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Proportional chambers with cathode readout in high particle flux environment. Michał Dziewiecki. The GSI. Temperature T (MeV). Density ρ / ρ 0. GSI – a heavy ion research facility, Darmstadt, Germany FAIR – Facility for Antiproton and Ion Research –

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the gsi
The GSI

Temperature T (MeV)

Density ρ/ρ0

GSI – a heavy ion research facility, Darmstadt, Germany

FAIR – Facility for Antiproton and Ion Research –

a futureheavy ion accelerator centre

CBM –Compressed Baryonic Matter Experiment –

Its main aim will be to obtain a quark-gluon plasmaat very high nuclear matter density but moderate temperatures.

It can be achieved by colliding heavy (Au) ions against an Au target.

detector setup of cbm

Silicon tracker

TRT 1 (Transition Radiation Tracker)

TRT 2

TRT 3

Heavy ion (Au) beam

25 AGeV

Beam hole

Target (0.3mm Au)

RICH (Ring Imaging Cherenkov Detector)

Detector setup of CBM

TRTs:

1st: 5.8 x 3.9 m

2nd: 8.7 x 5.8 m

3rd: 11.6 x 7.7 m

Each TRT module consists of 6 independent detectors.

Total TRTs’ area: 500 m2

trt at cbm
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 - 300

The detector can be also used for particle track reconstruction.

  • Total amount of 18 detectors

3 modules, 3 double detectors per module, inclined at angle of 0, +10 and –10 degrees vs the vertical orientaton

  • Main problem: high particle flux

High collision multiplicity (300-400 particles per collision), up to millions of collisions per second

trt operation principle

High energy electron

Electron + photon beam

Radiator

(a set of foils)

Straw chamber

TRT operation principle
  • 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 foils

Straws: 3-6mm diameter,

metalized Kapton

proportional counter single section of a straw chamber

Anode (+HV)

Cathode (GND)

Gas

Charged particle

or photon

Avalanche

Ionization cluster

Proportional counter –single section of a straw chamber

Charged particle producesionization clusters

A photon produces only one, but relatively large cluster

Electrons drift to the anode along the potential gradient

Near the anode the electric field is strong enough to cause a secondary ionization (electron avalanche).

Resultant ions drift to the cathode, causing measurable current flow through the counter.

position estimation a node readout

x

Amplified

signal

time

Δt = f(x)

Position estimationAnode 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
position estimation c athode readout

Straws

Cathode strips

Position estimationCathode readout
  • 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 calledfrequency effects.

It’s one of the most significant problems by cathode readout.

pad readout an enhancement of cathode readout
Pad readoutan enhancement of cathode readout
  • Cathode strips are split to relatively short pads
  • Errors induced by frequency effects are lower
  • There occurs a partial separation along the axis perpendicular to straws.
  • The disadvantage of this solution is a very big count of analog channels
  • The entire count of TRT pads in the CBM experiment can reach 1 million!
pad readout and frequency effects
Pad readoutand frequency effects
  • Each ionization causes a signal on every pad
  • Largest errors are caused by ‘near ionizations’ in the area of measured pads and in theirneighbourhood.
  • Frequency effects manifest as random errors on calculated position
  • There are two ways of reducing these errors:
    • Decreasing pad dimensions
    • Moreover, 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 resolution

The ADCs at CBM will be working at 20MHz sampling rate, thus the pulse width can not be shorter than 200ns.

computer simulation

20

18

16

14

12

Samples count

10

8

6

4

2

0

-0.15

-0.1

-0.05

0

0.05

0.1

Error[mm]

Exemplary error distribution (central part)

Computer simulation

A Monte-Carlo simulation was executed in order to analyze the problem

  • 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
simulation results and conclusions

Exemplary error distribution

101

100

error [mm]

5x50x2.5mm, 2100cz/mm^2*s

10-1

10-2

2

4

6

8

10

12

14

16

18

20

22

Particle flux[part·mm-2·s-1] ·100

Guarateed accuracy for 80, 90 and 95% of particles

Simulation resultsand conclusions
  • 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

BLR

ADC

BLR

ADC

BLR

ADC

BLR

ADC

HARDWARE

SOFTWARE

Pulse detection

Software BLR

Signal oversampling

Peak detection

HDD

Position calculation

Experimental verification of obtained results

A small model of chamber is being built to verify the simulation results.

  • A 40·40cm multiwire proportional chamber with pad readout
  • Four-channel amplifier – shaper – base line restorer – ADC circuitry
  • 250 ns pulse shaping, 20 MHz max sampling rate
  • A PC for data acquisition and processing
  • A multi-threaded Visual C++ program for better HT-processors utilization.