Development and evaluation of a signal analysis and a
138
DOCTORAL T HESIS Development and evaluation of a signal analysis and a readout system of straw tube detectors for the PANDA spectrometer Author: Pawel S TRZEMPEK Supervisor: prof. dr hab. Piotr S ALABURA Co-supervisor: Grzegorz KORCYL Jagiellonian University February 4, 2017
Development and evaluation of a signal analysis and a
Development and evaluation of a signal analysis and a readout
system of straw tube detectors for the PANDA spectrometerDOCTORAL
THESIS
Development and evaluation of a signal analysis and a readout
system of straw tube
detectors for the PANDA spectrometer
Author: Pawe STRZEMPEK
Co-supervisor: Grzegorz KORCYL
Oswiadczenie
Ja nizej podpisany Pawe Strzempek doktorant Wydziau Fizyki,
Astronomii i In- formatyki Stosowanej Uniwersytetu Jagiellonskiego
oswiadczam, ze przedozona przeze mnie rozprawa doktorska pt. „
Development and evaluation of a signal analysis and a readout
system of straw tube detectors for the PANDA spectrometer” jest
oryginalna i przedstawia wyniki badan wykonanych przeze mnie
osobiscie, pod kierunkiem prof. dr hab. Piotra Salabury. Prace
napisaem samodzielnie.
Oswiadczam, ze moja rozprawa doktorska zostaa opracowana zgodnie z
Ustawa o prawie autorskim i prawach pokrewnych z dnia 4 lutego 1994
r. (Dziennik Ustaw 1994 nr 24 poz. 83 wraz z pózniejszymi
zmianami).
Jestem swiadom, ze niezgodnosc niniejszego oswiadczenia z prawda
ujawniona w dowolnym czasie, niezaleznie od skutków prawnych
wynikajacych z ww. ustawy, moze spowodowac uniewaznienie stopnia
nabytego na podstawie tej rozprawy.
Kraków, dnia ....................................
.................................. podpis doktoranta
Doctor of Philosophy
Development and evaluation of a signal analysis and a readout
system of straw tube detectors for the PANDA spectrometer
by Pawe STRZEMPEK
In this thesis the prototype system for processing of the signals
generated in the straw tube trackers of the PANDA spectrometer is
proposed, built and evaluated. The full processing chain of signal
consists of programmable readout electronics, config- ware and
analysis methods. The proposed readout is based on the front end
electron- ics, equipped with the configurable PASTTRECv1 ASIC
(PANDA STT REadout Chip version 1 Application Specific Integrated
Circuit) and the readout board (TRBv3 - Trig- ger Readout Board
version 3) acting as data concentrator and time measurement de-
vice. The readout system performs full chain of data processing
consisting of analog shaping, digital conversion and data
transmission. A dedicated analysis methods have been developed to
extract track position and a particle energy deposit in the detec-
tors. Dedicated tests of the system by means of the cosmic rays and
proton beams have been performed. The results prove that the
spatial resolution better than 150 µm can be achieved. Furthermore,
particle identification based on time over threshold method can be
successfully applied in the momentum region below 800 MeV/c. The
last but not least goal was to show that it is possible to realize
complete readout system based on the concept presented in this
thesis which is capable of cope with the hit rates of the PANDA
spectrometer.
v
Rozprawa doktorska
Rozwój i ewaluacja systemu odczytu i analizy sygnaów z detektorów
somkowych spektrometru PANDA
Pawe Strzempek
W niniejszej pracy zosta opisany, zbudowany i przetestowany
prototyp systemu prze- twarzajacego sygnay generowanych przez
somkowe detektory sladu spektrometru PANDA. We wspomnianym
systemie, sciezka przetwarzania sygnau skada sie z pro- gramowalnej
elektroniki odczytu, oprogramowania wbudowanego oraz metod anal-
izy danych. Zaproponowana elektronika odczytu dzieli sie na
elektronike przednia, wyposazona w konfigurowalny ukad ASIC
(Application Specific Integrated Circuit) o nazwie PASTTRECv1
(PANDA STT REadout Chip version 1) oraz na pyte odczytu (TRBv3 -
Trigger Readout Board version 3), której zadaniem jest koncentracja
danych oraz pomiar czasu. System odczytu przeprowadza kompleksowe
przetwarzanie syg- naów poczawszy od ksztatowania sygnau
analogowego, jego konwersje do postaci cyfrowej a nastepnie
cyfrowej transmisji. Rozwiniete zostay metody analizy danych suzace
do okreslania sladów przelotów czastek przez detektor oraz
pozwalajace okreslic ilosc zdeponowanej przez nie energii. Ponadto
przeprowadzone zostay dedykowane testy systemu z promieniowaniem
kosmicznym oraz z wiazka protonowa. W wyniku tych testów okreslono
pozycyjna zdolnosc rozdzielcza detektora która spenia warunki (x ≤
150µm ) stawiane przez eksperyment PANDA. Co wiecej rozpoznawanie
czastek w oparciu o metode czasu nad progiem moze z powodzeniem byc
zastosowane w obszarze pedów czastek ponizej 800 MeV/c. Ostatnim z
wyznaczonych celów pracy byo wykazanie, ze mozliwa jest realizacja
penego systemu odczytu detektora, przy wykorzystaniu koncepcji
przedstawionych w niniejszej pracy, speniajacego stawiane wymagania
w eksperymencie PANDA.
vii
Acknowledgements I would like to thank my dissertation advisor
prof. Piotr Salabura for his time, help
and a lot of valuable advice. I would also like to thank my
dissertation co-advisor dr Grzegorz Korcyl who not only taught me a
lot but also was a great companion. Many thanks to my family and
especially my wife Agata who has always been there to support and
encourage me to finish this work.
Moreover, the presented work was supported by NCN based on decision
number: [DEC-2013/09/N/ST2/02180].
ix
Contents
1 Introduction 1
2 PANDA experiment 5 2.1 Overview . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 5 2.2 Physical motivation .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1 The Quantum Chromo Dynamics . . . . . . . . . . . . . . . . .
. 5 Charmonium spectroscopy . . . . . . . . . . . . . . . . . . . .
. . 6
2.2.2 Gluonic excitation . . . . . . . . . . . . . . . . . . . . .
. . . . . . 7 Hyperon physics . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 8
2.3 Infrastructure . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 8 2.4 Spectrometer overview . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 9
2.4.1 Targets . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 11 2.4.2 Magnets . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 12 2.4.3 Subdetectors . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 12
Micro Vertex Detector . . . . . . . . . . . . . . . . . . . . . . .
. . 12 Electromagnetic calorimeter . . . . . . . . . . . . . . . .
. . . . . . 13 Gas Electron Multiplier (GEM) . . . . . . . . . . .
. . . . . . . . . 13 Detector of Internally Reflected Cherenkov
light (DIRC) . . . . . 13 Ring Imagine Cherenkov light . . . . . .
. . . . . . . . . . . . . . 14 Muon detector . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 14 Central Straw Tube Tracker
(STT) . . . . . . . . . . . . . . . . . . . 14 Forward Tracker (FT)
. . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.4.4 Straw tube trackers . . . . . . . . . . . . . . . . . . . . .
. . . . . 16 Principles of operation and construction of PANDA
straws . . . 16 Purpose of the tracking system . . . . . . . . . .
. . . . . . . . . . 19 Data rates in the straw tube trackers . . .
. . . . . . . . . . . . . . 20
3 Architecture of the Readout System for the straw tube trackers 23
3.1 Data acquisition systems in nuclear and particle physics . . .
. . . . . . 23
3.1.1 Detector . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 23 3.1.2 Front end electronics . . . . . . . . . . .
. . . . . . . . . . . . . . . 25 3.1.3 Amplifiers, shapers and
digitizers . . . . . . . . . . . . . . . . . . 26
ADC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 27 TDC . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 27
3.1.4 Trigger system . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 28 3.1.5 Data concentrators and storage . . . . . . . .
. . . . . . . . . . . . 29 3.1.6 Event building and networking . .
. . . . . . . . . . . . . . . . . . 30 3.1.7 Data processing . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 32
3.2 PANDA DAQ system . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 32 3.2.1 DAQ overview . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 32
xi
3.2.2 The event building . . . . . . . . . . . . . . . . . . . . .
. . . . . . 33 3.2.3 The computing farm . . . . . . . . . . . . . .
. . . . . . . . . . . . 35
3.3 Concepts of signal processing for the STT and FT . . . . . . .
. . . . . . . 36 3.3.1 ADC based approach . . . . . . . . . . . . .
. . . . . . . . . . . . . 36 3.3.2 Time over threshold approach . .
. . . . . . . . . . . . . . . . . . 38
4 STT and FT signal processing 39 4.1 Front end electronics . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 39
4.1.1 PASTTREC chip . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 39 Analog part . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 39 Digital part . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 40 Difference between PASTTRECs
versions . . . . . . . . . . . . . . 41
4.1.2 Front end board for PASTTRECv1 . . . . . . . . . . . . . . .
. . . 41 4.2 TRBv3 as a readout platform . . . . . . . . . . . . .
. . . . . . . . . . . . . 42
4.2.1 Time to digital converter . . . . . . . . . . . . . . . . . .
. . . . . . 43 Zero suppression . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 45 Calibration . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 45 Time measurement resolution . .
. . . . . . . . . . . . . . . . . . . 46
4.2.2 Data transmission and networking . . . . . . . . . . . . . .
. . . . 46 Estimations of the maximal data bandwidth and limits of
the sys-
tem performance . . . . . . . . . . . . . . . . . . . . . . 48
Efficiency and performance tests . . . . . . . . . . . . . . . . .
. . 49
4.2.3 Integration of the slow control for the PASTTRECv1 . . . . .
. . . 50 4.3 Event building . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 50 4.4 Analysis software for the FT
prototype . . . . . . . . . . . . . . . . . . . . 52
4.4.1 Offline software design . . . . . . . . . . . . . . . . . . .
. . . . . 52 4.4.2 Offline methods and procedures of the analysis .
. . . . . . . . . 53
Time to radius calibration . . . . . . . . . . . . . . . . . . . .
. . . 55 Track reconstruction . . . . . . . . . . . . . . . . . . .
. . . . . . . 56 Track to wire distance and truncated mean . . . .
. . . . . . . . . 57
4.4.3 Online quality assessment . . . . . . . . . . . . . . . . . .
. . . . . 57
5 Measurements and tests 61 5.1 Laboratory equipment and setup . .
. . . . . . . . . . . . . . . . . . . . . 61 5.2 Investigation of
the PASTTREC configurations . . . . . . . . . . . . . . . 61
5.2.1 Optimal chip settings . . . . . . . . . . . . . . . . . . . .
. . . . . 63 5.2.2 Gain and baseline uniformity . . . . . . . . . .
. . . . . . . . . . . 64 5.2.3 Hit rate capability . . . . . . . .
. . . . . . . . . . . . . . . . . . . 68 5.2.4 Tests with 55Fe
source . . . . . . . . . . . . . . . . . . . . . . . . . 69
55Fe source . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 69 Dynamic range . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 70 Charge and TOT correlation . . . . . . . . . . . .
. . . . . . . . . . 71 Threshold position investigation . . . . . .
. . . . . . . . . . . . . 71
5.2.5 Evaluation of the prototype with the cosmic rays . . . . . .
. . . 72 Baseline optimization . . . . . . . . . . . . . . . . . .
. . . . . . . 73
5.2.6 Cosmic rays data analysis . . . . . . . . . . . . . . . . . .
. . . . . 73 Data sets and event filtration . . . . . . . . . . . .
. . . . . . . . . 74 Track reconstruction . . . . . . . . . . . . .
. . . . . . . . . . . . . 75 Detector efficiency . . . . . . . . .
. . . . . . . . . . . . . . . . . . 76 TOT separation . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 77
xii
6 In beam operation 79 6.1 Setup description . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 79 6.2 Measurement
conditions and data sets . . . . . . . . . . . . . . . . . . . . 80
6.3 Data analysis . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 80
6.3.1 QA plots and calibration . . . . . . . . . . . . . . . . . .
. . . . . . 80 6.3.2 Spatial resolution . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 82 6.3.3 Particle identification . . .
. . . . . . . . . . . . . . . . . . . . . . . 83
FT data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 84 STT data . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 86
7 Summary and conclusions 89 7.1 Discussion of the results . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 89 7.2 Readout
schematic for the STT and the FT . . . . . . . . . . . . . . . . .
. 90 7.3 Outlook . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 92
A The use of FPGA based development boards for testing electronics
95 A.1 FPGA units and development boards . . . . . . . . . . . . .
. . . . . . . 95 A.2 Signal routing and conversion . . . . . . . .
. . . . . . . . . . . . . . . . . 96
B Slow control for the PASTTRECv0 99 B.1 Slow control protocol . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
C Slow control for the PASTTRECv1 101 C.0.1 User interface . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 101 C.0.2
Configuration module . . . . . . . . . . . . . . . . . . . . . . .
. . 101
D Energy loss of charged particles 105 D.1 Bethe formula . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105 D.2
Most probable energy loss . . . . . . . . . . . . . . . . . . . . .
. . . . . . 105
Bibliography 109
List of Figures
2.1 Formation of charmonium in e+e− (a) and in two different
scenarios of pp annihilation (b,c). . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 6
2.2 The charmonium spectrum. Black lines denote charmonium states,
and red dots indicate charmonium-like states. Blue lines indicate
the thresh- olds at which states can decay into a pair of D mesons.
Adapted from [5]. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 7
2.3 Illustration of the FAIR complex. The red color marks the part
of the facility which is not yet constructed. Source: [1]. . . . .
. . . . . . . . . . 9
2.4 Schematic view of the HESR. The place of the beam injection,
experimen- tal installations and devices for the beam cooling are
marked. Source: [7]. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 10
2.5 The PANDA spectrometer. . . . . . . . . . . . . . . . . . . . .
. . . . . . 11 2.6 Left: Photography of one STT module. Right: View
of the STT from the
beam direction. The red color indicates straw tubes skewed by
−2.9o
whereas the blue by 2.9o. . . . . . . . . . . . . . . . . . . . . .
. . . . . . 15 2.7 The model of the FT1 and FT2 station. The hole
inside the module is
planned for the beam pipe. . . . . . . . . . . . . . . . . . . . .
. . . . . . . 16 2.8 The straw tube construction on the example of
STT tracker. . . . . . . . . 17 2.9 Simulated drift time in respect
to the distance from the anode wire with-
out (left) and with (right) magnetic field presence. Adapted form
[14] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 18
2.10 Simulated drift path of the electrons originating form the
ionization pro- cess. The case without (left) and with (right)
magnetic field is presented. Adapted form [14]. . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 19
2.11 Separation power in the STT detector for the energy bands
built with particles all tracked with the same muon mass
hypothesis. Source: [11]. 20
2.12 Simulation of pp reactions at 15GeV/c giving the number of
hits per event and per cm along the tubes in the inner most layer
of the STT PANDA [11]. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 21
2.13 Number of counts per second expected in the individual straws
placed in the X location. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 21
3.1 Schematic representation of the front end electronics functions
with two concepts of signal shaping: A - with analog circuitry, B -
inside the signal processor. . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 25
3.2 The three levels of the ATLAS trigger and their event rates and
process- ing time. Source: [21] . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 29
3.3 Schematic of the DAQ general concept. The data streams from FEE
to the DAQ endpoints, Concentrators and finally gets to the event
builders via network. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 30
3.4 A schematic view of the HADES network. The number refers to the
number of boards in the system. Adapted from: [26]. . . . . . . . .
. . . 31
xv
3.5 The general view of the PANDA DAQ architecture. The FEE is con-
nected to the Data Concentrators which receive packets from SODANet
network. The event building and event filtering is done in the
Compute Node matrix. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 33
3.6 Left: The schematic view of the CN architecture. Its basic
building blocks are XILINX Virtex 5 FPGAs. The four central
Virtexes are used as processing units and the fifth one is used to
provide connection to FP- GAs from other CN via the ATCA backplane.
Right: The photography of the µTCA compliant daughter board of CN.
Source [29] and [27]. . . 34
3.7 The SODANet topology. Three types of protocols are marked. The
red one represents the SODANet connection which main task is to
distribute the clock and time information. The blue one is the
Ethernet standard used for data transfer from DCs to EB. The black
one is a custom protocol individually selected for each subsystem.
Adapted from [31] . . . . . . . 35
3.8 Left: The representation of the real to conformal space
transition per- formed by the Hough transform . Right: The example
of the the con- formal space for the line Hough transform. The
local maximum corre- sponds to the line (particle trajectory) in
the real space. Source [34]. . . . 36
3.9 The algorithm representation of clusters finding in the EMC
detector. On the left the list of hits registered in one event. On
the right upper the two dimensional space on which the hits are
mapped. Arrows indicate the direction of the neighboring hits
search. On the right bottom the order of the neighboring hits
search. Source [33]. . . . . . . . . . . . . . . . . . 37
4.1 The micrograph of produced prototype of the PASTTREC. . . . . .
. . . 40 4.2 Schematic of the internal PASTTREC architecture. The
following com-
ponents are presented: preamplifier with first stage shaper
(CR-RC), two stage tail cancellation circuit, baseline holder
circuit and discriminator. . 40
4.3 Photography of the FEB and the high voltage decoupling board.
The straw tubes are connected to the pads located on the right side
(respect to the photography) of the decoupling board. . . . . . . .
. . . . . . . . . 42
4.4 Left: the TRBv3 photography. Right: schematic representation of
the TRBv3 components. . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 43
4.5 Schematic representation of time acquisition by TDC. Each TDC
chan- nel stores information in ring buffer which is filled by
header (1 word), epoch counter (1 word) and consecutive hits (2
words per hit). . . . . . 44
4.6 The fine time calibration step function which translates the
TDC bin in- formation (x-axis) into time information (y-axis) . . .
. . . . . . . . . . . 45
4.7 The stretcher offsets values for each of the channel in one
TDC. . . . . . 46 4.8 The RMS values of the lead time difference
between two consecutive
channels of the FEE and TDC. . . . . . . . . . . . . . . . . . . .
. . . . . . 47 4.9 Schematic representation of tree like
architecture of the TRBv3 system.
The black line connecting the HUBs indicates TRBnet protocol. Each
endpoint can have FEBs connected but for the simplicity only
several FEBs were drawn. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 47
4.10 A simplified schematic of TRBv3 architecture. . . . . . . . .
. . . . . . . . 48 4.11 Overall (payload and headers) data transfer
rate from TRBv3 depending
on the read request frequency. The hit rate per channel was set to
1 MHz. 49
xvi
4.12 The correlation between the multiplicity of hits and the hit
rate. Above 250kHz the constant hit multiplicity of∼ 50 is present
which corresponds to the saturation of the ring buffer. Slight
increase of the number of recorded hits appears due to the
decreasing number of epoch counter words with the increasing data
rates. The data was taken for a single channel. . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 50
4.13 Multiplicity of registered hits coming from one generation
series pro- duced by Virtex5. One generation series consists of
100001 simulated hits. The read request in TRBv3 was generated with
internal generator set to 5kHz. The histogram shows number of hits
collected by a single channel. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 51
4.14 Schematic representation of the class designed, implemented
and used in analysis software. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . 53
4.15 The GUI of the event display designed for the FT prototype. On
the canvas the prototype detector geometry is displayed together
with one event. The black and the red straight lines represents
reconstructed tra- jectories of the particle. . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 54
4.16 Schematic representation of the data analysis stages. On the
left side the input files for the macros which runs the analysis.
On the right the data structure of the output files. The meaning of
the names is as following: globEvNum - global event number, chNum -
channel number, ftTDC2 - drift time, TOT - time over threshold,
driftR - drift radius, X - x coordi- nate of the straw that fired,
Z - z coordinate of the straw that fired, a, b, a_err, b_err - the
parameters of the prefit, a_mi, b_mi - the parameters of the Minuit
fit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 55
4.17 The hit multiplicity per event for cosmic rays data which was
taken with the setup described in section 5.1. The hits number
maximizes at 6 what corresponds to the number of the straw tube
layers used in the measure- ment. . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 55
4.18 Exemplar calibration curve done for the data collected with
the cosmic rays. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 56
4.19 Generation of the signal inside the straw. Particle crossing
the straw close to the anode wire (left) leaves more charge than
the one crossing close to the wall (right). . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 58
4.20 TOT simulation with single straw response (top) and truncated
aver- aged on 24 straws (bottom) for proton and charged pion and
kaon with 0.5 GeV/c momentum. Solid red lines show the Gaussian fit
to the dis- tributions. The protons, charged kaons and pions are
well distinguished using distance corrected TOT after applying the
truncated mean by dis- carding the 30% of the largest values. [14]
. . . . . . . . . . . . . . . . . . 59
4.21 The graphical user interface of the go4. . . . . . . . . . . .
. . . . . . . . 59
5.1 A: Prototype detector setup installed at Jagiellonian
University in Krakow. The setup consist of 3 vertically oriented FT
modules (96 channels in to- tal). B: Schematic view of the detector
arrangement with rough dimen- sions marked. C: Block diagram
showing connection of the detector, FEBs and the TRBv3. . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 62
5.2 Set of analog pulse shapes taken with the delta pulse for all
combinations of the preamplifier gain settings (0.67, 1, 2, 4
mV/fC) and the peaking time settings (10, 15, 20, 35 ns). . . . . .
. . . . . . . . . . . . . . . . . . . 64
xvii
5.3 Left: Amplitudes of 16 output signals versus input charge for
the same ASIC configuration (see text for details). Right:
Distribution of the base- line levels accumulated from 198
channels. . . . . . . . . . . . . . . . . . 68
5.4 Gain measurements for delta pulses for four settings of
preamplifier gain parameter (K). . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 69
5.5 The analog output of the PASTTREC chip responding to the pulse
cou- pled to the FEB input test. The input pulse frequency equals 2
MHz.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 69 5.6 The TOT spectrum of 55Fe taken with high
voltage set to 1700 V. The
strong right peak corresponds to the full absorption of the 5.9 keV
X- rays, and it is clearly separated from the 2.9 keV argon escape
peak on the left. . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 70
5.7 TOT and charge correlation. The charge was obtained by taking
into account the number of primary electron production amplified
according to gas gain function. The TOT values come from Gaussian
distribution fitted to TOT spectrum which was collected with 55Fe
source. . . . . . . 72
5.8 Correlation between separation parameter and threshold
position. Sep- aration parameter calculated according to the
equation 5.1. The data taken with the iron radioactive source at
straws high voltage set to 1700V. 73
5.9 Left: TOT spectrum versus channel number for selected channels
of the detector before the baseline tune procedure was applied. The
white bars corresponds to disconnected straw tubes. Right: TOT
spectrum versus channel number for selected channels of the
detector after the baseline tune procedure was applied. The
channels above 32 have less statistic as they belong to the second
module of the straw tubes which was located 10 cm farther from the
iron source (see section 5.1). . . . . . . . . . . . . . 74
5.10 Left: Drift time spectrum versus channel number for all the 96
channels of the system. The white bars correspond to disconnected
straw tubes. Right: Projection of the drift time spectrum for all
the 96 channels. . . . 75
5.11 Left: TOT versus drift time correlation for 1800V and
threshold 21 mV. Data for all the hits in the detector. Right: TOT
versus drift time correla- tion for 1800V and threshold 21 mV for
the reconstructed tracks. Divid- ing number of entries by 6
(average hit multiplicity) results in estimation of number of
reconstructed tracks (∼6500) in the data set. . . . . . . . . .
76
5.12 The histograms of residuals for different cosmic ray data
sets. . . . . . . 76 5.13 The FT prototype straw tubes placement.
Example event is drawn which
contributes to the detector inefficiency. . . . . . . . . . . . . .
. . . . . . . 77 5.14 Left: Efficiency versus hit position for two
different threshold levels and
the same voltage (1800 V) at the detector. Right: Efficiency versus
hit position between two different high voltages at the detector
(1700 and 1900V) and the same threshold 21 mV. . . . . . . . . . .
. . . . . . . . . . 78
5.15 Comparison of the TOT mean spectra for cosmic rays for
different high voltages applied to the detector and two thresholds:
5mV (left), 21mV (right). . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 78
6.1 The schematic representation of the detector modules placement
during the proton beam tests. . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 80
6.2 Left-upper: The drift time spectra for all 96 channels of FT
system. Right- upper: TOT spectrum for all 96 channels of FT
system. Lower: Drift time spectrum for one selected straw tube. . .
. . . . . . . . . . . . . . . . . . 81
xviii
6.3 Left: Drift time versus TOT correlation for all hits. Middle:
Drift time versus TOT correlation for the hits belonging to the
successfully recon- structed tracks. Right: Drift time versus TOT
correlation after perform- ing the TOT geometrical calibration. . .
. . . . . . . . . . . . . . . . . . . 82
6.4 Residual distribution as a function of the drift time of the
reconstructed proton tracks (proton momenta 750 MeV/c) . . . . . .
. . . . . . . . . . . 82
6.5 Left: Residual distribution for different PASTTRECv1 settings
(see text for details) and different beam momenta. Right: Residual
distribution for different thresholds, default PASTTRECv1 setting
and different beam momenta. . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 83
6.6 Energy loss measurement for different particles at different
momenta. Plot was done from data taken with time-projection
chambers by the PEP4 experiment. Adapted from [12]. . . . . . . . .
. . . . . . . . . . . . 84
6.7 TOT truncated mean 20% for different proton momenta. Data taken
with the FT detector, threshold equal 10 mV and setting2. . . . . .
. . . . . . 84
6.8 Dependence of TOT measurement on thresholds. Left: TOT
truncated mean for four beam momenta. Middle: TOT truncated mean
resolution for the four beam momenta. Right: Separation power
calculated for each beam momenta with respect to the minimum
ionizing protons. . . . . . 85
6.9 Dependence of TOT measurement on settings of the PASTTRECv1.
Left: TOT truncated mean for four beam momenta. Middle: TOT
truncated mean resolution for the four beam momenta. Right:
Separation power calculated in respect to MIPs (p=3GeV/s). . . . .
. . . . . . . . . . . . . 85
6.10 Separation power as function of βγ for protons, pions and
kaons. Result obtained with default settings of the PASTTRECv1 and
threshold set to 10 mV. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 86
6.11 TOT truncated mean for the protons with 750 MeV/c momentum ob-
tained for different event ranges saved in a single file. The
significant differences in the spectrum shapes may indicate that
the measuring con- ditions were not stable in time. . . . . . . . .
. . . . . . . . . . . . . . . . 87
6.12 TOT truncated mean 40% for different proton momenta. Data
taken with the STT detector, threshold equal 10 mV and setting2. .
. . . . . . . . . . 87
6.13 Left: Cumulative distribution function and probability of
identification proton with momentum 550 MeV/c and pion with
momentum 550 MeV/c mimic by the minimum ionizing proton (3 GeV/c).
Right: Cumulative distribution function and probability of
identification proton with mo- mentum 550 MeV/c and kaon with
momentum 550 MeV/c mimic by the quasi minimum ionizing proton (1
GeV/c). . . . . . . . . . . . . . . 88
7.1 Schematic of the suggested readout of the STT detector. The
readout is based on the TRBv3 boards and the FEB equipped with the
PAST- TRECv1 chip. Compare with figure 3.7. . . . . . . . . . . . .
. . . . . . . 91
A.1 Architecture of single logic block. . . . . . . . . . . . . . .
. . . . . . . . . 95 A.2 Full development path of the configuration
file for the FPGA. Code writ-
ten in VHDL is synthesized into logic which is later mapped to the
avail- able resources inside FPGA. In the zoom the single logic
blocks are visible. 97
B.1 The graphical user interface for the prototype FEB. . . . . . .
. . . . . . . 99 B.2 The command word format used to control the
prototype front end board.100
xix
C.1 The snapshot of the currently used graphical user interface for
the FEE setup. The GUI is integrated with the TRBv3 system. . . . .
. . . . . . . 102
C.2 Waveform of the TRBv3 to PASTTRECv1 communication. The callback
line is used for back transmission of saved value in the register.
. . . . . 102
D.1 Mean energy loss rate in different materials. . . . . . . . . .
. . . . . . . . 106 D.2 Function of energy loss (here denote as /x)
in silicon material for 500
MeV pions, normalized to unity at most probable value. Adapted from
[12] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 106
D.3 Energy loss in a straw tube (blue dashed histogram) compared
with the sharper Landau distribution (black histogram). Simulation
done for a 1 GeV/c pion crossing tube filled with a ArCO2 (90/10)
gas mixture at 2 bars. Adapted from [52]. . . . . . . . . . . . . .
. . . . . . . . . . . . . . 107
xx
List of Tables
2.1 Size, placement and number of straws in each FT station. . . .
. . . . . . 16 2.2 Properties of the argon and the carbon dioxide.
Ex and Ei are the ex-
citation and ionization energies. Wi is the minimal energy
necessary to produce one electron-ion pair in the gas. dE/dx is the
most probable energy loss of the minimum ionizing particle in the
gas. Np and Nt are the number of primary and total electrons per
cm, respectively. X0 is the radiation length. Adapted from [12] . .
. . . . . . . . . . . . . . . . . . . 18
4.1 Description of the PASTTRECv1 registers. . . . . . . . . . . .
. . . . . . . 41 4.2 The data format of the time data word [42]. .
. . . . . . . . . . . . . . . . 51 4.3 Steps of a data processing
form binary file to user’s result file. . . . . . . 52
5.1 Three optimal settings found for preamplifier gain equal 1
mV/fC and peaking time equal 15 ns. . . . . . . . . . . . . . . . .
. . . . . . . . . . . 65
5.2 Three optimal settings found for preamplifier gain equal 1
mV/fC and peaking time equal 20 ns. . . . . . . . . . . . . . . . .
. . . . . . . . . . . 65
5.3 Three optimal settings found for preamplifier gain equal 1
mV/fC and peaking time equal 35 ns. . . . . . . . . . . . . . . . .
. . . . . . . . . . . 66
5.4 Three optimal settings found for preamplifier gain equal 2
mV/fC and peaking time equal 15 ns. . . . . . . . . . . . . . . . .
. . . . . . . . . . . 66
5.5 Three optimal settings found for preamplifier gain equal 2
mV/fC and peaking time equal 20 ns. . . . . . . . . . . . . . . . .
. . . . . . . . . . . 67
5.6 Three optimal settings found for preamplifier gain equal 2
mV/fC and peaking time equal 35 ns. . . . . . . . . . . . . . . . .
. . . . . . . . . . . 67
5.7 Complete set of data collected with the cosmic rays. . . . . .
. . . . . . . 74
C.1 The command word structure send from the PC to the
configuration module implemented inside FPGA. The lower nibble (19
LSB) is for- warded to the PASTTRECv1. X – not important bits, R –
reset bit. If set to 1, both PASTTRECv1 chips placed on FEB, which
address is indicated by the CC bits, are reset, C – cable connector
number. 00 – connector 1, 01 – connector 2, 10- connector 3, 11 –
connector 4 (which should not be used as one TDC can measure time
form maximally 3 FEE), H – header, should be 1010. After ASIC
decodes this bit sequence it starts to decode the rest of the
command word, S – select ASIC. One FEE board has two ASIC chips, in
order to distinguish them each has individual address 10 or 01, O –
read or write operation. 1 – read, 0 – write. Once read oper- ation
has been selected the addressed register is only read (its value is
transmitted over the SPI return line), whereas write operation
alter the register and previous value of register is transmitted
over SPI return line, A – 4 bit ASIC register address, V – 8 bit
ASIC register value. . . . . . . 103
xxi
List of Abbreviations
ADC Analog to Digital Converter ASIC Application Specific
Integrated Circuit CFD Constant Fraction Discrimination CN Copute
Node CTS Central Trigger System DAQ Data AcQuisition DC Data
Concentrator DIRC Detector of Internally Reflected Cherenkov light
EMC Electro Magnetic Calorimeter FADC Flash Analog to Digital
Converters FEB Front End Borad FEE Front End Electronics FPGA Field
Programmable Gate Arrays FT Forward Tracker FS Forward Spectrometer
GEM Gas Electron Multiplier HLM High Luminosity Mode HRM High
Resolution Mode IP Interaction Point MDT Mini Drift Tubes MIP
Minimum Ionizing Particles MVD Micro Vertex Detector PCB Printed
Circuit Board PID Particle IDentification PT Peaking Time RICH Ring
Imagine CHerenkov SODANet Synchronization Of Data Acquisition
Network STT Straw Tube Tracker TC Tail Cancellation TDC Time to
Digital Converter TOT Time Over Threshold TRB Trigger Readout Board
TS Target Spectrometer VHDL Very high speed Hardware Description
Language QA Quality Assessment
xxiii
Introduction
In 1896 Henri Becquerel discovered that samples of uranium ore
leave marks on the photographic plates. His discovery gave a
foundation for new scientific discipline - nuclear physics.
Becquerel’s work inspired Marie Curie Sklodowska who followed his
findings and together with her husband (Pierr) have discovered
radium and polonium, two radioactive chemical elements extracted
from uraninite ore sample.
A huge step forward in the investigation of radioactivity was done
by Ernest Ruther- ford who is called the father of nuclear physics.
He discovered the half-life period prin- ciple but the most famous
experiment that he performed was irradiation of golden foil with
alpha particles which proved that the atoms consist of orbital
electrons moving around protons concentrated in the center of the
atom forming nucleus.
Another mile stone was reached at the beginning of 1932, when James
Chadwick discovered the neutron which as a neutral particle could
more easily than proton reach the nucleus of atom. This effect was
used by Enrico Fermi who has proven that neutron bombardment can
lead to artificially induced radioactivity. No more than five years
passed and neutron induced uranium fission was performed which led
to construction of nuclear reactors and uranium-fission
bombs.
Till 1960s people believed that protons and neutrons were the
smallest possible particles. The situation has changed after the
investigation of deep inelastic scattering at Stanford Linear
Accelerator Center which confirmed existence of quarks proposed by
the quark model introduced in the 1964. This discovery led to
general acceptance of the Standard Model (SM) which had been
evolving since several years at that time.
The SM is a theory unifying description of the electromagnetic,
weak and strong interactions among the particles. Even though the
theory is not full (does not include the gravitation interaction)
it has many successful experimental confirmations like for example
discoveries of the W and Z bozons existence or charm and bottom
quarks. In order to be able to compare further SM predictions with
the underlying physical prop- erties of the nature the physicists
need to construct more advanced and sophisticated detecting
machines which are able to record particles created in the
collisions of high energy particle beams with stationary target or
another counter beam.
An example of such a sophisticated detector is the one being built
for the PANDA (anti-Proton ANnihilation at DArmstadt) experiment
which will work at the high en- ergy storage ring accelerator at
FAIR facility in Darmstad. The detector will operate with a fixed
proton target where proton-antiproton collisions will take place at
the beam momentum in range 1.5 - 15 GeV/c. The research program
aims to study the physics of the strong interactions described by
the Quantum Chromo Dynamics.
The spectrometer, designed for the PANDA experiment, will measure
different re- action products by means of many sub-detectors. One
of them is a tracking system which is responsible for
reconstruction of the particle tracks inside a dedicated mag- netic
field. Second task of the tracking system is the energy loss
measurement which is
1
intended for the particle identification. This thesis work focuses
on investigation and development of a dedicated readout
system for the PANDA tracking detectors. The main these of the work
is defined as following: "A dedicated readout system for the
signals generated by straw tubes de- signed for the PANDA
experiment can be built with the signal amplifying, shaping and
discriminating front end electronics based on the PASTTRECv11 chip
connected to the Triggered Readout Board (TRBv3). Such a readout
schematic fulfills the PANDA spectrometer requirements for 150 µm
spatial resolution and enables particle identifica- tion based on
energy loss measurement with Time-Over-Threshold method". The main
goals of the work which are required to prove the quoted these can
be characterized as:
• Prove that the readout system together with the straw tubes
detector is capable of recording trajectories of particles with
spatial resolution no worse than 150 µm.
• Prove that the concept of the Time-Over-Threshold method for
energy loss mea- surement is sufficient for particle identification
in the beam momentum range below 800 MeV/c.
• Present a proposition of readout architecture for the whole
tracking system.
In order to fulfill the above goals the following additional tasks
need to be targeted:
• Preparation and tests of individual hardware components of the
system.
• Preparation of the data acquisition, control software and
firmware for the system.
• Construction of the prototype readout and its performance tests.
Measurements with the prototype and proton beams of various
momenta.
• Development and commissioning of the analysis methods needed for
verification of the requirements.
The research on the specified goals has been completed and the
results are pre- sented in the chapters which are structured as
following.
Chapter 2 gives the physical motivation and research overview of
the PANDA ex- periment. The characteristic of the spectrometer is
described with special attention to the construction and purpose of
the tracking system based on straw tubes detectors.
Chapter 3 is dedicated to general description of data acquisition
systems in nuclear and particle physics and its comparison with the
concept proposed for the PANDA. This section ends with discussion
of two general approaches of digitization the PANDA straws chosen
by the collaboration.
Chapter 4 presents all components of the full chain signal
processing consisting of: PASTTREC which amplifies, shapes and
discriminates analog signals, the TRBv3 readout board which
measures the time of the arrival and the time over threshold and
transmits the data to the PC for storage and a dedicated analysis
software.
Chapter 5 contains description of tests performed with the front
end electronics as well as with the readout board. Tests of whole
prototype system combined with the detector are shown in the final
sections.
Chapter 6 focuses on the results obtained with the detector and
proton beams with a different momenta.
The discussion of the results from laboratory tests and in the beam
measurements are summarized in chapter 7 which presents also a
proposition of the readout system
1PANDA Straw Tube Tracker REconfigurable Chip
Chapter 1. Introduction 3
of the PANDA tracking system. Whole work is closed with the summary
and future plans.
Chapter 2
PANDA experiment
2.1 Overview
The PANDA (AntiProton ANnihilation at DArmstadt) is one of the most
important ex- periment planned to operate in the Facility for
Antiproton and Ion Research (FAIR) at GSI, Darmstadt [1]. The core
of the FAIR will be a new synchrotron SIS100 providing primary
proton (with maximum energy of 29 GeV) or heavy ion (with maximum
en- ergy 1.5 GeV/u for U28+) beam. Anti-protons beams, of very high
intensity and quality, will be provided as secondary beams and
stored in the dedicated High Energy Storage Ring (HESR) (see
section 2.3 for more details). They will interact with stationary
target. The expected available center-of-mass energy will be in
range 2.3-5.5 GeV which will enable the spectroscopy of charm above
the open charm threshold (3.74 GeV) as well as double and triple
hyperons.
The following sections of the chapter are dedicated to more detail
description of some selected aspects of the physics program of the
PANDA, the accelerator complex and the spectrometer which is under
construction and it will be used to fulfill the re- search
program.
2.2 Physical motivation
The PANDA experiment posses very rich scientific research program
which is shortly presented in this section. More complete
information can be found in [2].
2.2.1 The Quantum Chromo Dynamics
The Quantum Chromo Dynamics (QCD) is a very successful theory which
describes the strong interaction occurring between the quarks. The
interactions force between the QCD objects can be explained with
the boson exchange (gluon), similar to the Quantum Electromagnetic
Dynamics theory (QED), where the same mechanism exists and the
exchanged boson is the photon. The quarks as well as gluons carry
strong charge which is called the color. In terms of the QCD the
nature is colorless, it means that no free quarks or gluons can be
observed. Instead these particles are confined in the states of
total color charge equal zero. The confinement, which is the
consequence of the gluon self interaction, is characterized by a
coefficient called the coupling constant (αs) which depends on the
distance between the QCD objects.
At low energies (short distances) the coupling constant is large
thus quarks and gluons are closed in the hadrons. In this region
the QCD may be approximated with the Chiral Perturbation Theory,
whereas in the high energy region, where αs is small and quarks can
be considered as free particles, the perturbative theory is
applicable.
5
6 Chapter 2. PANDA experiment
The perturbative methods fail to describe the medium energy domain,
where more ex- perimental data (hopefully delivered by PANDA) is
needed to evaluate proper theory.
Charmonium spectroscopy
The charmonium spectroscopy begun in 1974 with the discovery of the
J/ψ resonance by the two experimental groups at Brookhaven and SLAC
[3],[4]. Soon after this dis- covery the latter group found also
the ψ′ resonance. New particles were identified as being bound
states of not observed before charmed quark and its antiquark (cc)
which mass mc = 1.27GeV/c2 is much bigger than the mass of u,d and
s quarks. A bound state with the hidden charm are called quarkonia
and in case of cc we speak of char- monium.
In the particle physics mesons which contain one c quark (or anti-c
quark) are called D mesons and are considered as having an open
charm. It has been shown that char- monium spectroscopy is a very
convenient tool for studying the QCD bound system. Thanks to the
big mass of the c quark it is possible to use the non-relativistic
quark potential models which predict the bound states and narrow
resonances.
Both SLAC and Brookhaven groups were using the e+e− annihilation in
their ex- periments. In such a system direct creation of charmed
mesons is possible only for the JPC = 1−− states. Such a limitation
is not present in the antiproton-proton collisions where proton
quarks annihilate with the anti-proton quarks and thus make it
possible to directly form states with all possible quantum numbers.
It is demonstrated with the Feymann diagrams presented in figure
2.1. This is the reason why the PANDA experiment plans to use the
anti-proton beam.
e+
e-
γ
c
a) b) c)
FIGURE 2.1: Formation of charmonium in e+e− (a) and in two
different scenarios of pp annihilation (b,c).
In figure 2.2 the charmonium spectrum is shown. It can be divided
into two regions: the one below and the one above the open charm
threshold (3.73 GeV). The part of the spectrum below 3.73 GeV is
almost completely theoretically understood and is well explored
with the experiments. Above the threshold there is a lot of new
states called XYZ which were discovered at B-factories (decays of B
mesons which consist of at least one b quark). Here the situation
needs further exploration.
Operating at the D meson production threshold (corresponding to the
beam mo- mentum of 6.4 GeV/c) PANDA will measure the well-known D
and Ds mesons. A large quantity of D mesons can be produced with
favorable background conditions, as the phase space for additional
hadrons is small. Thanks to the narrow beam momen- tum spread it
will be possible to measure masses with accuracies of the order of
100 keV and widths to 10% or better by means of the fine
scan.
PANDA operating with the high luminosity, excellent beam momentum
resolution, detector with great spatial coverage and precise
magnetic field will be able to deeply
Chapter 2. PANDA experiment 7
FIGURE 2.2: The charmonium spectrum. Black lines denote charmo-
nium states, and red dots indicate charmonium-like states. Blue
lines in- dicate the thresholds at which states can decay into a
pair of D mesons.
Adapted from [5].
explore the charmonium mass region for the well established states
and the newly discovered XYZ states which are expected to be
classified concerning their quantum number configuration.
2.2.2 Gluonic excitation
Beside a relatively simple mesonics states (qq) the QCD allows also
existence of the objects with excited glue. Such states consist of
the quark, antiquark and addition- ally gluon resulting in qqg
resonance called hybrid. The excited gluonic component contributes
to the bound state quantum numbers (JPC) what increases the degree
of freedom in comparison to the qq states. Thanks to the exotic
quantum numbers of hybrids, which cannot be accessed by mesonic
states, the mixing of qqg and qq is not possible and therefore
hybrids are predicted to be rather narrow and easy to identify
experimentally.
Furthermore, the QCD foresees the existence of colorless gluonic
hadrons consist- ing only from gluons (ggg), which are called the
glueballs. The glueballs fall in two categories: the ones without
exotic quantum numbers and the ones with exotic quan- tum numbers
(oddballs). It is possible that due to different spin structure of
oddballs in comparison to glueballs, the difference of the
properties of the two will reveal insight into the unknown glueball
structure.
8 Chapter 2. PANDA experiment
The hybrids and gluons are interesting objects as they are
described by the low energy features of the QCD and their
investigation will put light to the studies of the QCD vacuum
structure.
Hyperon physics
The hypernuclei is created when the proton or neutron of the nuclei
is replaced by one or more hyperons. So far only single (with one Λ
hyperon) and double (with two Λ hyperons) hypernuclei were
discovered. The use of antiproton beam (as planned at PANDA) will
lead to efficient production of hypernuclei of higher order which
will enable the studies of nucleon-hyperon interactions as well as
extend the nuclear struc- ture spectroscopy studies. Not to mention
the importance of this research for other disciplines like
astrophysics, where hyperon-nucleon interaction is essential for
the un- derstanding of the neutron stars equation of state.
In the PANDA experiment a double target is foreseen for the
hypernuclear studies. The signature of the reaction is the
antiproton which interacts with the light target producing a pair
of e.g. Ξ−Ξ
+. The double strange Xi particle is rescattered and it is captured
in the secondary target where it forms Ξ−hypernuclei which
eventually is transformed into a double Λ-hypernuclei. Such a
nucleus is not stable and the Λ de-excitation in nuclear potential
can be registered via γ−rays production.
Also systematic spectroscopy of Hyperons containing two and three
strange quarks is foreseen. Presently only a few ground states of
such baryons are known while quarks models predict many excited
states which needs to be discovered.
2.3 Infrastructure
Operation of the PANDA experiment could not be possible without the
advanced accelerator facility capable of ions and antiprotons
production. This demand is met by the forehead mentioned FAIR
complex which will be built at the site of the GSI Helmholtzzentrum
für Schwerionenforschung in Darmstadt. There are four research
pillars at FAIR, these are:
• Physics with High Energy Antiprotons - represented by the
PANDA,
• Atomic, Plasma Physics and Applications (APPA) which focus on
investigations of properties of QED in a presence of strong
electric and magnetic fields and nuclear matter under extreme
conditions (i.e. high field, high pressure, high tem-
perature)
• Nuclear Structure, Astrophysics and Reactions (NUSTAR) which will
use the ra- dioactive beams for investigation of nuclear structure
and dynamics. NUSTAR includes also nuclear astrophysics
investigations.
• Nuclear Matter Physics - represented by Compressed Baryonic
Matter (CBM) ex- periments which goal is to explore the QCD phase
diagram in the region of high baryon densities.
The whole complex is shown in the Fig. 2.3. The red color marks the
parts of the facility which are still to be built. The beam
formation will start in the p-LINAC which will produce the protons
or ions which will be later accelerated by the SIS18 synchrotron to
the 4.7 GeV (protons) or 1 - 2 A GeV (ions). Then the beam will
enter
Chapter 2. PANDA experiment 9
FIGURE 2.3: Illustration of the FAIR complex. The red color marks
the part of the facility which is not yet constructed. Source:
[1].
SIS100 for further acceleration. In case of the protons the energy
obtained at this stage equals 29 GeV and the number of particles
per cycle reaches 2 x 1013 [6]. The next stage is the conversion of
the protons to antiprotons which will be done on the target shield
possibly made of light metal (i.e. copper, iridium) in order to
avoid shield melting upon high energy proton irradiation on the one
hand but heavy enough to produce sufficient amount of antiprotons
on the other hand. The production rate of 5 x 10−6 antiproton per
proton is foreseen. The extracted antiprotons are gathered by the
Collector Ring, which performs stochastical pre-cooling and later
are forwarded to the High Energy Storage Ring (HESR).
The total circumference of the HESR will be 574 m and it will have
two curved parts and two linear parts (132 m). One of them will be
occupied by the stochastic cooling system and electron cooling
system. The other linear part will be partly occupied by the
experimental hall dedicated for the PANDA. The hall will have 43m x
29m floor space and 14.5m height. The HESR will be able to
ac-/decelerated antiprotons to the desired momentum.
The HESR will be able to operate in two modes: low luminosity mode
with high momentum precision p/p = 4 × 10−5 (at luminosity 2 x
1031cm−2s−1) or high lumi- nosity (2 x 1032cm−2s−1) with comparably
higher momentum spread.
2.4 Spectrometer overview
The PANDA detector was designed to achieve 4π geometrical
acceptance, very good resolution for track reconstruction, high
efficiency of particle identification and being able to cope with
very high interaction rates (up to 2× 107/s). The design includes
also the versatile readout system with online event
selection.
There are several characteristics of the measured particles which
have to be estab- lished by the spectrometer. These are: event
topology, particle species and full dynam- ics. The complete
detector design is divided into many subdetectors responsible for
determination of one (or more) of the mentioned
characteristics.
10 Chapter 2. PANDA experiment
FIGURE 2.4: Schematic view of the HESR. The place of the beam in-
jection, experimental installations and devices for the beam
cooling are
marked. Source: [7].
Knowing the event topology means that the vertex (place of particle
origin/creation) is well defined. The event topology is known based
on the tracking procedure which allows to reconstruct whole flight
trajectory of the particle. Mainly the following sub- detectors are
used for the tracking (see figure 2.5): Micro Vertex Detector
(MVD), Straw Tube Tracker (STT) and Gas Electron Multiplier (GEM).
These three modules are called the Central Tracker covering central
region around the target. There is also tracking system localized
at the forward direction, Forward Tracker (FT), as a part of
Forward Spectrometer , described in details below.
The second characteristic is particle species, which can be
determined based on energy loss measurement (which depends on the
particle velocity) or by simultane- ous velocity and momentum
measurement. For the energy loss measurement mainly Muon system and
Electromagnetic Calorimeter (EMC) will be used but also STT may
bring important input here. For the velocity measurement the Ring
Imagine Cherenkov (RICH) and the Detector of Internally Reflected
Cherenkov (DIRC) is planned to be used in the range of high
momentum whereas in the low momentum region the Time of Flight
(TOF) system is proposed. The momentum will be extracted from the
investi- gation of the particles’ trajectories bending in the
magnetic field.
The last characteristic is the full dynamics which gives
information about the to- tal momentum and the energy of the
particle. Therefore the dynamics of an particle is described by the
four-momentum which is formed from the measured momentum (deduced
from the tracking) and the detectors used for energy deposit
measurement (EMC and Muon System).
In order to estimate the mentioned characteristics, the PANDA
detector will be composed of two parts: the Target Spectrometer
(TS) and the Forward Spectrometer (FS). The first one will cover
the space around the interaction point (IP), which is the place
where high energy beam meets the stationary target. The FS will
register the particle going in the forward direction in the angles
down to 5o in the vertical and 10o
in the horizontal plane.
Chapter 2. PANDA experiment 11
The TS will consist of many subdetectors arranged in multiple
layers surrounding the IP. The closets to the IP will be the MVD
which main responsibility will be recon- struction of the primary
vertex. After the MVD the STT module is planned to be placed. EMC
will be the last component in the closets vicinity of the IP and it
will be placed in the electromagnetic field created by the
superconductive magnet coils which will be placed just after it but
before Muon system. The TS is closed with the front end cap
including GEM, Disk DIRC and EMC detectors.
The FS will be situated behind the TS looking from the beam
direction. It will be responsible for registration of the particles
emitted at low angles (±5o in the vertical and 10o in the
horizontal plane). The FS will consist of RICH, EMC, Muon system,
TOF and FT. Important element will be the dipole magnet causing the
particles deflection needed for the momentum reconstruction.
The model of proposed spectrometer with all the subdetectors marked
is presented in the Fig. 2.5. More detailed description of the most
important subdetectors is placed later.
FIGURE 2.5: The PANDA spectrometer.
2.4.1 Targets
The particles registered by the spectrometer originating from
interactions between the beam and the target. There are two main
concerns regarding the target realization which are vital for
successful experiment: the target thickness and its position. The
position is crucial for the definition of the IP and the target
thickness has direct impact on the number of interactions. There
are two different types of the targets foreseen for the PANDA. The
first one is the hydrogen gaseous targets used in proton-antiproton
studies and the second one is the solid target in form of foil or
thin wire used for studies of antiproton nucleon reactions. For the
latter studies also gaseous target with heavier gases can be
used.
When it comes to the gaseous targets, there are two main methods of
their forma- tion that are currently under evaluation. The first
one is a cluster jet target which is realized by injecting the
pressurized hydrogen into the vacuum via a special nozzle. The
hydrogen forms a narrow jet clusters which are seen as high
homogeneous density
12 Chapter 2. PANDA experiment
targets. The drawback of cluster jets is high lateral spread
leading to uncertainty of the IP definition.
The second method produces the target in the form of frozen
hydrogen drops (pel- lets) falling freely and crossing the
antiproton beam. The pellet target assure high effec- tive
densities but have no uniform time distribution (discrete pellets
crossing the beam line). Therefore the beam must be extended to
guarantee crossing with many pellets which forces finding the
balance between average luminosity and the beam spread.
In both solutions it seems to be possible acquiring the target
density in the order of 4 × 1015 hydrogen atoms per square
centimeter which together with the number of antipronts (1011)
hitting the target every 2 µs, results in the desired beam
luminosity in the order of 2× 1032 cm−2s−1.
Due to the limitation on the material budged and space, the target
production sys- tem, including vacuum pumps, will be installed on
the top of the TS and the jets or pellets will be delivered to the
IP via 100 mm in diameter tube going through the sub- detectors of
the TS as well as the solenoid magnet.
2.4.2 Magnets
The magnets play an important role in the PANDA spectrometer as
they bend the charged particles trajectories which enables their
momentum calculation. There are two magnets foreseen: a solenoid
type and a dipole type. The solenoid magnet will have 2.8 m length
and inner diameter equal 90 cm and will be installed in the TS. The
field will amount to 2 T and its fluctuations will not exceed ±2%.
The second magnet is of the dipole type and will be installed in
the FS. It will occupy 1.6 m in the beam direction starting from
3.9 m downstream of the target. The magnet will cause the
deflection of the beam. To minimize this effect there will be the
possibility of dumping down the filed for the time of acceleration
of the antiprotons and also two correcting dipole magnets are
planned to be used around the spectrometer. More comprehensive
description of the magnets can be found in [8].
2.4.3 Subdetectors
In this section all the subdetectors comprising the PANDA
spectrometer are shortly described. The straw tube based modules
(STT and FT) are described in more details at the end of the
chapter. More complete description of a the subdetectors can be
found in [2].
Micro Vertex Detector
The Micro Vertex Detector is the pixel detector similar to the one
used in the ATLAS experiment. It bases on the silicon pixels in
which upon particle crossing the electrons and holes pairs are
created. As the pixel is polarized, the pairs are pulled in
opposite directions until they reach the pads which connect the
pixel with the readout electron- ics.
The detector is planned to be divided into 4 barrel layers
surrounding the IP and six discs placed in the forward direction in
respect to the beam. The first barrel will be placed 2.5 cm away
from the IP and the most outer one will be situated at the distance
of 12.5 cm. The last two barrels and the last two disc are made of
silicon strips and the remaining part of the MVD is equipped with
the pixels of 100 x 100 µm.
Chapter 2. PANDA experiment 13
The main goal of designing the MVD detector is the reconstruction
of the primary vertex and the secondary vertex with the resolution
better than 100 µm which is re- quired for identification of the
short lived particles. Additionally, the MVD can con- tribute to
the particle identification via some energy loss measurement and it
can bring improvement of the momentum resolution.
Electromagnetic calorimeter
The electromagnetic calorimeter is a detector dedicated to energy
measurement by the total particle absorption and additionally
designating the place of the absorption. The decay channels
containing the photons or leptons are of high importance therefore
two types of calorimeters have been foreseen in the final
spectrometer design.
The first one is located inside the magnetic field of solenoid what
limits the radial thickness of the calorimeter. It has a barrel
shape and two end-caps providing hermetic- ity. The total number of
15552 crystals are planned to be used, each 200 mm long. The
crystals have to be able to produce fast responses to incident
particles in high count- ing rate environment, deliver sufficient
energy resolution and detection efficiency of the photons and
leptons in a high range of energy. Also the radiation hardness has
been taken into consideration while searching for optimal crystals
material. The lead tungsten (PbWO4) crystals are chosen as a high
density inorganic scintillator [9].
The second EMC is of a shashlik type, which means that the
detection modules (plastic scintillators) are interleaved with the
lead plates. It is situated in the FS after the dipole magnet and
it consists of 1404 modules 55x55 mm2 cell size each.
Gas Electron Multiplier (GEM)
The main part of the GEM detector is a very thin polymer foil which
is metal-coated on both sides. There are many micro dimensional
holes in the foil (typically 50-100 mm2). The large difference of
electrical potential, applied to the both sides of the foil,
generates strong electric field inside the holes. Strong field
causes each electron inside the hole to create an electromagnetic
avalanche containing typically 100-1000 electrons. Multi- plied
electrons are then led to the readout plates from which the front
end electronics can capture the signal.
In the spectrometer, three planar plates of the GEM detectors are
planned to be installed. All of them situated downstream of the
target at 1.1 m, 1.4 m and 1.9 m position. The main purpose of GEM
installation is detection of the particles escap- ing the STT
detector in the forward direction (below 22o). The very high
counting rates (3× 104cm−2s−1 close to the beam pipe) are not
acceptable for the drift chambers, which would suffer from aging
effect due to too high occupancy, therefore use of the GEMs in this
region is an optimal choice because of their large rate
capabilities.
Detector of Internally Reflected Cherenkov light (DIRC)
The DIRC detector is planned to be used as particle identification
device. The parti- cle traversing medium, with refraction parameter
n, emits the photons in Θ angles in respect to movement direction
where Θ = arccos(1/nβ) and β is the speed of the par- ticle.
Therefore by measuring the Θ angle one can determine the speed (β)
of incident particle. Comparison of momentum (measured by other
detectors) and speed lead to designation of the particle mass and
its identification.
14 Chapter 2. PANDA experiment
An example of the material, which suits for the Cherenkov light
detector designed for the PANDA, is artificial quartz with the
refraction index n=1.47 which enables the pion-kaons separation in
momentum range 0.8-5 GeV/c.
Long quartz crystals of 1.7 cm thickness surround the STT detector.
The light will travel along the crystal and it will be distributed
over micro-channel plate photomulti- plier tubes via dedicated
lenses.
Exactly the same concept is foreseen for the forward DIRC disk
which will be 2 cm thick and it will have a radius of 110 cm. It
will be positioned directly upstream of the forward end-cap
calorimeter.
Ring Imagine Cherenkov light
In order to make possible π/K and K/p separation in a broad
momentum region (2-15 GeV/c) the Ring Imagine Cherenkov detector is
proposed to be placed in forward di- rection (below 220). It is
planned to use similar construction to the HERMES RICH [10] and
even to reuse parts of it. The detector will consist of two
symmetrical parts: lower and upper. The incident particles will go
through aluminum window, they will enter silica aerogel (with
n=1.0304) and later the C4F10 gas (n=1.00137). The photons emitted
in two different materials will be later reflected by a spherical
mirror array located at the end. The light cones will be focused
onto a focal surface located above and below the active volume. In
case of the HERMES RICH the focal surface is covered almost
completely (∼ 92% ) by the light collecting funnels ended with the
photomultipliers.
Muon detector
The Drell-Yan processes, D-meson decays and J/ψ decays can be
examined by the de- tection of the primary muons. The challenge for
the muon detection is very high back- ground coming from pions and
decay muons. By applying filters and by looking for the correlation
of the signals from all independent systems one should achieve
desir- able level of signal purity for muons. Mentioned
correlations become more important especially with the lower beam
energy.
The muon detection system will consist of four components: a Muon
Barrel which will surround the TS, a Muon End Cap which will close
the TS downstream of the target, a Muon Filter which will be
situated right after the end cap and a Muon Range System which will
be placed at the end of the FS.
The general detection concept of all PANDA muon systems is similar
for all its components and it will be based on rectangular aluminum
Mini Drift Tubes (MDT) interleaved with the iron absorber. In the
TS the total number of 2751 MDTs for Muon Barrel and End Cap and
424 MDTs for the Muon Filter is foreseen to be used. In the End Cap
more material is needed for absorbtion due to the higher momenta of
the incident particles.
The main task of the Muon Range System is the discrimination of
pions from muons and detection of pion decays. Additionally energy
determination of neutrons and anti- neutrons with moderate
resolution will be possible. The Muon Range System will have 576
MDTs and it will be placed 9 m downstream from the target.
Central Straw Tube Tracker (STT)
The STT will have a cylindrical shape and is planned to be placed
at -550 mm to 1100 mm in z-direction relative to the target. It
will have an inner radius of 150 mm and
Chapter 2. PANDA experiment 15
the outer one of 420 mm. It will consist of 4224 straw tubes of 10
mm diameter and 1500 mm length. More detailed description of the
straw tubes can be found in 2.4.4. The whole STT will be made of 6
modules (one module is presented in figure 2.6 (left)). The first 8
layers of each modules will be placed parallel to the beam z-axis.
Then there will be 4 double layers, the first and the third one
skewed by 2.9o the second and the fourth one skewed by −2.9o. The
module ends with 11 parallel straw tube layers with decreasing
number of straws (see figure 2.4.4 (right)).
The STT module will be placed in the solenoid magnetic field to
enable momentum determination of the particles crossing its
sensitive volume. The skewed straws are needed to enable three
dimensional trajectory reconstruction.
All the six modules of the STT will be fixed to an aluminum
supporting frame. The gas will be supplied to the straw tubes from
the one side going through one straw and returning through the
another straw. Also the front end electronics will be attached from
one side of the module.
FIGURE 2.6: Left: Photography of one STT module. Right: View of the
STT from the beam direction. The red color indicates straw tubes
skewed
by −2.9o whereas the blue by 2.9o.
Forward Tracker (FT)
The Forward Tracker is responsible for the reconstruction of the
trajectories of the par- ticles going in the forward direction at
small Θ angles. It will consists of 6 stations distributed before
the dipole magnet (2 pieces), inside (2 pieces) and after (2
pieces) the magnetic field (see table 2.1). Such a placement allows
precise parametrization of the particles tracks bending inside the
magnetic field.
Each station will be made of four double layers of the straw tubes.
The first and the last layer are planned to be placed vertically
whereas the two middle ones will be skewed by ±5o. The straws,
included in the station, will be grouped in modules. Each module
will have 32 straw tubes and it will be equipped with the
individual power supply, gas system, high voltage and front end
electronics. The individual modules allow easy replacement of
malfunctioning parts of the detector.
The size of the tracking station placed before the dipole magnet
will be 134 cm horizontally and 64 cm vertically. The further
stations will have larger dimensions as indicated in the table 2.1.
The total number of straws foreseen to be used in the
16 Chapter 2. PANDA experiment
final installation amounts to 12224. In figure 2.7 the first and
the second FT station is presented. The hole inside the station is
foreseen for the beam pipe.
FIGURE 2.7: The model of the FT1 and FT2 station. The hole inside
the module is planned for the beam pipe.
Tracking station z_min - z_max [mm] Active area
Number of straws width [mm] height [mm]
1 2954-3104 1338 640 4x288 = 1152 2 3274-3424 1338 640 4x288 = 1152
3 3945-4245 1782 690 4x384 = 1536 4 4385-4685 2105 767 4x448 = 1792
5 6075-6225 3923 1200 4x824 = 3296 6 6395-6545 3923 1200 4x824 =
3296
TABLE 2.1: Size, placement and number of straws in each FT
station.
2.4.4 Straw tube trackers
The readout system presented in this thesis is considered as a
common solution for the both tracking stations. Therefore, in order
to understand requirements defined on the system, underlying
principles of the straw tube detectors are introduced in next
section in more details.
Principles of operation and construction of PANDA straws
The basic building block of the straw tube trackers, used in the
PANDA, are the straw tubes which are cylindrical mini drift
chambers filled with the over pressured gas mix- ture. In the
center of a straw tube a 20 µm anode wire is stretched along the
cylinder axis. The wall of the straw tube is made of aluminized
Mylar foil of 27 µm thickness. The length of the tubes varies
between: 150 cm used in the STT and 68 cm used in the FT. A single
complete STT straw weighs 2.5 g. The figure 2.8 presents stages of
the STT tube assembly together with its components. Ready tubes are
glued together forming layers which are then used to form STT and
FT detectors. The layers of the straws are self-supporting due to
the filling by over pressurized gas. This is a very desirable fea-
ture as it limits the supporting frame needed for the detector and
therefore limits the radiation length.
The straw tubes are a good choice for the drift chambers
construction as they posses the following advantages:
Chapter 2. PANDA experiment 17
FIGURE 2.8: The straw tube construction on the example of STT
tracker.
• tight arrangement of the straws resulting in mechanically robust
system,
• very high spatial resolution of the reconstructed tracks (σ <
150µm),
• reliable electrostatic configuration with shielding walls of the
straw tubes which protects other straws in case one is
damaged,
• a high detection efficiency for a single particle hit (reaching
99% [11]),
• a capability of handling high hit rates (1-2 MHz per straw
tube),
• a small radiation length (X/X0 ∼ 0.05%) resulting in negligible
particle scatter- ing.
A dedicated gas mixture, Ar : CO2 (90:10), is used for the straw
tubes filling. The incident particle ionizes the gas molecules
producing electron-ion pairs. The high volt- age applied to the
anode wire cause the electrons to drift towards the anode and the
ions move toward the cathode wall. The electric signal which is
induced on the anode by the moving charge is then captured by the
front end electronics.
Due to the demand on the spatial resolution of the tracking
detectors the gas mix- ture, used in the straws, should enable a
high amplitude anode signal even for single electron cluster.
Unfortunately a high gas gain (closely connected with the high
volt- age) reduces the life time of the chambers and therefore a
balance between performance and durability has to be found.
Improper straws filling may also lead to electrons quenching or
their velocity saturation preventing them from creating electron
showers in the anode vicinity. Having in mind all the mentioned
considerations the simulation was performed and the results have
pointed to Ar : CO2 (90:10) gas mixture [11]. The properties of the
mixture components are give in the table 2.2.
The number of created electron-ion pairs, during the ionization,
depends on the energy loss of the incident particle. Using the
Bethe-Bloch formula it can be shown that the energy deposition of
the minimum ionizing particles (MIP) in the Ar : CO2
(90:10) equals 2.5 keV/cm at standard temperature and pressure. As
straw tubes op- erate at 2 bars therefore the energy loss of
incident particle equals 5 keV/cm. The interaction with the gas
atoms leading to electron-ion pairs creation is called primary
ionization. The primary ionization depends on incident particle
characteristics (en- ergy and charge) and gas properties (atomic
number, density, ionization potential of the gas). The ionization
caused by the primary electron-ion pairs is called secondary
18 Chapter 2. PANDA experiment
Gas Ex [eV] Ei [eV] Wi [eV] dE/dx [keV/cm] Np cm−1 Nt cm−1 X0 m Ar
11.6 15.7 26 2.44 23 94 110
CO2 5.2 13.7 33 3.01 35.5 91 183
TABLE 2.2: Properties of the argon and the carbon dioxide. Ex and
Ei
are the excitation and ionization energies. Wi is the minimal
energy nec- essary to produce one electron-ion pair in the gas.
dE/dx is the most probable energy loss of the minimum ionizing
particle in the gas. Np
and Nt are the number of primary and total electrons per cm,
respec- tively. X0 is the radiation length. Adapted from
[12].
ionization. The total number of the primary electron-ion pairs can
be calculated as nt = E/W where E is the energy deposition in the
gas volume and W is the average effective energy necessary to
produce one pair. Taking the values from the table 2.2 one can
estimate the number of total ionization pairs to ' 94 cm−1 [13] and
' 188 cm−1 for the 2 bar over-pressure present inside the PANDAs’
straw tubes and the MIPs.
The electrons created in the ionization process move randomly and
collide with the gas molecules with an average thermal energy
3
2kT ' 0.035eV . Once the electric field is applied the electrons
and ions gain additional velocity towards the anode and the cathode
respectively. The electrons move through the gas and lose part of
their energy in the collisions with the molecules. As the mass of
the electron is small in respect to the mass of the molecule the
energy loss in the impact is small in contrast to the ions
colliding with the molecules. Therefore the electrons velocity is
higher (104 times) than the one of ions whose relatively slow
motion contributes to the analog signal in form of a long tail (1
µm). The figure 2.9 shows results of the simulation of the drift
time in respect to the distance from the anode wire for the PANDA
straw. In case of magnetic field absence the maximal drift time,
corresponding to the ionization close to the cathode wall (5 cm),
equals 130 ns.
FIGURE 2.9: Simulated drift time in respect to the distance from
the anode wire without (left) and with (right) magnetic field
presence.
Adapted form [14] .
If the straw tube is placed in the magnetic field then the movement
of the electrons is affected by the Lorenz force causing bending of
the drift path and increase the maximal drift time. The behavior of
the electrons has been simulated [14] and their paths inside
Chapter 2. PANDA experiment 19
the tube, with and without magnetic field applied along the wire,
are presented in figure 2.10.
FIGURE 2.10: Simulated drift path of the electrons originating form
the ionization process. The case without (left) and with (right)
magnetic
field is presented. Adapted form [14].
The ∼200 (more precisely 188, what was shown before) electrons
coming from the ionization, caused by the minimum ionizing
particles, posses the charge of 3.2×10−17C which is far below the
detecting threshold of the electronics. For that reason a high
voltage is applied to the anode wire which induces the electric
field inside the straw tube. The intensity of the field increases
dramatically close to the anode wire what causes the electrons to
create a secondary electron showers multiplying the total charge.
The capability of the charge multiplication is called detector gain
and is described for the PANDA by the following formula:
G = e0.009U−5.3525
where U (in units V) denotes the high voltage applied to the anode
wire. This func- tion will be used later in the chapter 5 to
estimate the total charge deposited by the radioactive sources or
particles.
Purpose of the tracking system
As it was mentioned before, the tracking system will consist of
MVD, STT and GEM subdetector systems in the Target Spectrometer and
the FT in the Forward Spectrome- ter. There are three main goals of
the tracking system:
• Vertex reconstruction - the determination of the primary vertex
is crucial for iden- tification of the particles which disintegrate
into a couple of product particles.
• Momentum measurement - the precise particle trajectory
reconstruction is the key to determine the momentum of a charged
particle by measurement of its bending in the magnetic field.
• Energy loss measurement for the particle identification.
The particle identification based on the energy loss measurement
which will be done mainly in the STT detector by means of 27 straw
tube layers (energy measure- ments). For the PANDA the distinction
between π and e/K/p and between K and p
20 Chapter 2. PANDA experiment
should be possible in the momentum range below 800 MeV/c. In the
[11] the separa- tion power between two particles, assuming
Gaussian distributions for dE/dx distri- bution, has been defined
as:
S = |E1 − E2| σ1/2 + σ2/2
. (2.1)
The simulation of the STT has been performed to check what is the
expected separation power for the different particle couples in the
low energy regime. The results, which clearly demonstrates the
particle identification capabilities of the STT, are shown in
figure 2.11.
FIGURE 2.11: Separation power in the STT detector for the energy
bands built with particles all tracked with the same muon mass
hypothesis.
Source: [11].
Data rates in the straw tube trackers
The design of the straw tube trackers readout should be well suited
to the hit rates in the detector. The simulations show that in the
high luminosity mode the event rate will reach up to 2× 107 s−1.
From the point of view of the readout load the interesting
parameter is the number of straw tube responses per second, which
can be defined as intensity. The intensities for individual straws
will vary depending on the straw location. For the STT straws the
simulation of the average hit probability, per straw unit length
per event for the most inner layer, is presented in figure 2.12.
The simulation was performed assuming p− p collisions at 15 GeV/c.
In order to roughly estimate the intensities, which will occur on
individual straws, one can estimate the average hit rate per straw
as:
150[cm] ∗ 0.3 ∗ 10−3 ∗ 2 ∗ 107 = 0.9MHz
Chapter 2. PANDA experiment 21
where 0.3 ∗ 10−3 was taken from figure 2.12 as the average number
of hits per event and per cm.
-40 -20 0 20 40 60 80 100 0
0.1
0.2
0.3
0.4
0.5
0.6
1 layer, 2 atm
FIGURE 2.12: Simulation of pp reactions at 15GeV/c giving the
number of hits per event and per cm along the tubes in the inner
most layer of
the STT PANDA [11].
Also for the FT the simulation of intensity per straw was performed
assuming the high luminosity mode. The result for all six tracking
stations is shown in figure 2.13. Smaller counts number per second
close to the x=0 is caused by the beam hole presence. The average
intensity for the FT1-2, FT3-4 and FT5-6 equals 350, 310, 90
kHits/s/straw and was obtained by integration of the 2.13 for each
station and dividing the result by the number of straws in the
layer (for more details of straw number see table 2.1).
FIGURE 2.13: Number of counts per second expected in the individual
straws placed in the X location.
Chapter 3
Architecture of the Readout System for the straw tube
trackers
3.1 Data acquisition systems in nuclear and particle physics
The times when progress in physics could be achieved in small
laboratories with no use of complex electronic devices has ended
many years ago. Nowadays the mod- ern experiments in physics (not
only in nuclear and particle physics) are carried out with great
help of most up-to-date technologies, which deliver advanced
sensors, data transmission protocols and means for processing and
storage of large data volumes. Computers help scientists to analyze
the data and conclude about investigated physic cases.
The general organization of a system which consists of detector
sensors, signal pro- cessing chain (analogue and digital), data
storage and some logic units which decide to store or abort given
event is called Data AcQuisition - DAQ. The DAQ systems can be
understand as small, compact device, like for example personal
health monitoring or as a large scale system, like a power plant
control and monitoring. In case of nuclear and particle physics DAQ
systems are very complex. Its exact form depends on many aspects of
undertaken experiment (e.g. scale, physical phenomena to be
investigated) but it always consists of:
• detectors, which are sensors sensitive to particles (neutral or
charged),
• amplifiers and shapers, which are electronic circuits that
amplify and change the signal originating from the detector, to
convenient form for further processing,
• digitizers, which translate the analog signal to its digital
representation,
• data concentrators / data transmitters, which forward the data
from digitizers to a data storage,
• trigger logic units which decide if given event should be stored
or rejected,
• event building combined with a data storage.
Each part of the DAQ is described in more details in sections
below.
3.1.1 Detector
A sensor, sometimes called a transducer, converts interaction of a
particle with a detec- tor material into a measurable electrical
signal. The type of sensor defines its electrical output which can
be any electrical attribute that varies over time (e.g. voltage,
cur- rent, resistance). Most of the detectors require additional
components and circuitry to properly produce a signal (e.g.
application of high voltage).
23
24 Chapter 3. Architecture of the Readout System for the straw tube
trackers
In case of nuclear and particle physics we can follow [15] which
defines several considerations regarding choice of a detector for
defined purpose. These are:
• Sensitivity It defines a capability of a detector to produce
measurable signal for a given type of radiation and energy range.
Some detectors are prepared to register charged particles and they
cannot register particles with neutral charge (are not sensitive to
neutral particles). In order to register neutral particles (for
example photons or neutrons) one utilizes interactions which
results in production of charged parti- cles. If energy of
particles is beyond the range specific for sensitive volume of the
detector then the measured signal from the detector may be too
small for efficient detection.
• Type of detector response Beside measuring an event of the
irradiation by ionization of the sensitive vol- ume, most of
detectors are also able to give information about the amount of
ionization which is proportional to the impact energy of the
particle. In most of the cases, the signal which is obtained from
the detector is a current pulse. A correlation between the
amplitude of that pulse and the energy deposited by a particle
passing sensitive volume is called detector response. It is desired
that the response function is linear (or almost linear) in whole
operation range of the detector, which gives equal conditions of
measurement for different particles.
• Energy, time resolution The energy and the time resolution are
quantities which characterize ability of a detector to disti