1. 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

2. 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

3. 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: ps
Efficiency: >90%
* The multi-gap structure: E. Cerron Zeballos, et al., Nucl. Instr. and Meth. A 374 (1996) 132.

4. 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

5. 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

6. Motivation of the simulationx
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

7. 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
J.L. Hern´andez-´Avila, E. Basurto, J. de Urquijo, J. Phys. D 35, 2264 (2002), and references therein

8. 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

9. 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)

10. 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,
[2] S.F. Biagi, Nucl. Instr. and Meth. A 421 (1999) 234Ð240

11. 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)

12. 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)

13. 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)

14. Garfield++: i-C4H10, Alpha and Ve
Magboltz (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

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

16. 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

17. 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

18. 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)

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

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

21. Cross-sections
C2H2F4
dissociation

22. 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