*Corresponding author. Tel.: #39-081-676128; fax: #39-081-676246.

E-mail address: iacovacci@na.infn.it (M. Iacovacci).

Nuclear Instruments and Methods in Physics Research A 456 (2000) 35}39

Study of RPC gas mixtures for the ARGO-YBJ experiment

B. Bartoli, R. Buonomo, E. Calloni, S. Catalanotti, B. D'Ettore Piazzoli,G. Di Sciascio, M. Iacovacci*

Dipartimento di Scienze Fisiche dell+UniversitaH di Napoli and INFN, Sezione di Napoli, Complesso Universitario di Monte S. Angelo,Edixcio G, Viale Cintia, Napoli, Italy

Abstract

The ARGO-YBJ experiment consists of a Resistive Plate Counter (RPC) carpet to be operated at the Yangbajinglaboratory (Tibet, China), 4300m a.s.l., and devoted to the detection of showers initiated by photon primaries in theenergy range 100GeV}20TeV. The measurement technique, namely the timing on the shower front with a few tens ofparticles, requires RPC operation with 1 ns time resolution, low strip multiplicity, high e$ciency and low single countingrate. We have tested RPCs with many gas mixtures, at sea level, in order to optimize these parameters. The results of thisstudy are reported here. ( 2000 Elsevier Science B.V. All rights reserved.

PACS: 95.85.PW; 96.40.Pq; 95.55.Ka; 95.55.Vj

Keywords: Gamma-ray astronomy; Extensive air shower; ARGO-YBJ; RPCs

1. Introduction

The ARGO-YBJ experiment [1] is under wayover the next few years at the Yangbajing HighAltitude Cosmic Ray Laboratory (4300ma.s.l.,606 g/cm2), 90 km North to Lhasa (Tibet, China).

The aim of the ARGO-YBJ experiment is thestudy of fundamental issues in cosmic ray and as-troparticle physics including c-ray astronomy,GRBs physics at 100GeV threshold energy and themeasurement of the antip/p at TeV energies.

The apparatus consists of a full coverage detectorof dimension 71]74m2 realized with a single layer

of Resistive Plate Counters (RPCs). A guard ringpartially (about 50%) instrumented with RPCs,surrounds the central detector, up to 100]100m2;it improves the apparatus performance by enlarg-ing the "ducial area for the detection of showerswith the core outside the full coverage carpet.

A lead converter 0.5 cm thick will cover uniform-ly the RPC plane in order to increase the number ofcharged particles by conversion of shower photonsand to reduce the time spread of the shower par-ticles.

The measurement technique, namely the timingon the shower front with a few tens of particles,requires RPC operation with 1 ns time resolu-tion, low strip multiplicity for good energy estima-tion at low energies, high e$ciency and lowsingle counting rate to trigger e$ciently at lowmultiplicity.

0168-9002/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 0 0 ) 0 0 9 5 8 - X R&D

Fig. 1. E$ciency, time resolution, single rate and mean strip multiplicity for gas mixtures with.(a) 60% of Ar; (b) 40% of Ar.

Keeping in mind all these needs and thelow operating pressure (about 600 mbar) atYangbajing we started to investigate di!erent

gas mixtures, at sea level, in order to optimizethe detector performance. In fact, previousstudies have shown that the performance of the

36 B. Bartoli et al. / Nuclear Instruments and Methods in Physics Research A 456 (2000) 35}39

Fig. 2. E$ciency, time resolution, single rate and mean strip multiplicity for gas mixtures with.(a) 10% of i-But; (b) 2% of i-But.

B. Bartoli et al. / Nuclear Instruments and Methods in Physics Research A 456 (2000) 35}39 37

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Fig. 3. E$ciency, time resolution, single rate and mean stripmultiplicity for the gas mixtures Ar/i-But/TFE"15/10/70 andAr/i-But/TFE"15/5/80.

detector may be heavily a!ected by the reducedpressure [2].

Three gas components were used: argon, iso-bu-tane C

4H

10and tetra#uoroethane C

2H

2F4

thatwill be indicated in the following as Ar, i-But andTFE, respectively.

The set-up used for this study consists of a smalltelescope of four RPCs 50]50m2 area with 16pick-up strips 3 cm wide connected to the front-endelectronics board. The front-end circuit contains 16discriminators, with about 70mV voltage thresh-old, and provides a FAST-OR signal with thesame input-to-output delay (10 ns) for all the chan-nels. The four RPCs were overlapped one over theother, three out of them were used to de"ne a cos-mic ray beam by means of a triple coincidence oftheir FAST-OR signals, the fourth one was used astest RPC. The three RPCs providing the triggerwere operated with a gas mixture of 60% Ar, 37%i-But and 3% TFE.

At any trigger occurrence the time provided bythe test RPC was read by means of a LECROYTDC of 0.25 ns time bin, operated in commonSTART mode; the number of "red strips was readby means of a CAEN module C187. The singlecounting rate was read by a CAEN scaler C243.

2. RPC performance

The RPCs were operated in streamer mode, asforeseen for the experiment, at the ARGO laborat-ory of the Physics Department of the Naples Uni-versity. Many gas mixtures have been tested,including mixtures with a high percentage of Ar(40}60%) which represent a reference point for theperformance of the detector. The results are shownin Fig. 1 where in the "rst column (a) are reportedthe three mixtures with Ar kept at 60%; the num-bers associated to each line like 60/30/10 refer,respectively, to Ar/i-But/TFE. It can be seen thatchanging the relative percentage of the two quench-ing components, i-But and TFE, does not result inany strong or evident di!erence of the detectorperformance. In fact, the operating voltage does notchange substantially, neither the e$ciency; the timeresolution stabilizes around 1 ns and the strip mul-tiplicity ranges from 1.1 to 1.3 depending on the

38 B. Bartoli et al. / Nuclear Instruments and Methods in Physics Research A 456 (2000) 35}39

applied voltage. This is con"rmed by the set ofmeasurements done with Ar kept at 40% (columnb). Only the mixture 40/50/10 shows some di!er-ence, but it is likely that there was some uncontrol-led change or problem during the data taking.

In order to check gas mixtures with i-But contentaround and below the #ammability threshold(10%) we have performed measurements with mix-tures where the i-But fraction was kept at 10% inone set and 2% in another set. The results arereported in Fig. 2(a) and (b). Both samples ofmeasurements show that the Ar percentage playsan important role in de"ning the operating voltage(about 800V above the knee-voltage) and in#uen-ces the strip multiplicity. The mean value of thestrip multiplicity remains anyway below 1.3; alsothe single rate changes according to the Ar percent-age and this can be understood as we work at "xeddiscrimination threshold. E$ciency and time res-olution are not a!ected. This means that i-But,traditionally used to `quencha the discharge byabsorption of ultraviolet photons, can be sub-stituted almost completely by TFE which is elec-tronegative and captures electrons of the plasma.

A solution to the problem of low pressure isincreasing the density of the gas mixture. An in-crease of TFE concentration at the expense of Arconcentration should therefore increase the pri-mary ionization, thus compensating for the 40%reduction caused by the lower gas target pressure(600mbar) and reduces the afterpulse probability.We have tested two mixtures with high content ofTFE, namely Ar/i-But/TFE : 15/10/75 and 15/5/80;the second one is a non #ammable mixture. Theexperimental results (Fig. 3) do not exhibit evidentdi!erences between them. The reduction of the ar-gon concentration in favour of the TFE results ina clear increase of the operating voltage as expectedfrom the large quenching action of TFE. The e$-ciency, 98%, is comparable to the values obtainedwith the mixtures having low percentage of TFE;might even be higher but this, given the experimental#uctuations, cannot be stated with certainty. More-over, the time resolution does not show anyworsening, the single rate keeps below 400Hz/m2

and also the strip multiplicity stays below 1.1. Themixture 15/10/75 has been already used successfullyat Yangbajing in the ARGO test [3,4].

3. Conclusions

We have studied the performance of RPCs withgas mixtures made of Ar, i-But and TFE. Wechecked mixtures with a high percentage of Ar,mixtures with low content of i-But, and "nallymixtures with low and high percentage of TFE. Forall of them, apart from those with a high percentageof TFE, we can summarize as follows: (1) the e$-ciency is 95}97%; (2) the voltage where the e$cien-cy plateau starts (V

,/%%) and the working voltage

(typically V,/%%

#800 V), depends strongly on theAr fraction, showing an increase of about 500V perevery 10% Ar reduction; (3) the single rate showsa plateau in the frequency range 400}600Hz/m2; (4)the time resolution at working voltage is typically1.0}1.3 ns; and (5) the mean value of the strip multi-plicity at the working voltage is 1.15}1.25 depend-ing on the Ar percentage. The mixtures with higherTFE content result in slightly better performance,giving 98% e$ciency, lower strip multiplicity andsingle rate. We could not "nd any parameter whichis able to discriminate the di!erent mixtures. Pre-liminary measurements show that in mixtures withhigher TFE fraction a smaller charge per track isdeveloped. Tests are going on to investigate theanalog read-out of the detector operated instreamer mode.

References

[1] M. Abbrescia et al., Astroparticle Physics with ARGO,Proposal (1996). Download this document at the URL:www1.na.infn.it/wsubnucl/cosm/argo/argo.html

[2] S. Natali et al., RPC performance at low pressure, in: S.P.Ratti, M. Merlo (Eds.), Proceedings of the InternationalWorkshop on Resistive Plate Chambers and Related De-tectors } RPC95, Pavia, 1995.

[3] C. Bacci et al., Nucl. Instr. and Meth. A 443 (2000) 342.[4] C. Bacci et al., Results from the ARGO-YBJ test experi-

ment, Astropart. Phys. (2000), submitted for publication.

B. Bartoli et al. / Nuclear Instruments and Methods in Physics Research A 456 (2000) 35}39 39

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