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