CMS Note Mailing address: CMS CERN, CH-1211 GENEVA 23, Switzerland

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1 Available on CMS information server CMS NOTE 1998/065 The Compact Muon Solenoid Experiment CMS Note Mailing address: CMS CERN, CH-1211 GENEVA 23, Switzerland 21-st Oct 1998 Results of tests of Inverted Double Gap RPC at the CERN GIF facility H. Czyrkowski, M. Ćwiok, R. Da browski, W. Dominik, J. Królikowski, P. Majewski Institute of Experimental Physics, Warsaw University, ul. Hoża 69, PL Warsaw, Poland M. Górski Soltan Institute of Nuclear Studies, ul. Hoża 69, PL Warsaw, Poland Abstract A medium size prototype of Inverted Double Gap RPC made of low resistivity bakelite plates was tested in the GIF facility at the CERN SPS. The chamber efficiency as a function of the source intensity and its timing properties were studied and found to be excellent. This work was partially financed by Polish Committee for Scientific Research under grants KBN 115/E-343/SPUB/P03/119/96 and KBN 115/E-343/SPUB/P03/004/97.

2 Introduction The CMS is one of the detectors being prepared for the LHC p-p accelerator at CERN [1]. The purpose of this detector is the detailed study of high energy p-p interactions with special stress being put (among other features) on the precise and efficient reconstruction of muon tracks. Such a detector needs a good muon trigger. The design of the trigger has been undertaken and results of prototype tests were reported in [2]. It is foreseen that the central part (jj<1) will be equipped with four muon stations, each of them comprising a high precision muon detector (DTBX drift tubes) and one or two layers of the Resistive Plate Chambers as the dedicated trigger detector. In the forward part Cathode Strip Chambers (CSC) will be used. The RPC trigger detectors will be extended up to (jj=2.1). The purpose of the RPCs is to provide a fast muon trigger with bunch crossing identification. They were choosen because of low production cost and good timing properties. The detailed simulation of the CMS detector [3] indicates, that the incident flux at the positions foreseen for the RPCs will not be higher than 1 khz/cm 2 at the LHC peak luminosity, and for the currently envisaged shielding. The flux comes from several sources: muons produced directly at the interaction point originating from heavy quark and W/Z decays, muons from in-flight decay of light particles, hadronic punchthrough particles, low energy neutrons and photons coming from the machine related sources. Most of the signal rate is due to the background. The particles arrive continuously in time, so it is preferable to test the RPC s in such an enviroment which is provided at the CERN Gamma Irradiation Facility. 1 THE AIM OF PRESENT STUDY In order to be able to use the RPCs arranged in the double gap structures as the basis of the muon triggering system of the CMS detector we need to study their properties. Since we envisage mass production of the chambers (about 3400 m 2 ), we need to make sure, that they are as simple and cheap as possible without compromising their performance. The main aspects addressed were the chamber efficiency in high incident flux, the timing properties of signal, and the multiplicity of responding strips. 2 REQUIREMENTS IMPOSED ON RPC S The RPC chambers used as the basis of the CMS triggering system should possess following properties (according to [4]): Good efficiency in high incident flux. Since the trigger is provided by requiring a pattern of hits in four chamber planes, even a small decrease in a single plane efficiency may have serious consequences for the overall trigger performance. The single chamber efficiency at expected incident rate should be in excess of 95%. Good timing properties. The bunch crossing at the LHC occurs every 25 ns, therefore if the trigger should identify a given beam crossing, the time resolution of the RPC should not exceed 3 ns (RMS) and at least 98% of the signals should be contained within a 20 ns window. The average timing of the response should not vary with changing incident rate or external conditions such as temperature and pressure by more than 5 ns. Low multiplicity of responding strips. The trigger estimates the transverse momentum (p t ) of a muon by comparing the pattern of hits recorded in the RPCs with the predefined set of patterns obtained by Monte Carlo study of muon propagation in the CMS. Each predefined pattern has a p t value associated with it. In case, where more than one match is obtained within a single segment of the trigger, the one corresponding to largest value of p t is chosen as a result. If the multiplicity of hits is large, the trigger algorithm tends to overestimate the p t value. Therefore we need to keep the multiplicity of responding strips low. 2

3 Uniformity of the chamber response (efficiency and timing) across the chamber surface. In order to obtain this uniformity the distance between the electrodes should be kept as constant as possible. The average cluster size should stay below 2 strips. Use of safe gases. The gas mixtures used in the CMS should not be flammable ones, and the Montreal convention prohibits usage of certain CFC s which were normally used in RPC s. The radiation hardness and acceptable ageing properties. We expect the radiation dose of 25 Gy in the bakelite during 10 years of the RPC operation at the LHC maximum luminosity and the total number of avalanches to be about =cm 2. 3 GENERAL PROPERTIES OF A RPC The classical RPC operating in the streamer mode consists of two high specific resistivity (about cm) bakelite plates separated by a gas gap of 2 mm width. Its principle of operation and limitations may be found in [5]. Let us recall here, that the efficiency of the standard RPC falls below 60% at incoming charged particle rate above 100 Hz/cm 2 [6], which makes it unsuitable for use in the LHC experiments. This efficiency drop is due to the streamer mode discharges, which cause serious drop of the electric field inside the gas gap. The high resistivity of the chamber plates prevents rapid transmission of electric charge through them. It has been shown by many authors [7, 8], that in order to improve the chamber efficiency at high rates one should operate it in the proportional mode, where the charge in a single avalanche is much smaller than in the case of streamer. This mode of operation at high charge gain may be reached by using gases containing high percentage of freons, which limit the formation of streamer discharges. Another important feature is the specific resistivity of the material used for production of the chambers. If the resistivity is too high, the electric field inside decreases with avalanche rate due to the voltage drop on the resistance of the electrodes, thus resulting in lowering of the gas gain. This effect causes the loss of the detection efficiency and poor timing at high incident fluxes. 4 CHAMBER CONSTRUCTION In this work we present results obtained with the chamber prototype constructed using the bakelite plates of specific resistivity equal to about cm 1), about two orders of magnitude lower than in the case of RPCs made by other groups [6, 9]. Our detector is an Inverted Double Gap chamber, where each subunit has its own plane of the readout strips situated on the outside (Fig. 1). The high voltage plates are facing each other and are separated by a screening plate and insulating films. The inner surfaces of the chamber were covered with linseed oil containing 1 % of manganese and lead salts hardener to improve their smoothness. Such a technique is frequently used in RPC production [9]. The chamber was a square 2525 cm 2 with mm wide readout strips at 12 mm pich. In order to ensure the gap width stability four spacers were put at 10 cm intervals. More details on the chamber construction may be found in [10]. The readout strips from both subunits were connected together and the signals were fed into front-end preamplifiers of 10 MHz bandwidth and charge sensitivity of about 1.5 mv/fc 2) The signals from the preamplifiers were fed into discriminators with 50 mv threshold and then into the LeCroy 2277 multihit TDC with a 1 ns resolution. 5 TEST CONDITIONS. The chamber was constructed in Warsaw and tested at CERN during September 1997 in the H2 beam. It showed excellent efficiency in excess of 98% even at beam intensities up to 7 khz/cm 2. The signal spread in time was less than 3 ns and the time walk from low to highest intensity was not larger than 2 ns. The efficiency plateau was at least 500 Volts wide and the average strip multiplicity was less than 2 strips. Those results are reported in [10]. The tests with the accelerator beam suffer from following drawbacks The chamber is irradiated only during the SPS spill time, which lasts for about 2.5 s followed by 12 s without beam. Therefore we are not testing the detector in conditions in which it will operate in the LHC 1) The electrode material was produced specially for us by IZO-ERG S.A., Gliwice, Poland. 2) Courtesy of the INFN group from Bari. More infomation about the amplifiers can be found in [11]. 3

4 environment. Further inconvenience is the variation of the beam intensity during the spill time, especially at high beam rates. The beam spot size is about 1010 cm 2 for the muon beam and about 44 cm 2 for the hadron beam. Again, it is preferable to illuminate all of the chamber surface with the same intensity. In order to overcome these drawbacks the chamber was tested in October 1997 in the GIF (Gamma Irradiation Facility) (Fig. 2), which simulates the situation expected at the LHC. A strong cesium source irradiates continuously all of the chamber surface with 661 kev gamma rays. The source intensity may be adjusted thanks to a system of absorbers. The SPS muon beam monitored by the set of scintillating counters and two position sensitive gas chambers passes through the detector enabling us to measure the chamber efficiency and timing properties at various irradiation levels. The beam intensity was very low, about 20 Hz/cm 2. The atmospheric pressure and temperature were measured during part of data taking in order to investigate possible dependence of chamber behaviour on those quantities. 6 RESULTS. In order to classify an event as a good one we search for a hit inside a 100 ns wide time window. The earliest signal found within this window is taken as the one corresponding to the chamber response. The chamber efficiency is then defined as the ratio of events within this window to the total number of events recorded. Therefore the chamber efficiency and timing properties studies come from the beam muon tracks. The chamber efficiency as a function of the high voltage is shown in Fig. 3 with the full source intensity and without source. It may be noticed that the efficiency curve with source is displaced by about 300 Volts, which suggests that this value corresponds to the voltage drop across the resistive electrodes. The effect would be more pronounced if we irradiated the chamber with minimum ionizing particles, each of them giving a signal in both chamber planes as opposed to low energy photons, which are seen by one plane only. In the following set of figures we present the chamber properties as a function of the nominal source intensity. The value 1 of source intensity corresponds to the source fully opened, i.e. all the absorbers moved away. The Fig. 4 shows the chamber efficiency variation with the increasing source intensity for HV values of 9400, 9600 and 9800 Volts. Only at 9400 Volts does the chamber show the efficiency drop to 91% at the highest source intensities. The expected maximum value of the hit frequency at the LHC is of the order of 1 khz/cm 2 per single plane, which in our case corresponds to 2 khz/cm 2 fot the double gap chamber. Therefore the conditions for which we area aiming correspond to no more than 20% of GIF source intensity, therefore we are confident that our chamber is a suitable detector for the CMS. In Fig. 5 the variation of the pulse arrival time with radiation rate is shown. Its shift over the full source intensity range is equal to about 5 ns, and it is less than 2 ns below 0.1 full intensity. The signal spread in time is shown in Fig. 6. In the relevant region of source intensities it stays below 2 ns for high voltage above 9600 V. Another important feature to be studied is the strip multiplicty. As in other studies [10, 11] we define the strip multiplicity to be equal to the number of strips which respond in the time window of 10 ns after earliest signal. The average strip multiplicity (Fig. 7) shows a steady decrease with source intensity, which may be attributed to the decrease of effective high voltage and stays below 2.5 strips, which corresponds to 3 cm size. Since our strip pitch is only 1.2 cm we expect the multiplicity to be much lower in case of final chamber design where strips will be wider. In Fig. 8 the hit rate per square cm is shown. The hit rate given in the figure is the rate seen by both chamber planes. The rate grows linearly with the source intensity, which should be expected from constancy of the chamber efficiency as a function of the source intensity. The current drawn by the chamber as a function of the nominal source intensity is shown in the Fig. 9. It may be seen, that the current dependence on the source intensity corresponds well to the measured hit rate. Another important feature of the chamber behaviour is its dependence on atmospheric conditions - namely the temperature and pressure. In Figs. 10, 11, 12 and 13 we show the variation of chamber s performance on the temperature. It may be deduced from those figures, that when the temperature increases, chamber behaves as if the operating high voltage were raised - efficiency and strip multiplicity increase and signal arrival time and its spread 4

5 decrease. We feel that this question should be further investigated in a more systematic way. We used the variation of the efficiency and average timing across the chamber as a measure of the plane distance. In this analysis the data where only one plane was powered were used. The chamber was divided into narrow regions of 1 mm width and in each one the efficiency and average signal arrival time were measured. We used then the dependence of the efficiency and timing (measured for whole chamber) in order to estimate the local value of the electric field. This value was then used to calculate the local distance between the electrodes. Data from several runs at different high voltage values are plotted together in Fig. 14 where the distance was estimated from timing and in Fig. 15 where efficiency was used. The vertical axis shows departure from nominal chamber thickness. Both figures show similar behavior and the distance variation does not exceed 50 m. The second plane showed much smaller variation. The variation of the pulse arrival time across the chamber does not exceed about 2 ns at the highest voltage used, while efficiency dependence on the position can be observed only at low voltages, where the efficiency does not exceed about 95 %. The measurement of the signal spread in time is performed relative to the trigger signal provided by the beam scintillator hodoscope. If this signal exhibits some jitter, this jitter will be added in quadrature to the measured time resolution. Since our measurements were performed together with measurements of Bari chamber [12], we may use both chambers in order to estimate the scintillator signal spread. We estimated the trigger spread in time to be about 0.5 ns, therefore we may conclude that it does not affect seriously our measurement of the chamber timing resolution. 7 CONCLUSIONS. We have shown that a RPC with resistivity about cm operates well in the high radiation environment similar to one expected at the LHC. Its main characteristics such as the efficiency, timing properties and strip multiplicity are all in the ranges required of such a detector. This smallness of the time walk with the radiation rate implies that no adjustment of timing will be necessary in the trigger electronics. Further questions which must be addressed in the future are: determination of optimal value of the specific resistivity, where efficiency is still high; systematic study of chamber properties as a function of temperature and pressure; longterm behaviour of the chamber, specially whether the chamber exhibits any ageing and if it does, whether it will not prohibit its use during several years of running at the LHC; longterm operation under continuous irradiation; construction and study of a full-scale prototype; equipping the chamber with final front-end electronics. Acknowledgements The authors are grateful to Fabrizio Gasparini for encouragement in starting the study and for continuous interest. We are indebted to Giuseppe Iaselli for very fruitful discussions and for providing kindly the amplifiers for readout. We wish to thank to the INFN groups from Bari and Pavia for pleasant cooperation. We thank Lisa Gorn and Ian Crotty for their support and participation during the tests at the GIF facility. References [1] CMS Technical proposal, CERN/LHCC/ [2] M. Andlinger et al., Pattern Comparator Trigger (PACT) for the muon system of the CMS experiment. Nucl. Instr. and Methods, A370 (1996) 389. [3] M. Huhtinen, Radiation Environment simulations for the CMS Detector, CERN CMS TN/

6 [4] CMS Muon Technical Design Report, CERN/LHC/ [5] Proceedings of the III International Workshop on The Resistive Plate Chambers and Related Detectors, Pavia 1995, Scientifica Acta, vol. 11, no. 1. (1995). [6] L. Pontecorvo, Proceedings of the II International Workshop on The Resistive Plate Chambers in Particle Physics and Astrophysics, Pavia 1993, Scientifica Acta, vol. 8, no. 3. (1993) p [7] R. Cardarelli, Proceedings of III International Workshop on the Resistive Plate Chambers in Particle Physics and Astrophysics, Pavia 95, Scientifica Acta, vol. 8, no. 3. (1995) p.159. [8] H. Czyrkowski et al., Proceedings of the International Workshop on Resistive Plate Chambers and Related Detectors, Pavia 95, Scientifica Acta, vol. 11, no. 1. (1995) p.197. [9] M. Abbrescia et al., Nucl. Instr. and Methods A394 (1997) 13. [10] H. Czyrkowski et al., Proceedings of the IV-th International Workshop on Resistive Plate Chambers and Related Detectors, Napoli, Italy, October [11] M. Abbrescia et al. Beam Test Results on Resistive Plate Chambers for the CMS Experiment, CMS NOTE 1997/062. [12] M. Maggi et al., Proceedings of the IV-th International Workshop on Resistive Plate Chambers and Related Detectors, Napoli, Italy, October

7 Structured plate Honey-comb Strip plane Insulating film Resistive layer GND RPC gas gap Resistive layer HV Insulating film Screening plate Insulating film Resistive layer HV RPC gas gap Resistive layer GND Insulating film Strip plane Structured plate Honey-comb Figure 1: Cross section of an Inverted Double Gap RPC. RPC Beam mwpcs Scintillators SPS beam Filters Cs source GIF FACILITY AT CERN Figure 2: Schematic view of the GIF facility at CERN. 7

8 EFFICIENCY, % SOURCE OFF SOURCE ON HIGH VOLTAGE, [kv] Figure 3: Chamber efficiency as a function of the high voltage without and with source. EFFICIENCY, % HV=9.6 kv HV=9.8 kv E E SOURCE INTENSITY Figure 4: Chamber efficiency as a function of the source intensity. 8

9 <TDC>, ns HV=9.6 kv HV=9.8 kv E E SOURCE INTENSITY Figure 5: The signal arrival time as a function of the source intensity. σ<tdc>, ns HV=9.6 kv HV=9.8 kv E E SOURCE INTENSITY Figure 6: The signal arrival spread as a function of the source intensity. 9

10 <STRIP MULTIPLICITY> HV=9.6 kv HV=9.8 kv E E SOURCE INTENSITY Figure 7: The average strip multiplicty as a function of the source intensity. HIT FREQ [Hz/cm 2 ] HV=9.6 kv HV=9.8 kv 1.00* * * E E SOURCE INTENSITY Figure 8: Chamber hit rate seen by two planes as a function of the source intensity. 10

11 CHAMBER CURRENT [µa] HV=9.6 kv HV=9.8 kv E NOMINAL SOURCE INTENSITY Figure 9: The current drawn by one of the chamber planes as a function of the source intensity. EFFICIENCY, % SOURCE ON HV=9.1 kv HV=9.2 kv HV=9.3 kv HV=9.5 kv temperature, [ 0 C] Figure 10: Chamber efficiency as a function of the temperature. 11

12 <TDC>, ns SOURCE ON HV=9.1 kv HV=9.2 kv HV=9.3 kv HV=9.5 kv temperature, [ 0 C] Figure 11: The signal arrival time as a function of the temperature. σ<tdc>, ns SOURCE ON HV=9.1 kv HV=9.2 kv HV=9.3 kv HV=9.5 kv temperature, [ 0 C] Figure 12: The signal arrival spread as a function of the temperature. 12

13 <STRIP MULTIPLICITY> 1.80 SOURCE ON HV=9.1 kv HV=9.2 kv HV=9.3 kv HV=9.5 kv temperature, [ 0 C] Figure 13: The average strip multiplicty as a function of the temperature. plane 1 on, plane 2 off, efficiency delta dist, [µ] pos, [mm] Figure 14: The departure of the chamber thickness from nominal one calculated from efficiency. 13

14 plane 1 on, plane 2 off, timing delta dist, [µ] pos, [mm] Figure 15: The departure of the chamber thickness from nominal one calculated from timing. 14