elba, 27 may 2003werner riegler, cern 1 the physics of resistive plate chambers werner riegler,...

Elba, 27 May 2003 Werner Riegler, CERN 1

The Physics of Resistive The Physics of Resistive Plate ChambersPlate Chambers

Werner Riegler, Christian Lippmann

CERN


2mm gas gap

2mm Bakelite, 1010 cm

C2F4H2/Isobutane/SF6 96.7/3/0.3

HV: 10kV E: 50kV/cm

0.3mm gas gap

3mm glass, 2x1012 cm

2mm aluminum

C2F4H2/Isobutane/SF6 85/5/10

HV: 3/6 kV E: 100kV/cm

0.25mm gas gaps (5+5)

0.4mm glass, 1013 cm

PCB with cathodes, anodes

C2F4H2/Isobutane/SF6 90/5/5

HV: 12.5kV E: 100kV/cm

Trigger RPC

R. Santonico, R. Cardarelli

Multi Gap RPC

M.C.S. Williams et al.

Timing RPCP. Fonte, V. Peskov et al.


[1] Detector Physics and Simulation of Resistive Plate Chambers,

NIMA 500 (2003) 144-162, W. Riegler, C. Lippmann, R. Veenhof

[2] Space Charge Effects in Resistive Plate Chambers,

CERN-EP/2003-026, submitted to NIM, C. Lippmann, W. Riegler

[3] Induced Signals in Resistive Plate Chambers,

NIMA 491 (2002) 258-271, W. Riegler

[4] Signal Propagation, Termination and Crosstalk and Losses in Resistive Plate Chambers,

NIMA 481 (2002) 130-143, W. Riegler, D. Burgarth

[5] Detector Physics of Resistive Plate Chambers,

Proceedings of IEEE NSS/MIC (2002), C. Lippmann, W. Riegler

[6] Static Electric Fields in an Infinite Plane Condenser with One or Three Homogeneous Layers,

NIMA 489 (2002) 439-443, CERN-OPEN-2001-074, T. Heubrandtner, B. Schnizer, C. Lippmann, W. Riegler

[7] Detector Physics of RPCs,

Doctoral Thesis, C. Lippmann, May 2003 (CERN)

Over the last years we have published several articles on RPC detector physics:

Simulation studies by others:E. Cerron Zeballos et. al

NIMA 381 (1996) 569-572

M. Abbrescia et al.,

NIMA 398 (1997) 173-179, NIMA 409 (1998) 1-5, Nucl. Phys. B 78 (1999) 459-464, NIMA 471 (2001) 55-59

P. Fonte,

NIMA 456 (2000) 6-10, IEEE Trans. Nucl. Science Vol. 43 No. 3 (1996)

A. Mangiarotti, A. Gobbi,

NIMA 482 (2002) 192-215

G. Aielli

Advanced Studies on RPCs (Doctoral thesis Dec. 2000)


Motivation for the WorkMotivation for the Work

For 0.3mm gas gap RPCs using pure Isobutane or a C2F4H2 gas mixture one finds 75% efficiency which requires about 100 primary clusters/cm and a Townsend coefficient of 1000/cm.

A ‘popular’ value for Isobutane that is found in literature is 50 clusters/cm.

Even in case the above values were real, the expected average avalanche charge would be 107 pC, while one measures 5 pC.

Can a space charge effects provide such a large suppression factor ?

Eds along the gas gap is constant:

If there is a region in the avalanche where the electric field is low, there will also be a region where the field is high. Therefore one expects a ‘limited’ region for space charge suppression before the avalanche ‘explodes’.

In order to solve the problems, speculations about ‘strange new effects’ where started.


Simulation Input Simulation Input

RPC material: FLUKA

Primary ionization: HEED (Igor Smirnov)

Townsend, attachment coefficient: IMONTE (Steve Biagi)

Diffusion, drift velocity: MAGBOLTZ 2 (Steve Biagi)

Avalanche fluctuations: Werner Legler (1960)

Space charge field: Analytic Solutions [6]

Frontend electronics + noise: Analytic

[1]

Avalanche mode operation opens the possibility of a detailed simulation.

We assume that the gas is fully quenching.


Secondaries in RPCs: FLUKASecondaries in RPCs: FLUKA

Probability that the Pion is accompanied by at least one charged particle is 4.92%

(H. Vincke, CERN). This should have only a small effect on the efficiency.

hadron showers

electrons, photons

[1]


Primary Ionization: HEEDPrimary Ionization: HEED

C2F4H2 gas:

9. 5 clusters/mm for 7GeV Pion

105m between clusters

CERN-77-09

CERN-77-09

C2F4H2 gas:

2.7 electrons/cluster, long tail

Rieke et al., Phys. Rev. A 6 (1972) 1507

Rieke et al.

[1]


Gas Gain, Attachment: IMONTEGas Gain, Attachment: IMONTE

2mm Trigger RPCs, 50 kV/cm:

Effective Townsend Coefficient

10/mm

0.3mm Timing RPCs, 100 kV/cm:

Effective Townsend Coefficient

110/mm

[1]


Driftvelocity: MagboltzDriftvelocity: Magboltz

Isobutane

C2F4H2

E. Gorini, 4th workshop in RPCs (1997)

2mm Trigger RPCs, 50 kV/cm:

130 m/ns


210 m/ns

[1]


Avalanche FluctuationsAvalanche Fluctuations

Avalanches started by a single electron:

The very beginning of the avalanche decides

on the final charge.

W. Legler, 1960: Die Statistik der Elektronenlawinen in elektronegativen Gasen bei hohen Feldstärken und bei grosser Gasverstärkung

Assumption: ionization probability independent of the last collision

[1]


Approximate Time ResolutionApproximate Time Resolution

Time resolution is in the correct range

[1]

We expect:

• Time resolution depends only on effective

Townsend coefficient and drift-velocity.

• Dependence on threshold is weak.

Trigger RPC:

v 130 m/ns, - 10/mm,

t 1ns

Timing RPC:

v 210 m/ns, - 110/mm,

t 56ps


Approximate EfficiencyApproximate Efficiency


d=0.3mm, 0.105mm, 123/mm,

13/mm, Qt=20fC, Ew/Vw 1.48/mm

73%

Efficiency is in the correct range

[1]


Monte Carlo ResultsMonte Carlo Results

Monte Carlo

Measurement, P. Fonte, VIC 2001

Formula

Monte Carlo

Measurement,

P. Fonte et al., NIMA 449 (2000) 295

4x 0.3mm quad gap RPC0.3mm single gap RPC

Efficiency and time resolution are reproduced quite nicely

[1]


Expected Signal ChargesExpected Signal Charges

2mm Trigger RPC 10kV

Simulated Measured

Qtot 103 pC 40 pC

Qfast 102 pC 2 pC

0.3mmTiming RPC 3kV

Simulated Measured

Qtot 107 pC 5 pC

Qfast 105pC 0.5 pC

Discrepancy for timing RPCs is formidable

[1]


Space Charge EffectsSpace Charge Effects

Electric field of a point charge in an RPC

[6]


Space Charge EffectsSpace Charge Effects0.3mm timing RPC, 3kV

electrons, positive ions, negative ions, field

Avalanche is simulated by

dividing the development into time steps

and calculating the field at every point

within the avalanche at each step

Local field, Townsend coefficient,

attachment coefficient, driftvelocity

[2]



Simulation

P. Fonte et al., P. Fonte et al.,

Preprint LIP/00-04Preprint LIP/00-04

The detailed simulation indeed reproduces the small charges of a few pC

- compared to 107pC without space charge effect !

[2]

Measurement



Super thesis page 133

[2]

Electric field in a single electron avalanche, 0.3mm timing RPC, 2.8kV


Induced SignalsInduced Signals [3]

Theorems about signals induced on electrodes connected with arbitrary networks and embedded in a medium with position and frequency dependent permittivity and conductivity.

They allow analytic solutions of the influence

of the RPC material on the RPC signals:

E.g. Influence of carbon layer resistivity on the

RPC signal

T=electron drift time,

Rr0d

R ... Carbon Layer Resistivtiy (/)�

r … Material Permittivity

d… Gap Size


Crosstalk for Long StripsCrosstalk for Long Strips [4]

RPC with long readout strips is an inhomogeneous multi-conductor transmission line.

Signal on N-strips travels as an overlay of N different velocities (modal dispersion).

Crosstalk depends on the distance of the impact point from the amplifiers.

Signal termination is a complex issue N(N+1)/2 resistors.

All effects can be exactly calculated with elementary matrix transformations.


ConclusionsConclusions

Over the last three years we have systematically studied many aspects of RPC detector physics.

In our opinion, no strange effects have to be assumed in order to explain time resolution, efficiency and charge spectra.

Space charge effects are very prominent in this detector.

RPC signals and crosstalk can be studied with the help of very general theorems about signal induction and signal propagation.

In order to reproduce streamers, photon effects have to be included … there is more to do !

elba, 27 may 2003werner riegler, cern 1 the physics of resistive plate chambers werner riegler,...

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