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Systematic study of RPC performances in polluted or varying gas mixtures compositions: an online monitor

system for the RPC gas mixture at LHC

M. Capeans, R. Guida, B. Mandelli

CERN PH Department Detector Technologies Group

Abstract The importance of the correct gas mixture for the Resistive Plate Chamber (RPC) detector systems is fundamental for their correct and safe operation. A small change in the percentages of the gas mixture components can alter the RPC performance and this will rebound on the data quality in the ALICE, ATLAS and CMS experiments at CERN. A constant monitoring of the gas mixture injected in the RPCs would avoid such kind of problems. A systematic study has been performed to understand RPC performances with several gas mixture compositions and in the presence of common gas impurities. The systematic analysis of several RPC performance parameters in different gas mixtures allows the rapid identification of any variation in the RPC gas mixture. A set-up for the online monitoring of the RPC gas mixture in the LHC gas systems is also proposed.

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Table of Contents

1. Introduction 3

2. Experimental set-up 3

3. Analysis procedure 4

4. Changes to the classical RPC mixture composition 5 4.1. RPC behavior with different percentages of SF6 5 4.1. RPC behavior with different percentages of iC4H10 7

5. Effects of the presence of impurities in the gas mixture 8 5.1. RPC sensitivity to the presence of Air 8 5.2. RPC sensitivity to the presence of CF4 9 5.3. RPC sensitivity to the presence of Argon 9 5.4. RPC sensitivity to the presence of CO2 10

6. Discussion of results 10

7. Possible application of the results: a RPC monitoring system 12

8. Bibliography 14

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1. Introduction In the CMS and ATLAS RPC muon systems the gas mixture used is 94.7% C2H2F4/5% iC4H10 /0.3% SF6. The relative humidity of the gas mixture is adjusted to about 40%. Connecting a a gas chromatograph (GC) to the gas system allows monitoring the gas mixture composition, in terms of component’s concentration and presence of impurities. However it does not provide direct information about possible effects on the RPC performances and sensitivity, which can rapidly change with a slightly different gas mixture or in presence of some impurities. An example happened in April 2011 due to a failure of the SF6 Mass Flow Controller (MFC) in the CMS-RPC gas system. The technical hitch was not found immediately and the SF6 concentration in the gas mixture changed, reaching a value of 0.45%. This variation, even if small, caused a visible change in the RPC performances. The HV working point had to be increased by about 120 V, as shown in Fig. 1.

Fig. 1 Histogram of the applied high voltage for the CMS-RPC barrel region. A shift of about 120 V with respect to

the 2010 data is visible. A similar HV shift can cause serious damages on the RPC detectors. This example shows the importance of delivering the correct gas mixture to RPC detectors and of having different methods for monitoring its stability. The aim of the present work is to study several RPC parameters in order to find their sensitivity to different gas mixtures. Furthermore this work permits to propose a monitoring system of the RPC mixture in the LHC experiments that could come abreast of the GC.

2. Experimental set-up Fig. 2 shows a schematic view of the experimental set-up assembled to carry out this systematic study. Two standard high pressure laminate RPCs (in the following RPC0 and RPC1) with gas gap 2 mm wide and a surface of 80 × 100 cm2 were used. A set of three scintillators SC1, SC2 and SC3 were employed to trigger on cosmic muons. For simplicity the signal read out was made on only one strip per RPC. Data were acquired using a DT5724 Desktop Waveform Digitizer and collected at a frequency of about 1 Hz. The RPC performances were studied for several mixture compositions. Namely, concerning the main components, the SF6 and the iC4H10 concentrations were varied from the nominal values during two separate tests. The impact of common impurities, like Air, Ar, CO2 and CF4 were studied in dedicated tests adding several levels of concentration with a specific mass flow controller.

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Fig. 2 Schematic view of the experimental set-up.

3. Analysis procedure Fig. 3 shows a typical RPC signal obtained with the digitizer: avalanche and streamer pulses are clearly visible. The measurements were taken for different high voltages and different gas mixtures. Three different histograms were plotted in order to analyze the RPC signal, each giving different parameters that can be used to extract information about the RPC features and performances:

§ pulse height § pulse integrated charge § event time

The pulse height of the signals is shown in Fig. 3. In case of mixed pulses (avalanche and streamer), the maximum pulse height has been considered to be the one of the streamer. Thus the maximum height histogram gives information about the presence and development of the avalanche towards higher pulses. For the analysis of the pulse charge it is necessary to separate the avalanche and streamer regions. In the streamer regime the RPC signal has a much larger charge with respect to the avalanche regime. The charge spectrum for different applied high voltages has been plotted and divided into two zones: one for the avalanche and one for the streamer signals. In order to extract a simple synthetic indication, a fixed separation between the two regions was introduced: signals below 5 pC are identified as avalanche, while above this limit the signals are considered streamers.

Fig. 3 RPC signal obtained with the digitizer for a gas mixture of 94.7% C2H2F4 / 5% iC4H10 / 0.3% SF6 at 9800V. The avalanche and streamer signals are visible. The line pattern indicates the integration of the signal (pulse charge).

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In the present study, the event time is defined as the time of the most negative value reached of the signal as shown in Fig. 3. The time resolution for the set-up is the standard deviation of the histogram representing the RPC signal time distribution. In the second step of the analysis the previously defined parameters are used to obtain information on the RPC detector sensitivity to changes in the gas mixture composition. The information obtained are:

§ efficiency § average charge for the avalanche and streamer region § average total charge § event frequency for the avalanche and streamer region § time resolution

4. Changes to the classical RPC mixture composition The monitoring of the gas mixture in the RPC system plays a fundamental role and it can be done in two complementary ways: by means of the gas chromatogram, and by checking the RPC performances. A systematic study changing the percentage of the SF6 or iC4H10 has been done in order to understand the effects on the RPC and to obtain the RPC sensitivity to these changes.

4.1. RPC behavior with different percentages of SF6

SF6 is fundamental for the RPC gas mixture even in small concentration (currently, as low as 0.3%). It is an electronegative gas that captures free electrons, thus it decreases the average pulse charge and the streamer probability. A set of measurements was taken with different SF6 concentrations. It has been found that the efficiency curve for 0% SF6 is considerably shifted and steeper with respect to the other curves: without SF6 a lower high voltage is needed to reach the same efficiency of the other SF6 percentages. With the increasing of the SF6 percentage the efficiency curves move towards higher high voltage values; this behavior is consistent with the physical processes inside the RPC detector: the capture of free electrons by the SF6 is limiting the charge development and therefore a higher electric field is needed to produce the signals. To understand this point, a comparison between the charge distributions for different SF6 concentrations at the same efficiency (0.8) is useful, as shown in Fig. 4. With the increasing of the SF6 the number of streamer signals is reduced and also the integrated charge.

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Fig. 4 Comparison of the charge distribution for different SF6 concentrations. The data are taken at the same

efficiency. The average charge for the avalanche and streamer signals has been plotted as a function of the high voltage for the different SF6 concentrations (Fig. 5). As in the previous case, the data for the 0% SF6 are shifted towards lower high voltages and a higher streamer charge is also reached. The streamer pulse charge is about 12 pC for the 0% while it decreases with increasing SF6 concentration at equivalent efficiency. This behavior is due to the capture of electrons that prevents the streamer formation. It is interesting to study the probability of having a large charge pulse (streamer) during the operation of the detector in different conditions: Fig. 6 shows the frequency of avalanche and streamer signals. With increasing high voltage the streamer signals became predominant while at low voltages only the avalanche signals are present.

Fig. 5 Average charge for the avalanche (continuous line) and streamer (dashed line) signals as a function of the

high voltage for different SF6 concentrations.

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Fig. 6 Fraction of avalanche (continuous line) and streamer (dashed line) charge signals versus the applied high

voltage for different SF6 concentrations.

4.1. RPC behavior with different percentages of iC4H10

Another important gas component in the RPC mixture is the iC4H10, which is present at a concentration level of 5%. The iC4H10 is a quencher, which absorbs UV photons in a wide energy range reducing the production of secondary avalanches. A set of measurements was taken with different percentages of iC4H10 around the classical mixture composition. The curves representing the streamers and avalanches frequency at different iC4H10 concentrations are very close to each other (Fig. 7): this shows that a little variation of the iC4H10 does not affect the streamer presence in a significant way.

Fig. 7 Fraction of avalanche (continuous line) and streamer (dashed line) charge signals versus the applied high

voltage for different iC4H10 concentrations. The plot of the average charge for the avalanche and streamer modes confirms the low sensibility to a change in the iC4H10 percentage of the RPC performances (Fig. 8). The avalanche charge is substantially the same for all percentages and all high voltages and it is about 1 pC (in the SF6 scan study the charge changes slightly as a function of the high voltage and SF6

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concentration). There is not a substantial and systematic effect for the different iC4H10 percentages on the streamer charge.

Fig. 8 Average charge for the avalanche (continuous line) and streamer (dashed line) signals as a function of the

high voltage for different iC4H10 concentrations.

5. Effects due to the presence of impurities in the gas mixture After a detailed analysis on the effects of different SF6 and iC4H10 concentrations in the classical mixture, it is useful to study the impact of the presence of impurities. Air is a typical contaminant in the gas mixture and can be present in large systems for several reasons: low quality of supplied gas, possible leaks in the gas system or detectors, etc. As seen in previous studies [1], the return gas of RPCs contains varied gas impurities and therefore it is interested to see how the RPC working parameters are affected. In the present study the effect of CF4 was studied. The RPC detector can work with different gas mixture compositions. In the ALICE experiment at CERN the RPCs are operated in a very saturated avalanche regime with the addition of 50% of Argon to the classical gas mixture. A systematic study of the RPC working parameters has been done with different percentages of Ar and CO2 in the gas mixture. Even if for some impurities the present test will not show significant effects on the detector performances, this does not exclude the appearance of possible long term ageing phenomena [3].

5.1. RPC sensitivity to the presence of Air

In the RPC gas systems at the LHC, Air is usually present in the order of 100 − 500 ppm. Nevertheless incidents can occur causing higher contamination. Air can enter in the system in different ways, due to leaks, during maintenance operations of the gas system, or simply because of poor quality of the primary gas supplies, as it has happened for example, with the C2H2F4 primary containers. In this study a controlled quantity of Air (from 1000 ppm to 12000 ppm) was injected in order to study the RPC behavior in case similar concentrations would be accidentally present in a LHC gas system. Air is primarily composed of N2 (78%), O2 (21%) and 1% of Ar, CO2, which all are less electronegative than SF6. The presence of less F in the gas mixture allows the formation of a higher number of streamers, therefore if Air is present in a relatively large amount, the number of streamer signals will increase, as well as the total signal charge. The plot in Fig. 9 confirms this theory: at equal high voltage (approximately equal efficiency) the number

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of streamers increases if a higher quantity of Air is present. The presence of large quantities of Air in the gas mixture is also affecting the total average charge: it increases with the increasing concentration of Air because Air does not prevent the streamer formation.

Fig. 9 Fraction of avalanche (continuous line) and streamer (dashed line) signals versus the applied high voltage for

different concentrations of Air.

5.2. RPC sensitivity to the presence of CF4

The following study was done in order to verify the sensitivity of the RPC working parameters to one of the gas impurities found in irradiated RPCs [3]. The aim of these studies is to factorize effects due to impurities, by verifying if small CF4 concentrations, typically below 1%, can affect key RPC parameters. As in the SF6 case, the efficiency decreases with the increase of the CF4 concentration. However it has been noticed that the efficiency curves are not well separated: the presence of relatively small concentration of CF4 does not produce a clear change in the efficiency. Also in the case of the total average charge the curves are similar, but an effect is noticeable if the streamer and avalanche charge are considered separately in the former’s curves. The streamer charge is higher with the CF4 concentration because the CF4 is capturing less effectively the free electrons (the CF4 electronegativity is lower than the SF6 one).

5.3. RPC sensitivity to the presence of Argon

Ar is a noble gas and behaves in the opposite way than SF6, i.e. it does not capture electrons. Ar is used to operate the RPC in streamer or in a very saturated avalanche regime. The choice of this gas gives some advantages, such as the easiness of ionization and the low cost of the associated read-out electronics since signals of higher amplitude are obtained. However it is resulting in a lower rate capability (about 0.1 kHz with respect to 1kHz as it is the case for Freon based mixtures). The poor separation for the curves with similar Ar concentrations is visible for the plots of the avalanche and streamer frequencies and of the total average charge. Fig. 10 shows that the number of streamers increases with the increase of Ar concentration; with about 3.8% Ar the number of streamers exceeds the number of avalanches for an efficiency of 98% while in the SF6 and iC4H10 cases the number of streamers is always less than the avalanche one (of course except without SF6). Another point to underline is the total average charge: with the classical gas mixture the total charge reached is about 4pC, while in the case of 3.8% of Ar the charge is more than 6 pC.

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Fig. 10 Fraction of avalanche (continuous line) and streamer (dashed line) signals versus the applied high voltage

for different Ar concentrations.

5.4. RPC sensitivity to the presence of CO2

CO2 concentration was varied from 0% to 3.46%, but no effects on the efficiency were visible: the efficiency curves are not well separated and it is difficult to understand if a different concentration of CO2 in the gas mixture affecs RPC efficiency. Furthermore, the proof of a low sensitivity to changes in the CO2 concentration is visible in the plot of the average charge for the avalanche and streamer signals where the curves are not well separated (as for the other quencher gas analyzed, iC4H10). The avalanche charge is substantially the same for all concentrations and all high voltages and it is about 1 pC while for the streamer charge is not visible a systematic effect. The same behavior is obviously visible in the plot of the average total charge, where the lines representing the different CO2 concentrations are not completely distinguishable up to about an efficiency of 80%. This means that up to this value, the different percentages of CO2 do not change the process of signal formation in a significant way, as it was for the iC4H10 case (Section 4.1).

6. Discussion of results The aim of these studies was to understand the sensitivity of RPCs to small variations of the gas mixture composition or to the presence of common impurities. The studies have demonstrated that RPCs behave differently if the gas mixture changes: at constant efficiency the percentage of streamers as well as the total pulse charge, read-out by the strips, are function of the gas mixture. A systematic study has been done in order to quantify the sensitivity of the RPC working parameters to the different gas mixture composition analyzed before. Let’s consider the SF6 results for the different concentrations. As a summary of the test, the avalanche and streamer frequency versus the high voltage is the parameter most affected by any change of concentration. Therefore, in order to quantify the sensitivity, in the following only the data about the frequency are considered. Fig. 11a shows the high voltage corresponding to a fixed value of the avalanche to streamer ratio. The data corresponding to the 70% are chosen since they are in the region with higher derivative (i.e. higher sensitivity to any changes).

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a) b) Fig. 11 a) High voltage corresponding to the 70 : 30 percentage ratio of avalanche and streamer at different SF6 concentrations. b) RPC sensitivity (V/ppm) at different SF6 concentration. The high voltage variation with respect to the SF6 concentration is not linear and it is more pronounced for small concentrations. Varying the concentration from 0% to 0.15% the high voltage changes by 800 V while from 0.3% to 0.4% the high voltage changes by only 100 V. The next step is to plot the derivative of the previous graph as a function of the SF6 concentration, as shown in Fig. 11b. On the y-axis the high voltage change per variation of the SF6 concentration is reported: this parameter allows quantifying the sensitivity of the RPC working parameters to different concentrations. The variation of this parameter is extremely high, i.e. 0.6V/ppm, at low concentration (less than 1500 ppm), while it stabilizes to about 0.1 V/ppm from concentration of the order of 4000 ppm. Therefore, the fraction of large signals revealed to be sensitive to a few hundreds ppm variation of the SF6 concentration. The same analysis has been applied to the other gases considered in the previous sections. Fig. 12a shows the summary plot obtained for all the impurities and gas composition variations. The greater sensitivity is observed for SF6 changes in the concentration while for the other gas (Air and CF4), smaller but still measurable effects are visible. However for the iC4H10 and CO2 cases the changes are very difficult to detect. A zoom of Fig. 12a is visible in Fig. 12b in order to show the RPC sensitivity to changes in the concentration for other gases. iC4H10 and CO2 do not show significant variations, indicating how relatively low change in their concentrations are not affecting significantly the RPC working parameter. However a new test with higher statistic will be performed in order to understand if nevertheless there is a measurable effect. CF4 and Air show more significant variation even if they are about one order of magnitude lowers than the SF6 case.

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b) Fig. 12 a) Summary of the RPC sensitivity for different gases. b) Detailed view of the RPC sensitivity without the

SF6 data. The gas studied can be divided into two main groups: SF6 and iC4H10 have a positive sensitivity while the others have negative sensitivity. A positive sensitivity means that the high voltage corresponding to a fixed avalanche to streamer ratio is increasing with increasing concentration. Thus an increase of concentration, for instance of SF6, has to be compensated applying a higher high voltage in order to maintain constant the avalanche to streamer ratio. A negative sensitivity, as for Air, indicates that with the increase of the Air concentration the same avalanche to streamer ratio is reached at lower high voltage. The increasing or decreasing trends in Fig. 12a-b show how the sensitivity changes at different concentrations. For example, the decreasing trend of the SF6 curve summarizes the fact that the sensitivity decreases with the increase of the SF6 concentration: i.e. the first addition from 0% to 0.15% has a lower effect than from 0.3% to 0.45% on the frequency of the streamer signals.

7. Possible application of the results: a RPC monitoring system The present study shows how crucial RPC parameters are sensitive to variations of the gas mixture composition. In particular, the results show that the most sensitive parameter is the avalanche to streamer ratio. This effect can be exploited to monitor the stability of the working conditions and of the gas composition in large RPC detector systems. An online gas mixture monitoring system installed in the large RPC systems of ATLAS and CMS can avoid problems that cannot be easily identified at present (Fig. 13). Even if the existing gas systems are controlled and monitored through a PVSS software interface, where alarms can be set for variations of several parameters, not all problems can be promptly detected. It is therefore useful to use a dedicated monitoring system that would verify the quality of the gas mixture in the RPC system, and would give a precise view of critical parameters of RPC performance; during LHC physics runs it is almost impossible to monitor closely the RPC performances, while a small dedicated set-up can detect any changes and be used to verify the impact on the performance of the real RPC system.

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Fig. 13 Schematic view of the CMS gas system for the RPC detectors (the ATLAS gas system is very similar). On the surface (SG) is already installed a set of devices (GC, O2 and H2O analysers) for the gas quality monitoring of all detectors. In parallel, the dedicated gas monitoring for the RPC will be installed. Even if in the present study a simplified set-up has been employed, especially concerning the read-out electronics, very many useful results have been obtained. For the ATLAS and CMS experiments, an evolution of this set-up will be prepared and tested in the coming months. The

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new system will have three sets of RPCs, with the same characteristics of the ATLAS and CMS RPCs, operated with the mixture sampled from three different points, as shown in Fig. 14: - Point 1 (fresh mixture): it allows monitoring the fresh gas mixture injected into the system (usually in a measure of about 10% on the total flow) - Point 2 (before purifier): it allows checking the gas quality of the return mixture from the RPC system in term of gas mixture composition and presence of impurities. - Point 3 (after purifier): it allows verifying the effectiveness of the purifiers modules; after that the gas is injected again into the system. With these three sampling points, it will be possible to monitor all possible variations of the gas mixture travelling inside the complex RPC system (both in terms of gas and detector systems). The new system will allow a constant monitoring of RPC performances (efficiency, avalanche to streamer ratio, etc.). In case any variation is detected, a warning will be send to the operator (both RPC shifter and personnel of the CERN gas service team). Following a warning, the CERN gas service team will perform dedicated gas chromatographic analysis to verify the gas mixture. After these steps, the potential problem should be found and fixed.

Fig. 14 Possible set-up for the monitoring of changes in the gas mixture.

Furthermore the set-up will be positioned in a gas-tight box where absolute pressure is kept constant. This will cancel the contribution of any atmospheric pressure change. Temperature effects will be easily controlled if the set-up is installed in a room with controlled temperature. In this condition, the proposed set-up will allow to spot any variation of the RPC working condition: any variation of the avalanche to streamer fraction could be an indication that a working condition is changed. A detailed study is on going and a first prototype will be tested in the coming months.

8. Bibliography [1] B. Mandelli, Optimization of the RPC gas recirculation system for the CMS experiment, Master thesis (2011).

[2] B. Mandelli et al., Long-term study of optimal gas purifiers for the RPC systems at LHC, CERN PH-EP-Tech-Note-2012-001.

[3] R. Guida et al., Optimization of a closed-loop gas system for the operation of Resistive Plate Chambers at the Large Hadron Collider experiments, Nucl.Instr. and Meth. A 661 (2012) 214-221.