RPC working gas (C2H2F4/i-C4H10/SF6): Simulation and measurement Jingbo Wang Department of Engineering Physics, Tsinghua University, Beijing, China 10th RD51 Collaboration Meeting, Oct 4th, 2012, Stony Brook

Outline Multi-gap Resistive Plate Chamber (MRPC) Motivation of the simulation Experimental measurements C2H2F4, J de Urquijo i-C4H10, I.B. Lima, P. Fonte SF6, L. G. Christophorou Mixtures, G. Chiodini, A. Colucci Simulations of the swarm parameters Garfield++ Magboltz 8.9.2 Summary

Multi-gap Resistive Plate Chamber (MRPC) differential pre-amplifier particle standard PCB with read-out strips on one side HV distribution by a medium resistivity layer (e.g. Graphite) transparent to the induced signals Rin HV insulator +HV gas gaps (~0.22 mm) -HV Resistive electrodes (glass. bakelite) HV coating with R~2 MΩ/□ Time resolution: 20 - 100 ps Efficiency: >90% * The multi-gap structure: E. Cerron Zeballos, et al., Nucl. Instr. and Meth. A 374 (1996) 132.

MRPCs @ Tsinghua Rate capability: >20 kHz/cm2 STAR-TOF STAR-MTD 4032 modules for STAR-TOF 120 modules for STAR-MTD CBM-TOF Y. Wang, J. Wang, et al., Nucl. Instr. and Meth A 613 (2010) 200–206 Y. Wang, et al., Nucl. Instr. and Meth A 640 (2011) 85–90 J. Wang, et al., Nucl. Instr. and Meth. A 621 (2010) 151. Rate capability: >20 kHz/cm2

MRPCs @ Tsinghua 50cm * 50cm ~1010 Ωcm Low-resistivity doped glass MRPC Workshop modules for STAR-TOF modules for STAR-MTD Modules for CBM-TOF: rate capability up to 70kHz/cm2

Motivation of the simulation x Ions Electrons n(t) increases exponentially α*Ve dominates the time resolution. Timing RPC is working in avalanche mode, under space charge regime RPC wroking gas: C2H2F4/i-C4H10/SF6 First step: the latest electron swarm parameters Region: A, B, C, D, E 1.5-D model (Lippmann): a factor of 2 discrepency in the charge spectrum intrinsic time resolution: sT ~ 50 ps rate capability: R ~ 0.5 – 25 kHz/cm2 W. Riegler, et al., Nucl. Instr. and Meth A 500 (2003) 144–162 C. Lippmann, et al., Nucl. Instr. and Meth. A 517 (2004) 54–76

Experimental measurement: C2H2F4 J. de Urquijo The initial electrons were released by a UV flash. Pulse Townsend technique! Current fit The displacement current was fitted by the expression: Te J. de Urquijo, et al., Eur. Phys. J. D 51, 241–246 (2009) J. de Urquijo, et al., 1999 J. Phys. D: Appl. Phys. 32 41 J.L. Hern´andez-´Avila, E. Basurto, J. de Urquijo, J. Phys. D 35, 2264 (2002), and references therein

Experimental measurement: i-C4H10 P. Fonte I.B. Lima Chamber Current fit P. Fonte, et al., Nucl. Instr. and Meth. A 613 (2010) 40–45 I.B. Lima, et al., Nucl. Instr. and Meth. A 670 (2012) 55–60

Experimental measurements: Mixtures A. Colucci, C2H2F4/i-C4H10 = 97/3, 90/10 G. Chiodini, C2H2F4/i-C4H10/SF6 = 94.7/5/0.3 The alpha in figure is performed with the empirical formula α*/P VS p/E A. Colucci, et al., Nucl. Instr. and Meth. A 425 (1999) 84-91 G. Chiodini, et al., Nucl. Instr. and Meth. A 602 (2009) 757-760

Simulations of the electron swarm parameters Garfield++ C2H2F4 Iso-butane SF6 Mixtures Magboltz 8.9.2 Different solutions in Magboltz Comparison between simulations and measurements Cross-sections [1] [2] [1] R. Veenhof, Garfield - simulation of gaseous detectors, http://garfieldpp.web.cern.ch/garfieldpp/ [2] S.F. Biagi, Nucl. Instr. and Meth. A 421 (1999) 234Ð240

Garfield++: C2H2F4, Ve Nice agreement! C2H2F4 / Ar mixture 100/0 50/50 20/80 10/90 Nice agreement! Data: J de Urquijo, et, al., Eur. Phys. J. D 51, 241–246 (2009)

Garfield++: C2H2F4, Alpha* C2H2F4 / Ar mixture 100/0 50/50 20/80 10/90 Nice agreement! Data: J de Urquijo, et, al., Eur. Phys. J. D 51, 241–246 (2009)

Garfield++: C2H2F4, Dl C2H2F4 / Ar mixture 100/0 50/50 10/90 20/80 Data: J de Urquijo, et, al., Eur. Phys. J. D 51, 241–246 (2009)

Garfield++: i-C4H10, Alpha and Ve Magboltz 2.8.6 (very old version) VS Garfield++ (latest version) [1] P. Fonte [2] I.B. Lima RPC working point: 400 Td [1] P. Fonte, et, al., Nucl. Instr. and Meth. A 613 (2010) 40–45 [2] I.B. Lima et, al., Nucl. Instr. and Meth. A670 (2012) 55–60 Nothing has changed with i-C4H10

Garfield++: SF6,Alpha*, Ve L. G. Christophorou, et, al., J. Phys. Chem. Ref. Data, Vol 29, No. 3, 2000

Garfield++: Mixtures, Alpha* and Ve Magboltz 7.0 Garfield++ / Magboltz 8.9.7 P. Fonte, not published Disagreement for RPC gas mixtures (C2H2F4, i-C4H10, SF6) D. Gonzalez-Diaz, et, al., Nucl. Instr. and Meth. A 661 (2012) S172–S176

Different solutions in Magboltz 8.9.2 SST: Steady-state Townsend Solution Drift velocity: Ve Transvers diffusion: Dt Longitudinal diffusion: Dl Townsend coefficient: Alpha Attachment coefficient: Att PT: Pulsed Townsend Solution Ionization rate: Ri Attachment rate: Ra TOF: Time-of-flight Solution Drift velocity: Wr, Ws Effective Townsend coefficient: Alpha-Att MC: Monte Carlo (theoretical) solution: Ve, Dt, Dl, Alpha, Att Experimental definitions A constant number of electrons is emitted at the cathode, which generates a steady stream of electrons in the uniform electric field between parallel plates. A group of electrons is released at the cathode and its growth observed by measuring the external current a function of time. The growth is observed as a function of both position and time. Garfield++ takes the SST solution for alpha (MC solution if no SST output), and the MC solution for the others. Y Sakai, et, al., J. Phys. D: Phy., Vol. 10, 1977

Magboltz 8.9.2: Alpha* MC solution SST solution C2H2F4 / Ar mixture MC solution SST solution Alpha*/N Space charge correction? Question: which solution is recommended? Data: J de Urquijo, et, al., Eur. Phys. J. D 51, 241–246 (2009)

Magboltz 8.9.2: Ve C2H2F4 / Ar mixture MC solution SST solution Ve

Magboltz 8.9.2: Dl C2H2F4 / Ar mixture MC solution SST solution NDL

Cross-sections C2H2F4 dissociation

Thanks for your attention Summary Garfield++ and Magboltz: C2H2F4: Fine i-C4H10: ? SF6: Tiny discrepancy Mixtures: ? Different solutions in Magboltz Measurements for RPC gas mixtures are needed. Thanks for your attention