf.jp/psf/sf.html.
[Kur96] N. Kurz, “VFTX2”, EPAC’96, Sitges, June 1996, p. 7984
[Lae79] E Laegsgaard. Position-sensitve semiconductor detectors. Nuclear Instruments and Methods, 162(1):93–111, 1979.
140
[Leo87] William R Leo. Techniques for Nuclear and Particle Physics Experiments:
A How-to Approach. Springer Verlag, 1987.
[Lev11] B.L. Leverington, M. Anelli, P. Campana, R. Rosellini . ”A 1 mm
Scintillating Fibre Tracker Readout by a Multi-anode Photomultiplier”.
arXiv:1106.5649v2 [physics.ins-det].
[Mos77] M. Moszynski and B. Bengtson, Nucl. Instr. Meth. 142 (1977) 417
[Neu11] Technical Design Report of NeuLAND: The High-Resolution Neutron Timeof-Flight Spectrometer for R3 B (2011)
http://www.fair-center.de/fileadmin/fair/experiments/NUSTAR/Pdf/
TDRs/NeuLAND-TDR-Web.pdf
[NuD14] NuDAT Homepage. http://www.nndc.bnl.gov/nudat2/chartNuc.jsp.
[Online; accessed 27.January 2014].
[Pat13] Marco Patrizio. Characterisation of a Position and Energy Sensitive Silicon
Detector . Bachelor-Thesis, TU Darmstadt, 2013.
[PNP06] PNPI High Energy Physics Division MAIN SCIENTIFIC ACTIVITIES
2002- 2006 (2006) http://hepd.pnpi.spb.ru/hepd/articles/progress report.pdf
(P. 334).
[Pos50] R.F. Post and L.I. Schiff, Phys. Rev. 80 (1950) 1113
[R3B05] Technical Proposal for the Design, Construction, Commissioning and
Operation of R3 B (2005)
http://www.gsi.de/onTEAM/grafik/1130224398/13-PROP-R3B-TP-07Dez2005.pdf
[Ros13] D. M. Rossi et al., Phys. Rev. Lett. 111, (2013) 242503
[Sal82] M.H. Salomon and S.P. Ahlen, Nucl. Instr. Meth. 195 (1982) 557
[Sch13] Henning Schaffner, 2013. private communication.
[Sub08] R. Subedi et al., Science 320 (2008) 1476.
[Tai96] M. Taiuti et al, Nucl. Instr. Meth. A 370 (1996) 429
[Wam14] F. Wamers et al., Phys. Rev. Lett. 112 (2014) 132502
[WAS] Proposal for the Wide Angle Shower Apparatus (WASA) at COSY-Juelich
- ’WASA at COSY’ 2004 (Preprint nucl-ex/0411038)
141
[Yan89] T Yanagimachi, T Doke, N Hasebe, T Imai, T Kashiwagi, et al. New twodimensional position sensitive silicon detector with good position linearity and
resolution. Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment, 275(2):307–
314, 1989.
[Yu03] KN Yu, CWY Yip, D Nikezic, JPY Ho, and VSY Koo. Comparison among
alpha-particle energy losses in air obtained from data of srim, icru and experiments. Applied radiation and isotopes, 59(5):363–366, 2003.
142

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