Study of resistive plate chambers for the alice dimuon spectrometer

Abstract The trigger system of the Alice dimuon spectrometer is based on RPC detectors. We present experimental results of a beam test about rate capability, time resolution and cluster size of single gap RPC detectors operating both in ‘streamer mode’ and in ‘avalanche mode’. We have compared the performances of small chambers (50×50 cm 2 ) built with bakelite of different resistivity, from ∼ 3 · 10 9 Ωcm to ∼ 3 · 10 11 Ωcm. For the low resistivity RPC we obtained a rate capability of several hundreds of Hz/cm 2 when it is operating in ‘streamer mode’, and of several thousands of Hz/cm 2 when it is operating in ‘avalanche mode’.


INTRODUCTION
The ALICE experiment will study nucleusnucleus interactions at the LHC in order to investigate nuclear matter at extreme energy density where the formation of the Quark-Gluon-Plasma QGP is expected. The ALICE dimuon spectrometer is dedicated to the study of the production of J and resonances: the suppression of these particles is a signature of the transition of nuclear matter to the QGP 1 . The spectrometer measures the complete spectrum of heavy quark mesons, i.e. J , ', , ' and ", via their muonic decay in pp and heavy ion collisions 2 . The trigger detectors of the ALICE muon spectromer will be Resistive Plate Chambers RPC 3,4 which are simple devices well suited to trigger purposes: fast response and good time resolution; futhermore they are industrially produced and suitable to cover large areas. For these reasons the RPC are in use in various experiments and they will be largely used at the LHC experiments for the muon trigger.
RPC tests on beam have been performed in order to optimize the detector's performances for the ALICE experimental conditions.

ALICE REQUIREMENTS FOR THE RPCs
The experimental layout of the ALICE dimuon spectrometer is shown in g.1. The angular acceptance goes from 2 o to 9 o 2.5 4.0. The main elements of the spectrometer are: three absorbers front absorber, beam shield and muon lter which reduce the partile ux into the muon spectrometer, the dipole magnet with a eld integral of 3 Tm, 10 planes of tracking chambers and 2 trigger stations, MC1 and MC2. The trigger stations are placed at the end of the spectrometer, respectively at 16 m and 17 m from the interaction point and cover an area of about 6x6 m 2 with a 0.6x0.6 m 2 opening in the centre to accomodate beam pipe and shielding. Each trigger station is made of 2 planes of 18 single gap RPCs read-out on both sides by means of strips in the two orthogonal directions. In the bending plane the strip widths are 1-2-4 cm, going from the inner to the outer part of the trigger stations, instead in the non-bending plane, only two different strip widths are used: 2 and 4 cm. With this con guration there are about 30000 read-out channels for the 4 RPC planes. The trigger logic is based on transverse momentum cut in order to select high p t muons coming from resonances decay J and from low p t background from and K decay. The rst level trigger perfoms a loose p t cut on the detected tracks, the second level trigger performs a more precise cut on the p t and on the invariant mass of the dimuon, using the full information of the trigger chambers. The trigger chambers of the dimuon spectrometer will operate in presence of an important background due to a large number ofmuons from pions, kaons, charm and beauty decay and to low energy particles coming out of the absorber and the beam shield. From FLUKA simulations 5 , which take into account all these backgrounds, the maximum rate to be tolerated by the RPCs in ALICE will be 40 Hz cm 2 in the Ca-Ca interaction at the LHC luminosity o f 1 0 29 cm ,2 s ,1 and 5 Hz cm 2 in the Pb-Pb interaction at the LHC luminosity o f 1 0 27 cm ,2 s ,1 . The other requirements on the RPCs for the AL-ICE experiment are: e ciency 95 for each plane, a time resolution of 2 ns r.m.s. and a cluster size as close to 1 as possible in order to optimize the trigger selectivity.

BEAM TEST
In principle all the experimental requirements, presented in the previous section, could be satised by RPCs operating in streamer mode or in avalanche mode. The possibility to operate in streamer mode has been thoroughly investigated and compared to the avalanche operation mode during beam tests of RPC prototypes performed at the SPS at CERN. As the electrode resistivity is a very important parameter for the rate capability of RPC operating in streamer mode, 3 small 50x50 cm 2 single gap RPCs with resistivity from = 3.510 9 cm to = 310 11 cm were studied. The low resistivity chamber = 3.510 9 cm was tested both in streamer mode and in avalanche mode: e ciency, cluster size and time resolution measurements were performed. The RPCs operating in streamer mode were uxed with a gas mixture made up of: 49Ar + 40C 2 H 2 F 4 + 7C 4 H 10 + 4SF 6 , according to the results obtained in previous laboratory test performed with cosmic rays 6 . The RPC operating in avalanche mode were uxed with a gas mixture made up of: 95C 2 H 2 F 4 + 3C 4 H 10 + 2SF 6 , which gives a good e ciency 95 for a large high voltage range 500V free from streamer contamination 7 . The RPC were tested separately on a defocussed pion beam of 120 GeV c with a simple experimental set-up showed in g.2. A large X-Y scintillator hodoscope of 40x40cm 2 placed behind the RPC covers almost all the chamber surface. The 2 scintillators CRH and CRV de ne a small area 2x2cm 2 in the center of the beam spot and are used to perform local e ciency measurements.
The strip signals were directly discriminated for the RPC operating in streamer mode, and they were ampli ed by a factor 300 before discrimination when the chamber is operating in avalanche mode. Constant fraction discriminators were used in order to perform a more precise time resolution measurement.

Streamer mode results
The RPC electrodes are made of bakelite planes whose resistivity is the main factor which a ects the rate capability of the RPC , as mentioned before. In fact the time necessary to recharge the electrode surface after a spark is determined by the current that can ow across them and therefore is strictly related to the electrode resistivity. In g. 3 i s s h o wn the RPC e ciency as a function of the particle ux for 3 di erent electrode resistivity. For each c hamber the operation high voltage was choosen to be 200V above the starting point of the e ciency plateau. The maximum ux tolerated by the RPC with a good e ciency 95 goes from 5 Hz cm 2 for the high resistivity chamber = 3.510 11 cm to 550 Hz cm 2 for the low resistivity c hamber = 3.510 9 cm: it increases in inverse proportion with the resistivity. The rate capability of the low resistivity chamber ful ll the Alice experimental requirements also with a large safety factor which has to be taken into account. Fig. 4a shows an example of cluster size distribution for the low resistivity RPC equipped with 2 cm wide strips and operating at 9 KV: a mean value of 1.1 strips was obtained. For the chamber equipped with 1 cm wide strips the measured cluster size mean value is 1.5 strips. The mean cluster size increases slightly with the operating high voltage, as shown in g. 4b, as the streamer charge increases with the operation voltage inside the gas gap. On the other hand an increase in the operation high voltage improves the RPC time resolution as shown in g.5b where the time distribution r.m.s. as a function of the operating voltage is presented. An example of the time distribution for low resistivity c hamb e r a t 9 K V i s s h o wn in g. 5a: it is evident that besides a narrow peak r.m.s. 1ns there is a tail of late signals which becomes more important for lower operating voltage, as pointed out before and for high uxes. However with the time resolution measured for uxes up to 250 Hz cm 2 the 98 of events are included in a gate of 20 ns, as it will be in the Alice experiment. The big charge, generated inside the chamberby streamers 0.5 nC streamer , a ects the RPC performances and could produce a possible ageing e ect of the chamber. The high current which o w inside a large RPC Figure 6. E ciency of low resistivity RPC operating in avalanche mode as a function of particle ux for 3 di erent high voltage.
due to background particles in the Alice environement, could raise the electrode resistivity and then deteriorate the RPC performances after a long operation period. New tests are planned in order to study this e ect.

Avalanche mode results
The avalanche mode is characterized by a saturated avalanche regime of the detector where the gas gain is not much dependent from the operating voltages, this region is not far from the streamer threshold and largely dependent from the gas mixture. The presence of streamers in RPC operating in avalanche mode must be reduced to very low v alue otherwise the large charge produced inside the chamber would increase the pick-up strip multiplicity deteriorating the cluster size. With the gas mixture used during the test, and with a discrimination threshold of 200 mV, the e ciency plateau 95 starts at 9.8 KV and the fraction of events with streamers was always very low, less than 4 for a 500 V wide voltage range. At an operation voltage of 10 KV only 1 of events produced a streamer. The important advantage of the avalanche operation mode, with respect to the streamer operation mode, is the greater rate capability which can reach v alues of several KHz cm 2 as the charge produced is smaller. In g. 6 the low resistivity c hamber e ciency is plotted as a function of the particle ux for three operation voltage. The maximum ux tolerated by the chamber with 95 is 3 KHz cm 2 , 7 KHz cm 2 and more than 10 KHz cm 2 respectively for 9.9 KV, 10 KV and 10.3 KV. Even if the presence of streamers is very low, as mentioned before, the cluster size mean value is rather large: from 1.2 to 1.5 strips strip width = 2cm, increasing with the operation voltage as shown in g. 7b. An exemple of cluster size distribution is presented in g. 7a for an operation voltage of 10 KV and a particle ux of 1.1 KHz cm 2 : a mean value of 1.4 strips was obtained, this value has to be compared to 1.1 strips obtained in streamer mode. The larger cluster size is mainly due to electronic problem such a s noise and cross-talk between strips. The avalanche signals have a v ery small time uctuation as shown in g. 8 where an example of time distribution for the low resistivity c hamber is presented: a r.m.s. of less than 1 ns was obtained. This value is quite stable and it is not dependent from the high voltage and the particle ux.

CONCLUSION
A study of RPC with surface of 50x50 cm 2 and with bakelite electrodes of di erent resistivity w as performed at the SPS in order to de ne the parameters of the chambers which will be used in the Alice dimuon trigger. Rate capability, cluster size and time resolution were measured for the RPCs operating in streamer mode and in avalanche mode. The low resistivity chamber = 3.510 9 cm tolerates uxes up to 550 Hz cm 2 with an eciency 95 when it is operating in streamer mode, and uxes up to 10 KHz cm 2 when it is operating in avalanche mode. The mean cluster size 1.1 strips with 2 cm wide strips and the time resolution 2.3 ns r.m.s. of this RPC operating in streamer mode satisfy the Alice experimental requirements. Nevertheless ageing e ects due to the high current o wing inside the chamber operating in streamer mode could deteriorate the RPC performances, therefore more tests are planned to study these e ects. The performances of RPC operating in avalanche mode satisfy well the experimental requirements for what it concerns rate capability and time resolution, but a larger mean cluster size 1.4 strips than that obtained in streamer operation mode was measured, this is mainly due to noise and cross-talk between the strips.