ARTICLE IN PRESS

0168-9002/$ - se

doi:10.1016/j.ni

�Correspondi+86 10 6278113

E-mail addre

Nuclear Instruments and Methods in Physics Research A 538 (2005) 425–430

www.elsevier.com/locate/nima

Study on the performance of multi-gap resistive plate chambers

Wang Yi�, Li Yuanjing, Cheng Jianping, Li Jin, Yue Qian, Lai Yongfang,Tang Le, Fu Jinhua

Engineering Physics Department, Tsinghua University, Beijing 100084, China

Received 3 December 2003; received in revised form 24 August 2004; accepted 17 September 2004

Available online 27 October 2004

Abstract

A kind of multi-gap resistive plate chamber (MRPC) was developed for the STAR experiment at RHIC. The MRPC

has excellent time resolution with high efficiency and can be used as the time-of-flight detector. In this paper, we present

some performance of MRPC tested by cosmic ray, and performances of MRPC working in different gas mixture and in

different temperature are also illustrated.

r 2004 Elsevier B.V. All rights reserved.

Keywords: MRPC; Cosmic ray; TOF

1. Introduction

The multi-gap resistive plate chamber (MRPC)has good time resolution (less than 100 ps) andhigh detection efficiency (higher than 95%) and isa good candidate for the time-of-flight (TOF)detector for the STAR experiment at RHIC [1,2].It is low cost and can be segmented according torequirements [3,4]. The MRPC consists of a stackof glass plates, spaced one from the other withspacers of equal thickness creating a series of gasgaps. Electrodes are connected to the outersurfaces of the two outer glass plates. A strongelectric field is generated in each subgap by

e front matter r 2004 Elsevier B.V. All rights reserve

ma.2004.09.026

ng author. Tel.: +8610 62794480; fax:

3.

ss: yiwang@mail.tsinghua.edu.cn (W. Yi).

applying a high voltage on the external electrodes.All the internal glass plates are left electricallyfloating, they take the voltage as defined byelectrostatics. Typical resistivity for the glass platesis on the order of 1012–1013O cm. The electrodesare made of resistive graphite tape and aretransparent to charge. Copper pickup pads areused to read out the signals.

2. Structure of MRPC module

Fig. 1 shows the structure of the MRPC module.The module consists of six pads. The size of eachpad is 3.1� 6.0 cm2. There is a 3mm intervalbetween each pad. The total active area of onemodule is 18.6� 6.0 cm2. The glass plate is

d.

ARTICLE IN PRESS

Fig. 1. Structure of the MRPC module. (a) Long side view, (b) short side view.

W. Yi et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 425–430426

0.52mm thick and the resistivity is around8� 1012O cm. There are six gas gaps of about220 mm defined by nylon fishing-line of thisdiameter. The electrodes are made of a graphitetape with a surface resistivity of 7.6� 105O/squarewhich covers the entire active area. We use a non-flammable gas mixture which contains 94.7%tetra-fluoro-ethane and 5.3% iso-butane. When acharged particle goes through the chamber theavalanche generates in the gas gaps. The inducedsignal on the pads is the average of possibleavalanches from all gas gaps.

3. Cosmic ray test system

Cosmic ray test system is used to measure theperformance of MRPCs. Fig. 2 shows the diagramof the test system. The system consists of three

25 cm� 5 cm� 5 cm plastic scintillation counterswith photo-multiplier tubes (PMTs). N415 dis-criminator (products of CAEN) is used fordiscrimination of signals from five PMTs. Thecoincidence signal between PMT1 and PMT5provides the cosmic trigger for the ADC gateand the stop signal for the TDC converter. Thestop time is determined by the leading edge of thepulse from PMT1. PMT1 determines the rate ofaccidentals and is also used to correct theefficiency accordingly. All signals from PMT2 toPMT5 are fed into a VME TDC (C.A.E.N V775,35 ps bin width is chosen) as a start signal fordigitization. The time resolution for averagetiming of four PMTs, (t2+t3�t4�t5)/4, is about80 ps. The MRPC is mounted in a gas tight boxflushed with the working gas, and connected to afront end electronics (FEE) card identical to thosein use on the TOF (time-of-flight system based on

ARTICLE IN PRESS

Scintillator 1 PMT1

Scintillator 2 PMT2 PMT3

Scintillator 3 PMT5 PMT4

MRPC1

MRPC2

PMT1

PMT2

PMT3

PMT4

PMT5

Discrim 1

Discrim 2

Discrim 3

Discrim 4

Discrim 5

T

D

C

6-cell MRPC FEE

A D C

V

M

E

stop

gate

delay

delay

six channel TDC

six channel ADC

delay

Fig. 2. The block diagram of the cosmic ray test system.

W. Yi et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 425–430 427

MRPC technology) system. The analog and digitaloutputs from the FEE card are connected to theADC (C.A.E.N V265) and TDC modules. Anelectrothermal wire is put in the box. Thetemperature of the MRPC in the box can becontrolled by applying adequate voltage on thewire. The VME system connects the PC computervia an optical fiber. Labview6.0 program is used tocontrol the ADC and TDC, read data from theADC and TDC and display their correspondingspectrum in real time.

4. Results

Ten MRPCs have been constructed and are alltested in the same condition (using the same

device, gas mixture and high voltage). Figs. 3–6show the performance of a typical MRPC with avoltage 14 kV and with the working gas thatcontains 94.7% C2H2F4 and 5.3% iso-butane. Fig.3 shows the amplitude spectrum of the MRPC.The charge per channel of V265 ADC is 0.2 pC,and the pedestal is about 250 with a gate width of100 ns. The signals produced in the MRPC are firstamplified by MAXIM3760 amplifier in FEE, andlater fed into the ADC. The average chargeproduced in the MRPC by cosmic ray andrecorded by the ADC is around 40 pC. Fig. 4shows time–amplitude relation (T–A relation)without correction. Six-order of polynomial isused to do T–A correction. Fig. 5 shows the T–A

relation after correction. The time distributionafter T–A correction is shown in Fig. 6. The

ARTICLE IN PRESS

Fig. 3. Amplitude spectrum.

Fig. 4. The T–A relation.

Fig. 5. The T–A relation after correction.

Fig. 6. Time spectrum. Time resolution is about 90 ps.

W. Yi et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 425–430428

distributions are fitted with Gaussians, and weobtain a variance, s; of 120 ps. After subtractingthe mean time jitter introduced by two scintillationcounters (4PMTs), we obtain 90 ps time resolu-tion.Certainly, the performance of MRPC depends

on the gas mixture and different temperatures.Fig. 7 shows efficiency with different workinggases. Fig. 8 shows average charge induced in theMRPC with different working gases. With theincrease of high voltage, detection efficiencyincreases. It can be seen that the MRPC chamberreaches its efficiency plateau from a HV 14.5 kV.The maximum efficiency of cosmic ray test system(90%) is lower than the experiment in CERN PS-T10 [5], this phenomenon is mainly caused by animperfect geometrical arrangement of the triggersystem. Fig. 9 shows time resolution with differentworking gases. Based on the same reason, the timeresolution is worse than in Ref. [5]. Fig. 10 showsnoise rate with different working gases. Forconvenience, we define pure C2H2F4 as gas A,94.7% C2H2F4+5.3% iso-butane as gas B and90% C2H2F4+10% iso-butane as gas C. From thefour figures, we can obtain the following results.

�

The high-voltage plateau (time resolutiono100 ps) of the module in gas A is shorter thanin gases B and C. This is a fatal disadvantage ofusing the MRPC detector.

�

Efficiency in gas C is much lower than in gasesA and B. C2H2F4 is a kind of electronegative

ARTICLE IN PRESS

Fig. 7. Efficiency with different working gases.

Fig. 8. Average charge with different working gases.

Fig. 9. Time resolution with different working gases.

Fig. 10. Noise rate with different working gases.

W. Yi et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 425–430 429

gas, it can prevent the progression of avalanche.The MRPC works in avalanche mode, when theapplied voltage is very high (for example thehigh voltage excesses 14.5 kV), a lot of ava-lanches will be developed to streamers. So arelatively larger amplitude is obtained. Iso-butane can quench the streamer. So adequateratio of iso-butane mixed with C2H2F4 (gas B)will reduce the output amplitude. In gas C, theoutput amplitude increases, but the efficiencydecreases, this can be seen in Figs. 7 and 8. Thisphenomenon is very interesting and it is difficultto give a reasonable explanation. We will dofurther research on this phenomenon.

�

If there is no iso-butane (gas A) or too much iso-butane (gas C) in the working gas, the noise ofthe MRPC module increases greatly. So anadequate fraction of iso-butane can reducenoise.

�

The MRPC module working in gas B has goodtime resolution and high efficiency. This ex-plains that suitable ratio iso-butane mixed withC2H2F4 is favorable for the performance of theMRPC module.

Figs. 11 and 12 show that noise and darkcurrent change as a function of temperature. Theworking gas is gas B and the voltage is 14 kV. It

ARTICLE IN PRESS

Fig. 11. Noise in different temperatures.

Fig. 12. Dark current in different temperatures.

W. Yi et al. / Nuclear Instruments and Methods in Physics Research A 538 (2005) 425–430430

can be seen that with the increase in temperature,noise rate and dark current increase. As we know,when the temperature increases, both the pressureof the working gas and the density also increase, asa result, the noise and dark current of the MRPCincrease. When the temperature exceeds 301C,noise and dark current increase sharply, so the

performance will degrade. On the one hand, timeresolution degrades, on the other, the increase ofdark current will augment the load of the highvoltage source. The most important result is thatthe MRPC module has to work in temperaturelower than 301C.

5. Conclusions

According to the results from the cosmic raytest, the performance of the MRPC can beconcluded as the following:

�

The MRPC module can work very well with gasmixture which contains 94.7% C2H2F4+5.3%iso-butane. If there is no iso-butane or thefraction of iso-butane is too large, the perfor-mances of the MRPC module degrade.

�

The MRPC module has to work in temperaturelower than 30 1C.

�

The MRPC module has good time resolution(o100 ps) and high efficiency (495%). Itprovides performances comparable to the scin-tillator-based TOF technology but offering asignificantly lower price per channel and itscompact mechanics and magnetic compatibility.It can be used as TOF detector for the STARexperiment at RHIC or other experiments.

References

[1] B. Bonner, G. Eppley, J. Lamas-Valverde, et al., Nucl.

Instr. and Meth. A 478 (2002) 176.

[2] A. Akindinov, F. Anselmo, et al., Nucl. Instr. and Meth. A

456 (2000) 16.

[3] M. Spegel, Nucl. Instr. and Meth. A 453 (2000) 308.

[4] M.C.S. Williams, E. Cerron, et al., Nucl. Instr. and Meth. A

434 (1999) 362.

[5] M. Shao, L.J. Ruan, et al., Nucl. Instr. and Meth. A 492

(2002) 344.