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Proportional chambers with cathode readout
in high particle flux environment
Michał Dziewiecki
The GSIGSI – a heavy ion research facility, Darmstadt, Germany
FAIR – Facility for Antiproton and Ion Research – a future heavy ion accelerator centre
CBM –Compressed Baryonic Matter Experiment – Its main aim will be to obtain a quark-gluon plasma at very high nuclear matter density but moderate temperatures. It can be achieved by colliding heavy (Au) ions against an Au target.
Density ρ/ρ0
Tem
pera
ture
T (M
eV)
Detector setup of CBM
Heavy ion (Au) beam25 AGeV
Target (0.3mm Au)
Silicon tracker
RICH (Ring Imaging Cherenkov Detector)
TRT 1 (Transition Radiation Tracker)
TRT 2 TRT 3
Beam hole
TRTs:1st: 5.8 x 3.9 m2nd: 8.7 x 5.8 m3rd: 11.6 x 7.7 m
Each TRT module consists of 6 independent detectors. Total TRTs’ area: 500 m2
TRT at CBM Large Area Tracker (23-89 m2) Main goal particle selection
The aim is to extract high energy electrons from a huge amount of pions crossing the detector plane.Requested pion suppression factor - 300The detector can be also used for particle track reconstruction.
Total amount of 18 detectors3 modules, 3 double detectors per module, inclined at angle of 0, +10 and –10 degrees vs the vertical orientaton
Main problem: high particle fluxHigh collision multiplicity (300-400 particles per collision), up to millions of collisions per second
TRT operation principle
Radiator (a set of foils)
Straw chamber
High energy electron
Electron + photon beam
Each transition of a high energy particle between radiator mediums (foil and air) invokes X-ray emission
The X-ray photons (and the particle itself) are detected through straw detectors
Pions (at relatively low energies) do not generate X-rays – this feature lets us determine what kind of particle crossed the detector.CBM:
Radiator: 250-300 foilsStraws: 3-6mm diameter,
metalized Kapton
Proportional counter –single section of a straw chamber
1. Charged particle produces ionization clusters
2. A photon produces only one, but relatively large cluster
3. Electrons drift to the anode along the potential gradient
4. Near the anode the electric field is strong enough to cause a secondary ionization (electron avalanche).
5. Resultant ions drift to the cathode, causing measurable current flow through the counter.
Charged particleor photon
Gas
Avalanche
Ionization cluster
Cathode (GND)Anode (+HV)
Position estimation Anode readout
The time between particle appearance and signal registration is utilized.
The coordinate perpendicular to straw axis is measured
There is a left-right indetermination problem
A TAC (Time to Amplitude converter) or TDC (Time to Digital converter) circuitry is used
An additional fast detector (trigger) is necessary to generate TAC start signal
x
Δt = f(x)
time
Amplifiedsignal
Position estimationCathode readout
Cathode stripsStraws
We take advantage of signal differences between the cathode strips.
The coordinate parallel to straw axis is measured
If two particles cross the detector in the same (or near) time, the readings of the positions will disturb each other – it’s called frequency effects. It’s one of the most significant problems by cathode readout.
Pad readout an enhancement of cathode
readout Cathode strips are split to relatively short
pads
Errors induced by frequency effects are lowerThere occurs a partial separation along the axis perpendicular to straws.
The disadvantage of this solution is a very big count of analog channelsThe entire count of TRT pads in the CBM experiment can reach 1 million!
Pad readout and frequency effects
Each ionization causes a signal on every padLargest errors are caused by ‘near ionizations’ in the area of measured pads and in their neighbourhood.
Frequency effects manifest as random errors on calculated position
There are two ways of reducing these errors: Decreasing pad dimensionsMoreover, we must reduce the diamater of straws to decrease effective pad dimensions. This leads to reduction of detection efficiency. Speeding up the electronics, thus we can enhance the time resolutionThe ADCs at CBM will be working at 20MHz sampling rate, thus the pulse width can not be shorter than 200ns.
Computer simulation
All steps of the signal development were considered:
•cluster generation•electrons drift•gas amplification•signal forming at cathode•signal shaping at the amplifier
The simulator calculated the difference between computed and real position of particles
Results were presented in form of error distribution graphs
-0.15 -0.1 -0.05 0 0.05 0.102468
101214161820
Error[mm]
Sam
ples
cou
nt
Exemplary error distribution (central part)
A Monte-Carlo simulation was executed in order to analyze the problem
Simulation results and conclusions
5x50x2.5mm, 2100cz/mm^2*s
2 4 6 8 10 12 14 16 18 20 2210-2
10-1
100
101
erro
r [m
m]
Particle flux[part·mm-2·s-1] ·100
Guarateed accuracy for 80, 90 and 95% of particles
-0.2 -0.15 -0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0
10
20
30
40
50
60
70
error[mm]
coun
t
Exemplary error distribution
The problem long error distribution tails (see figure)Error values spread from 0 to few mm, dependig on pad width; the tails are effect of „near ionizations”, where the readings of two avalanches run into one another.
Large tails cause that relatively many readings are too inaccurate to be accepted.
Predetermined accuracy (200 μm) will be hard to achieve.
Experimental verification of obtained results
• A 40·40cm multiwire proportional chamber with pad readout
• Four-channel amplifier – shaper – base line restorer – ADC circuitry250 ns pulse shaping, 20 MHz max sampling rate
• A PC for data acquisition and processingA multi-threaded Visual C++ program for better HT-processors utilization.
BLR ADCBLR ADC
BLR ADCBLR ADC
HARDWARE
SOFTWARE
Software BLR
Pulse detection
Signal oversampling
Peak detection
Position calculationHDD
A small model of chamber is being built to verify the simulation results.
Thank youfor your attention