Elba, 27 May 2003Werner Riegler, CERN 1 The Physics of Resistive Plate Chambers Werner Riegler, Christian Lippmann CERN

Elba, 27 May 2003Werner Riegler, CERN 2 2mm gas gap 2mm Bakelite,    cm C 2 F 4 H 2 /Isobutane/SF /3/0.3 HV: 10kV  E:  50kV/cm 0.3mm gas gap 3mm glass,   2x10 12  cm 2mm aluminum C 2 F 4 H 2 /Isobutane/SF 6 85/5/10 HV: 3/6 kV  E:  100kV/cm 0.25mm gas gaps (5+5) 0.4mm glass,    cm PCB with cathodes, anodes C 2 F 4 H 2 /Isobutane/SF 6 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 RPC P. Fonte, V. Peskov et al.

Elba, 27 May 2003Werner Riegler, CERN 3 [1] Detector Physics and Simulation of Resistive Plate Chambers, NIMA 500 (2003) , W. Riegler, C. Lippmann, R. Veenhof [2] Space Charge Effects in Resistive Plate Chambers, CERN-EP/ , submitted to NIM, C. Lippmann, W. Riegler [3] Induced Signals in Resistive Plate Chambers, NIMA 491 (2002) , W. Riegler [4] Signal Propagation, Termination and Crosstalk and Losses in Resistive Plate Chambers, NIMA 481 (2002) , 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) , CERN-OPEN , 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) M. Abbrescia et al., NIMA 398 (1997) , NIMA 409 (1998) 1-5, Nucl. Phys. B 78 (1999) , NIMA 471 (2001) P. Fonte, NIMA 456 (2000) 6-10, IEEE Trans. Nucl. Science Vol. 43 No. 3 (1996) A. Mangiarotti, A. Gobbi, NIMA 482 (2002) G. Aielli Advanced Studies on RPCs (Doctoral thesis Dec. 2000)

Elba, 27 May 2003Werner Riegler, CERN 4 Motivation for the Work For 0.3mm gas gap RPCs using pure Isobutane or a C 2 F 4 H 2 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 10 7 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.

Elba, 27 May 2003Werner Riegler, CERN 5 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.

Elba, 27 May 2003Werner Riegler, CERN 6 Secondaries 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]

Elba, 27 May 2003Werner Riegler, CERN 7 Primary Ionization: HEED C 2 F 4 H 2 gas:  9. 5 clusters/mm for 7GeV Pion  105  m between clusters CERN C 2 F 4 H 2 gas:  2.7 electrons/cluster, long tail Rieke et al., Phys. Rev. A 6 (1972) 1507 Rieke et al. [1]

Elba, 27 May 2003Werner Riegler, CERN 8 Gas 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]

Elba, 27 May 2003Werner Riegler, CERN 9 Driftvelocity: Magboltz Isobutane C2F4H2C2F4H2 E. Gorini, 4 th workshop in RPCs (1997) 2mm Trigger RPCs, 50 kV/cm:  130  m/ns 0.3mm Timing RPCs, 100 kV/cm:  210  m/ns [1]

Elba, 27 May 2003Werner Riegler, CERN 10 Avalanche 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]

Elba, 27 May 2003Werner Riegler, CERN 11 Approximate 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

Elba, 27 May 2003Werner Riegler, CERN 12 Approximate Efficiency 0.3mm Timing RPCs, 100 kV/cm: d=0.3mm,  mm,   123/mm,   13/mm, Q t =20fC, E w /V w  1.48/mm    73% Efficiency is in the correct range [1]

Elba, 27 May 2003Werner Riegler, CERN 13 Monte 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]

Elba, 27 May 2003Werner Riegler, CERN 14 Expected Signal Charges 2mm Trigger RPC 10kV Simulated Measured Q tot  10 3 pC  40 pC Q fast  10 2 pC  2 pC 0.3mmTiming RPC 3kV Simulated Measured Q tot  10 7 pC  5 pC Q fast  10 5 pC  0.5 pC Discrepancy for timing RPCs is formidable [1]

Elba, 27 May 2003Werner Riegler, CERN 15 Space Charge Effects Electric field of a point charge in an RPC [6]

Elba, 27 May 2003Werner Riegler, CERN 16 Space Charge Effects 0.3mm 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]

Elba, 27 May 2003Werner Riegler, CERN 17 Space Charge Effects Simulation P. Fonte et al., Preprint LIP/00-04 The detailed simulation indeed reproduces the small charges of a few pC - compared to 10 7 pC without space charge effect ! [2] Measurement

Elba, 27 May 2003Werner Riegler, CERN 18 Space Charge Effects Super thesis page 133 [2] Electric field in a single electron avalanche, 0.3mm timing RPC, 2.8kV

Elba, 27 May 2003Werner Riegler, CERN 19 Induced 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,   R  r  0 d R... Carbon Layer Resistivtiy (  /)  r … Material Permittivity d… Gap Size

Elba, 27 May 2003Werner Riegler, CERN 20 Crosstalk 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.

Elba, 27 May 2003Werner Riegler, CERN 21 Conclusions   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 !