Avalanche fluctuations within the multigap resistive plate chamber

Nuclear

Instruments

and Methods

Research

NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH

A 381 (1996) 569-572

Letter to the Editor

ELSEVIER

Avalanche

in Physics

fluctuations

SectIon A

within the multigap resistive plate chamber

E. Cerron Zeballos”‘b, I. Crotty”, D. Hatzifotiadouazb, J. Lamas Valverdea,b’c, R.J. Veenhofa2d, M.C.S. Williams”‘f’*, A. Zichichi”‘“.f “PPE division, CERN, Geneva, Switzerland ‘World Laboratory, ‘Universiw

Lnusanne, Switzerland

Louis Pasteur, Strasbourg,

“CEA, DAPNIAISPP,

France

CE Saclay, 91191 Gifsur-Yvette,

‘Universily

ofBologna,

‘INFN,

Received

Bologna,

Frame

Italy

Bologna, Italy

23 June 1996

Abstract The multigap resistive plate chamber (MRPC) was originally designed to have improved time resolution (compared to the wide gap RPC), but also to keep the good high rate behaviour and ease of construction associated with the wide gap RPC. However in addition we observed a very long efficiency plateau, even at high rates. Here we consider fluctuations in of these fluctuations can account for the enhanced performance avalanche growth, and show that the inherent “averaging” of the muftigap RPC.

1. Introduction Recently we developed a new type of RPC: the multigap resistive plate chamber (MRPC) [l]. The reason for this design was to improve the time resolution, without losing the other advantages of the wide gap RPC. However we also found that this type of RPC had a better than expected rate capability and longer efficiency plateau. In this paper we consider avalanche fluctuations and show that this would predict an enhanced performance for the multigap RPC.

2. The principle

of the multi-gap

CONVENTIONAL WIDE GAP RPC

RPC MULTI-GAP RPC

The cross section of a conventional mono-gap and a 3 gap multigap RPC is shown in Fig. 1. The internal plates used to create the 3 sub-gaps are of melamine-phenolic laminate, the usual material we use for resistive plates. No electrical connections are made to these plates; they are left to assume a voltage due to electrostatics. We have observed very stable performance even in a pulsed beam environment. This will be discussed in detail in a future

author. Tel. +41 22 767 75 84, fax +41 *Corresponding 785 02 07, e-mail [email protected]. 0168.9002/96/$15,00 PII

Copyright

SO168-9002(96)00766-S

01996

Elsevier

22

2

C;tbod; (-15 kV

-,

(

Primary ionisation produced in the 0.5 mm closest to the cathode ( generates detectable avalanches

\ Pick-up strips (0 V /

Fi$;. 1. Schematic diagram and principle of operation of multi-gap RF% compared to a conventional 9 mm single gap RPC.

Science B.V. All rights reserved

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E. Certm

Zebdlos

et al. I Nucl. Instr. nrd Meth. in Phys. Res. A 381 (1996) 569472

paper. Signals generated by gaseous avalanches are induced on external electrodes. In both types of RPC we use a gas which produces -4 clusters of primary ionisation/ mm for a through-going 8 GeV/c pion or muon. For the mono-gap RPC, the gas gain is set high enough such that detectable avalanches can be produced by a single electron, if the original position of this electron is within I mm of the cathode. Thus, on average, 4 detectable avalanches are produced from 4 clusters of primary ionisation; however. the limiting case is the avalanche signal from a single electron produced somewhere within the nearest mm to the cathode. The time jitter is caused by the drift speed of electrons in this first mm. For the 3 layer multigap, the gas gain is set such that an electron within the first 113 mm of a (sub)-cathode produces a detectable avalanche signal. Thus, on average, detectable avalanches are produced in all 3 sub-gaps for a through-going ionising particle; however the limiting case is a detectable avalanche in only one sub-gap. In this limiting case, the cluster of primary ionisation is somewhere in the closest 0.33 mm to a (sub-)cathode. In this way the time jitter has been reduced by a factor 3. We have built and tested such a multigap device and indeed the time resolution was much improved over a mono-gap device [1,2]. Somewhat unexpected was the longer efficiency plateau, the low dark current and good high rate behaviour. We have investigated possible causes for this good behaviour. In our opinion the most dominant effect is due to fluctuations in the growth of the avalanche; we will discuss this below.

3. Avalanche

fluctuations

We have simulated the expected charge spectrum generated by an RPC. We generated clusters of primary electrons with the distance between each cluster obeying Poisson statistics. For this study we assume an average of 4 clusters of primary ionisation per mm. The number of electrons in each cluster follows the distributions found in Ref. [3]; the effect of the various components of the gas mixture was weighted using the formulae given in Ref. (41. These distributions were extended up to clusters containing 2000 electrons by assuming a 1IN* fall-off for clusters containing more than 19 electrons. Each cluster of electrons was multiplied by ea.‘, where cr is the first Townsend coefficient and x the distance to the anode. The fast component of the induced charge is 1lcwD (-7%) of the total electron charge, where D is the total gap. We sum the avalanche signal generated by all clusters of primary ionisation. A comparison between data for an 8 mm gas mono-gap RPC, previously published [2] and this simulation is shown in Fig. 2. The gas gain was set in the simulation to produce a spectrum with a mean of 1.8 pC to match the measured charge spectrum. There is an obvious

2x0

0 100

0

I OPC

ml I

300

ADC Chan”eIS

I IOPC

Fig. 2. Comparison of measured charge spectrum [2] with a simulation that does not include avalanche fluctuations.

disagreement, the measured data is peaked towards low values of induced charge and has a much longer tail. It is well known that there are fluctuations in avalanche growth. The distribution of the size of single electron avalanches in weak uniform electric fields is of exponential form and the relative variance of the avalanche size is equal to unity; in strong fields there is reduced dispersion [5]. In Fig. 3 we show the simulated pulse height spectra for 3 cases. These cases are: (a) the simulation as shown in Fig. 2 with no avalanche fluctuation; (b) is obtained by applying an exponential fluctuation to each avalanche; (c) is obtained by applying this fluctuation to a single (merged) avalanche. In Fig. 4 we show a comparison between the distribution in Fig. 3c and the data for the 8 mm gap. There is good agreement; this implies that the avalanche grown in a parallel plate chamber is not a sum of many smaller avalanches, but one merged avalanche. The gas gaps in a multigap are physically separate. Therefore, since (on average) each gap produces a detectable avalanche, there will be an averaging of the avalanche fluctuations. In Fig. 5 the charge spectra are shown for the 3 X3 mm multigap, with the three cases of avalanche fluctuations as shown for the 8 mm mono-gap chamber shown in Fig. 3; however each subgap is treated independently. In Fig. 6 we show a comparison between the charge spectrum of a 3 X 3 mm multigap (published in Ref. [1]) and our simulation. Again the agreement is good. There is a significant change in shape in comparison to the mono-gap spectrum shown in Fig. 4, in that the most probable value has shifted away from zero. In Fig. 7 we show the results of the simulating the efficiency as a

571

E. Cerron Zeballos et al. I Nucl. lnstr. nnd Meth. in Phys. Res. A 381 (1996) 569-572

SIMULATION

8 mm monogap

c: yI

3 x 3 mm multigap

SIMULATION

9

CC)

r No Avalanche

fichlations

No AvaLmche fluctuafions @)

. . . . . . . . . .

Avalanche fluctuatkms q&xi to each avalanche

. . .

Avalanche fhxruations applied to each avalanche

0

2

4

6

8

Avalanche fluctuations applied fo Single merged avakmche (each subgab is separate)

t

Avalanche fluctWions app%d to single merged avabnche

10

Magnirude of ‘fast’ induced signal [PC]

0 ii

i

‘i

i

i

Magnitude of ‘fart‘ induced signal [PC]

Fig. 3. Simulated charge spectra for 3 cases for an 8 mm gas gap chamber; (a) is with no avalanche fluctuations; (b) is with avalanche fluctuations applied to each avalanche - with each cluster of primary ionisation producing an independent avalanche; (c) is with a single avalanche fluctuation applied to a single (merged) avalanche.

Fig. 5. Simulated charge spectra for a 3 gap multigap chamber; (a) is with no avalanche fluctuations; (b) is with avalanche fluctuations applied to each avalanche - with each cluster of primary ionisation producing an independent avalanche; (c) is with a single avalanche fluctuation applied to a single (merged) avalanche but

function of threshold for a 9 mm mono-gap RPC and a 3 X 3 mm multigap - in each case the mean of signal was set to be 2 PC. We see that at 1.5 fC threshold the multigap efficiency is close to 100% and drops to 95% at 15.5 fC. For the mono-gap, the 15 fC only gives an efficiency of

98%, dropping to 95% at 50 fC threshold. Thus the mono-gap is at least 3 times more sensitive to changes in gain than the multigap. Therefore, even though we have doubled the number of resistive plates (4 vs 2) in the

each sub-gap is treated independently.

2ooo

1500 %, N ? Y :

B

1000

B 2 g

500

t&

0 ADCbins

5

Fig. 4. Comparison of measured simulation shown in Fig. 3c.

charge

spectrum

[2] with the

Fig. 6. Comparison of measured simulation shown in Fig. 5c.

I

I

3 layer multigap

10 pc

[l] with the

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et al. I Nucl. Instr. and Meth. in Phys. Res. A 381 (1996) 569-572

SIMULATION 3 x 3 mm

0

50

150

100 Threshold

muhi@lp

200

[fC]

Fig. 7. Simulation of efficiency versus threshold for a 9 mm mono-gap and a 3X3 mm multigap chamber, with both devices producing an average signal of 2 PC.

of pulse magnitude so that constant fraction discriminators are essential if one wants to extract good timing information. For a 3 layer multigap, one has 3 physically separate avalanches so that there is an “averaging” of the avalanche fluctuations. This averaging has a dramatic effect on the shape of the charge spectrum - enabling one to work at higher thresholds with higher efficiencies. The drop in efficiency with increasing threshold is -3 times smaller for a 3 layer multigap than the corresponding mono-gap, thus the X2 increase in resistance (due to the extra resistive plates) is more than compensated. It should be noted that the fact that the individual avalanches merge together is a critical factor that controls the charge spectrum of the mono gap; if this merging did not happen there would be very little difference between the charge spectra of the multigap and monogap RPC. There are other additional effects - such as space charge - that enhance the performance of the multigap. This will be discussed in a future paper.

Acknowledgments

this is more than compensated by the improved shape of the charge spectrum. It should be noted that the pick-up strips used to read out resistive plate chambers usually have high capacitance; thus 15 fC may be close to the lowest practical threshold.

The initial drive to study avalanche fluctuations came from probing questions asked by Chris Fabjan and Silvia Schuh. Their interest has enabled us to further our understanding of resistive plate chambers.

4. Conclusions

References

multigap,

We have previously noted the good behaviour of a multigap RPC. We have now shown by examination of the charge spectrum that the avalanche process in a RPC acts as a single avalanche (rather than individual avalanches from each cluster of primary ionisation). The inherent fluctuations of avalanche growth is a major factor controlling the shape of the charge spectrum. This shape forces one to work with low thresholds -1% of the average charge if one wants to have high efficiency; i.e., a threshold of -15 fC. There is also a large dynamic range

111 E. Cerron Zeballos,

I. Crotty, D. Hatzifotiadou, J. Lamas Valverde, S. Neupane, M.C.S. Williams and A. Zichichi, Nucl. Instr. and Meth A 374 (1996) 132. 121 E. Cerron Zeballos, I. Crotty, D. Hatzifotiadou, J. Lamas Valverde, S. Neupane, V Peskov, S. Singh, M.C.S. Williams and A. Zichichi, Nucl. Instr. and Meth. A 373 (1996) 35. r31 H. Fischle, J. Heintze and B. Schmidt, Nucl. Instr. and Meth. A 301 (1991) 202. f41 F.F. Rieke and W. Prepejchal, Phys. Rev. A 6 (1972) 1507. 151 G.D. Alkhazov, Nucl. Instr. and Meth. 89 (1970) 155.