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Pure avalanche mode operation of a 2 mm gap resistive plate chamber
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Nuclear Instruments and Methods in Physics Research A 396 (1997) 93-102
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l!iB ELSEVIER
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH SectjonA
Pure avalanche mode operation of a 2 mm gap resistive plate chamber E. Cerron
J. Lamas Valverdea,b,c, Zeballos”*b, I. Crotty”, D. Hatzifotiadou”.‘, M.C.S. Williams”*‘**, A. ZichichPd.”
Received 17 March 1997 Abstract It is necessary to operate the resistive plate chamber (RPC) in avalanche mode to obtain high efficiency at elevated particle fluxes. We examine this mode of operation with a 2 mm gap RPC using gas mixtures containing C2FSHZ and C2FSH. In order to explain the data we propose that the avalanche growth is strongly limited by space charge effects.
1. Introduction Resistive plate chambers (RPC) have been in use in various experiments since the 1980s; however. in all cases the flux of particles has been rather low. In the original Letters of Intent for experiments at LHC. the prime candidate for implementation of the muon trigger is the RPC. The proposed type of RPC has a 2 mm gas gap bounded by 2 mm thick (linseed-oil treated) Bakelite plates. In order to operate at theexpected flux of particles at the LHC experiments, both ATLAS and CMS [1,2] proposed operating these devices in the ‘low-gas gain’ mode (nowadays this mode is usually referred to as avalanche mode). There has been an extended study to find a suitable gas mixture; the proponents [3-51 of this type of RPC have proposed mixtures containing a high fraction of C,F,H, (an ecologically acceptable freon). The basic problem is to operate the RPC in avalanche mode with high efficiency and with only a very low contamination of streamers (i.e. pure avalanche mode). Data from various groups show a variety of charge spectra measured from this device. Abbrescia et al. [3] show an exponential shape, Ammosov et al. [4] show a Landau shaped distribution; while Alviggi et al. [S]
*Corresponding
author. E-mail: [email protected]
show a Gaussian shaped distribution. The ease of operation of this detector depends on the exact shape of these spectra; thus. we started a series of measurements in order to understand these apparent differences between various groups. We have simulated the charge spectrum using the program described in Ref. [6]. We assume that the number of primary ionisation clusters is 75jcm (obtained by scaling up by the atomic number of the molecule C3F4H2 from data for Argon) and have set the gas gain such that the average fast signal is 1 pC (assuming that fast/total charge is 1jrD and the attachment coefficient is zero; a is the Townsend coefficient and D the size of the gas gap). We show the result in Fig. 1 for three cases: (a) no avalanche fluctuations; (b) exponential fluctuations are applied to individual avalanches formed from each cluster of primary ionisation: (c) exponential fluctuation applied to a single merged avalanche. In all cases the shape is exponential with a long tail of high values of fast charge. For the wide gap RPC and multigap RPC [6] there were marked differences in the spectra when the fluctuations described above were applied; in that study we found that case(c) best described the measured charge spectrum. Transition to streamer production should begin when the avalanche contains more than lo8 ions (i.e. for fast signals greater than 1 PC); however, this limit may be higher in the presence of a strong quencher such as the freons used in this study. This exponential shaped charge
016%9002;97iSI 7.00 Copyright ,(‘: 1997 Elsevier Science B.V. All rights reserved PII SO1 6%9003(97)0075 1-l
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(a) No avalanche
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(b) Individual avalanches smeared
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(c) ‘Merged’ avalanche smeared
t,,,,1,,“1,,“‘,““““J 0
z
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offast
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Fig. 1. Simulation of charge spectrum produced by a 2 mm gap RPC filled with C2F4H2. Three cases are shown: (a) no ava-
lanche fluctuations applied: (b) each cluster of primary ionisation creates an independent avalanche. the size of which is smeared by an exponential fluctuation: (c) all individual avalanches are merged into a single avalanche. which is then smeared by the exponential avalanche fluctuations.
spectrum (as generated by the simulation) indicates that (a) a very low threshold discriminator is needed and (b) already there is a significant number of large avalanches, which could trigger a transition to a streamer. However. this simulation does not agree with data presented by various groups, where they show that pure avalanche mode operation is possible; also the shape of our simulated charge spectrum is only consistent with data obtained by Abbrescia et al.
2. Experimental set-up The resistive plate chamber used in this study was constructed with resistive plates made from 0.8 mm thick melamine+phenolic laminate sheets of 32 x 32 cm’. The smooth melamine surface faced the gas, while the outer phenolic surfaces were painted with conductive paint (Nickel paint with surface resistivity l-2 0/n) to form pad electrodes of 24 x 24 cm”. The gas gap was bounded by a 2 cm wide and 2 mm thick PVC spacer around the perimeter. A layer of glue attached the resistive plates to these spacers, thus creating a gas gap between 2.1 and 2.2 mm. One electrode, the cathode, was connected with a 1 MQ resistor to a high voltage power supply; the signal developed on the cathode was read out through a 1 nF capacitor. This signal was fed into a fast current
amplifier, the output of which was inverted and split between an ADC and a discriminator. The other electrode was connected to ground either through the 1 MR input impedance of the oscilloscope or to an ADC with input impedance of 50 R. The exact set-up is described below where the measurements are discussed. We triggered on through-going cosmic rays with a series of scintillator counters selecting a 10 x IO cm’ area in the centre of the RPC. We performed two series of tests; for the first we used a gas mixture of 3% iso-C,H I ,) and 97% C2F,H,. Normally we add 1% water vapour to our gas mixtures to keep the resistivity of our melaminephenolic plates at a low value [7]. For the tests with this gas mixture we did not add water vapour. The voltage needed to reach the knee of the efficiency plateau slowly increased from 10.5 to 1 I.2 kV over the several week period of the tests. We attribute this rise to an increase in resistivity. thus generating an increasing voltage drop across the resistive plates. We have ignored this effect except to note that various plots will be labelled with voltage relative to the knee of the efficiency plateau so that comparisons can be made. For the second series of tests we used a gas mixture of 95% C,F,H (also ecologically friendly) and 5% iso-C,H,,bubbled through a water reservoir at 10 C in order to add 1% water vapour. For these tests the working voltage was more constant.
3. Measurement technique There are various types of measurements made with this chamber. We have measured the charge spectrum of the fast signal derived from the cathode pad. We have measured the time spectrum with the threshold of the discriminator set at 1OfC. We also measured the total charge produced by the avalanche or streamer. For this measurement we connected an oscilloscope directly to the anode, using the high impedance of the oscilloscope to connect the anode to ground potential. A typical oscilloscope display is shown in Fig. 2. The step is due to the movement of the electrons towards the anode; the long ramp is due to the drift of the positive ions toward the cathode. Eventually the voltage levels off when all the ions have been collected. The total voltage deviation can be converted to charge, as we have measured the total capacitance of the anode to ground. In this case it is 0.7 nF. We are interested in the ratio of fast charge/total charge, i.e. the size of the initial step to the total deviation. This is clearly visible in Fig. 2; however we are also interested in small avalanches, where the total deviation due to the collection of positive ions is several mV. Obviously, the sensitivity and noise of the system would make it difficult to measure the step directly. We. thus, also monitored the fast signal measured by a fast current amplifier attached to the cathode, having first calibrated
E. Cerrorl Zehallos et al. iNuc1. Imtr. and Meth. in Phy. Vertical scale
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Rex A 396 (1997) 93-102
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only the electron signal that produces the observed fast signal of the avalanche. Another factor that we have to take into account is the coupling between the cathode and anode. A positive going signal is generated on the cathode and negative on the anode; thus, any capacitive coupling between the two plates will lead to a reduction of the signal. This reduction will depend on how each electrode is decoupled to ground. Additionally there is capacitive coupling between the electrode and ground which will allow part of the signal to be lost rather than fed into the amplifier. Thus, we cross-check our calibration by (a) knowing the gain of the amplifier attached to the cathode and (b) measuring the change of the mean of the charge spectrum obtained from the cathode when the anode is either coupled to ground through a 50 R or I M!ZI resistor. Even after this calibration procedure the uncertainty in the exact charge we estimate to be - 20%.
Fig. 2. Typical avalanche signal observed by an oscilloscope directly attached to the anode with a I MR input impedance.
4. Results vertical scale
5 mV/div
Horizontal scale
lCOIlS/diV
Fig. 3. Avalanche signal on expanded time scale. We attribute the first fast signal to the movement of the electrons. the fast ramp to the movement of the negative ions and the slow ramp (almost flat on this time scale) to the drift of the positive ions.
the output of this amplifier to the step observed for large avalanches. However, it should be noted that this step is not always a clean step for calibration. An example is shown in Fig. 3. Here one can observe the fast signal due to the electrons, a fast ramp that could be caused by the drift of negative ions (since the gas is electronegative), finally, followed by the slow ramp as the positive ions drift to the cathode. On a slow time scale (shown in Fig. 2) one cannot distinguish between the ramp of the negative ions and the fast electron signal; however it is
In Fig. 4 we show the efficiency plot of the RPC with the two gases under test. We also show the probability of producing a streamer. In the upper plot we show two curves; one is for ‘prompt’ streamer and the other for ‘any’ streamer. By ‘prompt’ streamer we mean that there has been more than 5 pC of charge within 50 ns of the original avalanche. The probability for ‘any’ streamer is obtained by inspecting the output from the RPC for some microseconds with the oscilloscope. The lower plot refers to the C,F,H gas mixture. With this gas, the streamer was always ‘prompt’ it does not have this characteristic delay between the avalanche and streamer as first observed by Cardarelli et al. [S]. Also, as discussed more fully below, C?F,H is more ‘quenching’ than C,F,H,; thus, any streamer produced is less violent and the transition between avalanche and streamer happens for a higher avalanche charge. Thus, we do not distinguish between ‘prompt’ and ‘any’ streamer for the lower plot. However, we also plot the occurrence of a signal > 20 pC within our 50 ns gate; this is a mixture of streamers and large avalanches. In Fig. 5 we show the ratio of ‘fast signal/total signal’ plotted as a function of fast charge. Ideally, this should be 1jrrD (where c( is the Townsend coefficient and D the size of the gas gap) if the attachment coefficient is zero. This is shown as a dotted line superimposed on the figure. Obviously, this ratio is well below this l/stD value for small avalanches. We assume the cause is that the ‘electronegative’ freons have a non-zero attachment coefficient. When the attachment coefficient is non-zero, we expect a ratio of fast/total signal to be (x’/Y).(l,/r’D) where z’ = c! - q and ‘1 is the attachment coefficient. There is a clear rise of the ratio of fast/total for increasing avalanche size: we attribute this to space charge effects. The
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electric field experienced by the electrons at the head of the avalanche is affected by the positive ions produced during the growth of the avalanche. These positive ions reduce the electric field and thus limit the growth of the avalanche. This will effectively move the centre of gravity of the electrons in the avalanche away from the anode, thus leading to an increase of this ratio of fast/total charge. As a comparison, we also include a measurement of this ratio for freon 13Bl that we have made previously [9]. Even though the systematic uncertainty is different between the two measurements, it is instructive to see an increase of the ratio of fast/total charge. It should be noted that only events producing single avalanches in the chamber are used for this measurement. Events that have
an after-pulse or streamer have been eliminated; thus the large number of data points corresponding to a fast signal in the range lo-20 pC with the C,F,H gas mixture indicates that large and clean avalanches can be produced with this mixture (i.e. no transition to streamers or after-pulses). In Fig. 6 we show the total charge produced by a single through-going cosmic ray as a function of voltage for the two gases. It is clear that for the C,F,H, mixture, the total charge is dominated by the production of streamers above the knee of the efficiency plateau. The lower probability of streamer and the lower intensity of the streamer discharge with the C2F,H gas mixture leads to a large reduction in the amount of charge in the gas gap.
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Fig. 5. Ratio of fast signal to total signal plotted as a function of fast charge for the two gas mixtures. We also show the theoretical of I/ND. Additionally, we show a measurement made previously with a mixture containing CBrF, [9].
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Fig. 6. Total charge produced in the gas gap by a single through-going gas mixtures under test. The right-hand axis is marked with power 100 Hz/cm’.
cosmic ray plotted as a function of applied voltage for the two dissipated in the gas gap per square metre assuming a flux of
Knowing
bility, knowing the amount of charge produced and the resistivity of the plates. We show, in Fig. 7, the rate necessary to produce a 200 V drop, assuming a resistivity of 4 x 10” Q cm and a plate thickness of 2 mm for both electrodes. Obviously, the rapid change in efficiency with voltage as shown in Fig. 4 would make a 200 V drop excessive unless one is working above the knee of the efficiency plateau. The high fraction and large charge produced by streamers in the C,F,H, gas mixtures
the
total
charge,
we can
in the gas gap assuming
calculate
the
power
a flux of 100 Hz/cm’ over a 1 m2 area. This is marked on the right-hand axis. Although 1 W/m’ does not seem a large source of power, it will heat the gas contained in this 2 mm thick volume by over 300°C in 1 h. Since the resistive plates are not good thermal conductors, some thought has to be given to the extraction of the heat even at this relatively low flux of 100 Hz/cm’. One can also estimate the rate capadissipation
Particle flux corresponding to 2(x) V drop
Knee of efficiency plateau
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High Voltage [kV] Fig. 7. Rate needed to produce a 200 V drop across the resistive plates assuming a resistivity of 4 x 1O”‘Rcm and a plate thickness of 2 mm for each electrode.
would result in a rate capability - 50 Hz/cm’. However, the C,F,H mixture is capable of being operated at - 500 Hz/cm2. In Fig. 8 we show the time spectrum obtained with a discriminator set with a -10 fC threshold for the C,F,H gas mixture for four voltages (11.3, 11.5, 11.9 and 12.3 kV). In this figure we also show the Full Width at Base (FWAB), which is a window that contains 99% of all the events. It is clear that for optimal timing one has to work with the voltage set well above the knee of the efficiency plateau. Similar results were obtained with the C2F,H2 gas mixture (i.e. one has to work at 400 V above the knee of the efficiency plateau to obtain a FWHM of 4 ns). In Fig. 9 we show the time walk with voltage. This large time walk ( - 2.5 ns/lOO V) will be of concern in critical timing applications. In Fig. 10 we show the charge spectrum at four voltages 11.3, 11.5, 11.9 and 12.3 kV (i.e. 200V below the knee, the knee, 400 and 800 V above) for the C,F,H gas mixture. Similar results were obtained with the C,F,H, gas mixture. The principle effect is the change from the exponential shape for low voltages to a Landau shaped distribution at the knee and then to the apparent Gaussian shape 800 V above the ‘knee’ voltage; however, it should be noted that the histogram corresponding to 12.3 kV only contains 10% of all events. There is a long tail containing the other 90%; thus, in reality it is not Gaussian. The change from exponential to the Landau shape is due to this strong limitation in the growth of large avalanches due to space-charge effects.
5. Discussion Our initial interest was to understand the differences in the shape of the charge spectra obtained by various groups working with a 2 mm gap RPC with C2F,H,. We find that the shape changes as one varies the applied voltage. Thus, the different shapes presented by various groups is just a reflection of operating the RPC at a different gas gain. The mechanism that we believe is responsible for this change in shape is the space charge of the positive ions created as the avalanche grows. This could also explain our measured rise in this ratio fast/total signal. Reducing the voltage produces an exponentialshaped charge spectrum. It is very difficult to work with this exponential-shaped spectrum since it requires an ever lower discriminator threshold. In order to move away from this exponential-shaped charge spectrum, we are forced to operate the RPC in a mode where the gas gain of large avalanches is limited by space charge (luckily there exists this space-charge limitation; otherwise these avalanches would all grow to a size that would trigger a transition to a streamer). In this mode of space-charge-limited avalanches, we do not expect an increase in the window of safe operating voltage (range of voltage at full efficiency with - 0% probability of streamer) by increasing the gas gap (to 3 mm, for example). In fact, one may find a reduction in the window of safe operation, since the density of positive ions in an avalanche for a 3 mm gap RPC will be less than a 2 mm gap RPC. It is the density of ions that give rise to
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efficiency plateau
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Fig. 8. Time spectra measured with the C2F,H gas mixture for four voltages. The full width half maximum (FWHM) is shown and the window
necessary
to contain
99% of the total events. (FWAB).
space-charge effects, which plays the all-important role of limiting the avalanche growth. C2F,H is a more suitable gas than C,F,H,; it is denser and has one less hydrogen per molecule - which could be important in environments where there is a high background of neutrons. It is more quenching, thus the transition to streamer occurs at larger avalanche size. Additionally, any streamer produced has a much lower total charge. However, it appears to have a larger attachment coefficient, thus working at very low gas gains would be difficult. For an effective mechanism to limit the growth of large avalanches, the gas gain has to vary rapidly with a change in electric field, otherwise the gas gain will not be modified by space charge of the positive ions. (It
should be noted that the efficiency of the C2F,H gas mixture does rise more quickly with voltage than the C2FSHL mixture, thus the gas gain does vary more rapidly with voltage). However, the rapid change of gain with a small change in the electric field has adverse side effects; the gas gain will vary rapidly with a small change in the gap dimension; the probability of streamers will increase rapidly with applied voltage (once the threshold for streamer production has been passed). One may enhance the window of safe operation if one can find a gas mixture that allows one to operate the RPC at a low applied voltage. It is the interplay between the field due to space charge and the electric field across the gas gap that leads to the reduction in the growth of the avalanche. However, a gas mixture that operates at a lower electric field
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may also have a slower drift velocity of the electrons; thus the time resolution could be degraded. The time resolution together with our measurement of time walk indicates that the use of a single-gap RPC may have marginal timing properties for LHC experiments. The fact that the streamers are delayed for the C,F,H, allows one to operate slightly above the threshold for streamer production. One can use the preceding (and clean) avalanche signal to generate the trigger signal. The large cluster of hit strips associated with the streamer occurs later. Nonetheless, there may be problems due to the large amount of produced charge (i.e. limited rate capability) and the extra noise in the system. It is instructive to question why this gas (C,F,H,) has delayed streamer production, while others gas mixtures do not. The transition between avalanche and streamer is initiated by the high electric field around the cloud of positive ions [lo]. This high field can directly ionise the gas triggering the production of a streamer. The field around the positive ions is partially negated by the electrons at the head of the avalanche; however. these electrons disappear when they enter the anode plate; this leads to a sudden increase in field around the positive ions. Since these gases are electronegative there are also negative ions generated in the avalanche which are interspersed among the positive ions. These will slowly drift out towards the anode leading to a further increase in field around the positive ions. On reaching the anode, the extra electron carried by the negative ions should jump across onto the anode (and disappear) allowing the neutralised ion to drift off. However, it is not clear how instantaneous this process is and how long the negative ions may sit on the surface of the anode. Thus, we have two processes which depend on the species of negative ions (thus, can be gas dependent): the drift speed of the negative ions and the speed that the negative ions are discharged on reaching the anode. As shown in Fig. 3, we have a signal that we associate with the drift of the negative ions with the C2F,H2 gas. Finally. we show some typical oscilloscope traces of streamers in the two gases. For this measurement we directly connect the anode to the oscilloscope: the input of the oscilloscope is terminated by 50 R. For the C2FdH2 mixture shown in Fig. 11 we are operating at 300 V above the knee of the efficiency plateau; Fig. 12 shows streamers in C,F,H at a voltage 1400 V above the knee of the efficiency plateau. It is clear that the streamers in C,F,H are more heavily quenched. The LHC experiments plan on building many thousand of square metres of RPC, with each chamber being of some square metres in size. One of the critical parameters for the mass production of these chambers will be the tolerance of the gas gap. A change in the dimension of this gap causes a change in electric field. Thus. if ‘streamer-free’ and high-eficiency operation can be achieved for a 200 V range with an applied voltage of
Avalanche
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Fig. 11. Typical streamer signals with CzFIHz gas mixture. The anode was directly connected to the oscilloscope with input termination of 50 R. Voltage set at 300 V above ‘knee’ of efficiency plateau.
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Fig. 13. Typical streamer signals with CLFsH gas mixture. The anode was directly connected to the oscilloscope with input termination of SO!2 Voltage set at 14OOV above ‘knee’ of efficiency plateau.
10 kV, this corresponds to change of 40 Fern in the 2 mm gap (i.e. the chambers have to be constructed with a tolerance of 0 - 12 pm). However, there is a correction to be made since a wider gap indeed has a lower field, but the avalanche has some extra distance to develop. We can estimate this correction since Ammosov et al. [4]
compare the operating voltage of a 2 and 3.5 mm chamber filled with 97”/;, C2F,H, and 3% iso-C,H,,. They report that the operating voltage increases from 8 to 13 kV for this increase of gap width; thus, the required tolerance is 60 urn (i.e. 0 - 60/,/12 = 17 pm). This is still a very tight tolerance!
cost of C7FSH is a factor - 3 more than CIFIHL; this could also be a critical constraint for large systems.
References [l]
6. Conclusions We have tested a 2 mm gap RPC with two types of freon. For both we find an increase in fast/total charge for increasing avalanche magnitude; this leads us to speculate that the growth of large avalanches is limited by space charge. We believe that this mechanism is also responsible for the change in shape of the charge spectrum. The shape changes from an exponential at low gas gain to a ‘Landau shaped’ spectrum at high gas gain. We find that the gas C2F5H has a larger voltage window of ‘safe operation’ (full efficiency with negligible streamers) and thus is a more suitable gas for use in narrow gap RPCs. Additionally we find that any streamers produced in C?F,H are ‘prompt’ but less violent than in C2FJHI. We have measured a large time-walk with voltage, which could limit the time resolution for large RPCs systems. Additionally, one needs to operate above the knee of the efficiency plateau to obtain a time resolution acceptable for LHC applications; however, at this elevated voltage the rate capability will be in the order of tens of Hz/cm’ for the CZFSH2 gas mixture and - 100 Hz /cm’ for the C2F5H gas mixture. Finally, the
[2] [3]
[4]
[S]
[6] 177 [8] [9] [IO]
ATLAS Technical proposal. CERN,,LHCC.‘94-43. LHCCiPZ ( 1994). The CMS technical proposal, CERN;‘LHCC/94-38. LHCC’PI (1994). M. Abbrescia et al., Performances of a Resistive Plate Chamber operated in avalanche mode under 137Cs irradiation. CMS note 97;003. Nucl. Instr-. and Meth. A.. submitted. V.V. Ammosov. V.A. Gapienko. V.A. Sen’koo. E.A. Usenko, I.L. Vasiliev. V.A. Zaets, Comparison between 1 mm and 3.5 mm gap RPCs operated at low gas gain. September 1996. Protvino. Russia. Unpublished Report. M.G. Alviggi. P. Bagnaia, P. Camarri. R. Cardarelli. P. Creti, R. de Asmundis. A. Di Ciaccio. E. Gorini, M. Primavera. L. Pontecorvo. P. Oberson. R. Santonico, S. Veneziano, Resistive Plate Chambers in ATLAS. ATLAS Internal Note. MUON-NO-131, 1 October, 1996. E. Cerron Zeballos et al. Nucl. Instr. and Meth. A 381 (1996) 569. 1. Crotty et al.. Nucl. Instr. Meth. A 360 (1995) 511. R. Cardarelli. V. Makeev. R. Santonico, Nucl. Instr. and Meth. A 382 (1996) 470. E. Cerron Zeballos et al.. Nucl. Instr. and Meth. A 373 (1996) 35. P. Fonte. A Model of Avalanche to Streamer Transition in PPC/RPC Detectors, talk given at 3rd Int. Workshop on Resistive Plate Chambers, Pavia, October 1995. Scientifica Acta XI (1996) 25.
Nuclear Instruments and Methods in Physics Research A 396 (1997) 93-102
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l!iB ELSEVIER
NUCLEAR INSTRUMENTS 8 METHODS IN PHYSICS RESEARCH SectjonA
Pure avalanche mode operation of a 2 mm gap resistive plate chamber E. Cerron
J. Lamas Valverdea,b,c, Zeballos”*b, I. Crotty”, D. Hatzifotiadou”.‘, M.C.S. Williams”*‘**, A. ZichichPd.”
Received 17 March 1997 Abstract It is necessary to operate the resistive plate chamber (RPC) in avalanche mode to obtain high efficiency at elevated particle fluxes. We examine this mode of operation with a 2 mm gap RPC using gas mixtures containing C2FSHZ and C2FSH. In order to explain the data we propose that the avalanche growth is strongly limited by space charge effects.
1. Introduction Resistive plate chambers (RPC) have been in use in various experiments since the 1980s; however. in all cases the flux of particles has been rather low. In the original Letters of Intent for experiments at LHC. the prime candidate for implementation of the muon trigger is the RPC. The proposed type of RPC has a 2 mm gas gap bounded by 2 mm thick (linseed-oil treated) Bakelite plates. In order to operate at theexpected flux of particles at the LHC experiments, both ATLAS and CMS [1,2] proposed operating these devices in the ‘low-gas gain’ mode (nowadays this mode is usually referred to as avalanche mode). There has been an extended study to find a suitable gas mixture; the proponents [3-51 of this type of RPC have proposed mixtures containing a high fraction of C,F,H, (an ecologically acceptable freon). The basic problem is to operate the RPC in avalanche mode with high efficiency and with only a very low contamination of streamers (i.e. pure avalanche mode). Data from various groups show a variety of charge spectra measured from this device. Abbrescia et al. [3] show an exponential shape, Ammosov et al. [4] show a Landau shaped distribution; while Alviggi et al. [S]
*Corresponding
author. E-mail: [email protected]
show a Gaussian shaped distribution. The ease of operation of this detector depends on the exact shape of these spectra; thus. we started a series of measurements in order to understand these apparent differences between various groups. We have simulated the charge spectrum using the program described in Ref. [6]. We assume that the number of primary ionisation clusters is 75jcm (obtained by scaling up by the atomic number of the molecule C3F4H2 from data for Argon) and have set the gas gain such that the average fast signal is 1 pC (assuming that fast/total charge is 1jrD and the attachment coefficient is zero; a is the Townsend coefficient and D the size of the gas gap). We show the result in Fig. 1 for three cases: (a) no avalanche fluctuations; (b) exponential fluctuations are applied to individual avalanches formed from each cluster of primary ionisation: (c) exponential fluctuation applied to a single merged avalanche. In all cases the shape is exponential with a long tail of high values of fast charge. For the wide gap RPC and multigap RPC [6] there were marked differences in the spectra when the fluctuations described above were applied; in that study we found that case(c) best described the measured charge spectrum. Transition to streamer production should begin when the avalanche contains more than lo8 ions (i.e. for fast signals greater than 1 PC); however, this limit may be higher in the presence of a strong quencher such as the freons used in this study. This exponential shaped charge
016%9002;97iSI 7.00 Copyright ,(‘: 1997 Elsevier Science B.V. All rights reserved PII SO1 6%9003(97)0075 1-l
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(a) No avalanche
fluctuations
(b) Individual avalanches smeared
I
(c) ‘Merged’ avalanche smeared
t,,,,1,,“1,,“‘,““““J 0
z
1
Magnitude
3
offast
4
s
signal [PC]
Fig. 1. Simulation of charge spectrum produced by a 2 mm gap RPC filled with C2F4H2. Three cases are shown: (a) no ava-
lanche fluctuations applied: (b) each cluster of primary ionisation creates an independent avalanche. the size of which is smeared by an exponential fluctuation: (c) all individual avalanches are merged into a single avalanche. which is then smeared by the exponential avalanche fluctuations.
spectrum (as generated by the simulation) indicates that (a) a very low threshold discriminator is needed and (b) already there is a significant number of large avalanches, which could trigger a transition to a streamer. However. this simulation does not agree with data presented by various groups, where they show that pure avalanche mode operation is possible; also the shape of our simulated charge spectrum is only consistent with data obtained by Abbrescia et al.
2. Experimental set-up The resistive plate chamber used in this study was constructed with resistive plates made from 0.8 mm thick melamine+phenolic laminate sheets of 32 x 32 cm’. The smooth melamine surface faced the gas, while the outer phenolic surfaces were painted with conductive paint (Nickel paint with surface resistivity l-2 0/n) to form pad electrodes of 24 x 24 cm”. The gas gap was bounded by a 2 cm wide and 2 mm thick PVC spacer around the perimeter. A layer of glue attached the resistive plates to these spacers, thus creating a gas gap between 2.1 and 2.2 mm. One electrode, the cathode, was connected with a 1 MQ resistor to a high voltage power supply; the signal developed on the cathode was read out through a 1 nF capacitor. This signal was fed into a fast current
amplifier, the output of which was inverted and split between an ADC and a discriminator. The other electrode was connected to ground either through the 1 MR input impedance of the oscilloscope or to an ADC with input impedance of 50 R. The exact set-up is described below where the measurements are discussed. We triggered on through-going cosmic rays with a series of scintillator counters selecting a 10 x IO cm’ area in the centre of the RPC. We performed two series of tests; for the first we used a gas mixture of 3% iso-C,H I ,) and 97% C2F,H,. Normally we add 1% water vapour to our gas mixtures to keep the resistivity of our melaminephenolic plates at a low value [7]. For the tests with this gas mixture we did not add water vapour. The voltage needed to reach the knee of the efficiency plateau slowly increased from 10.5 to 1 I.2 kV over the several week period of the tests. We attribute this rise to an increase in resistivity. thus generating an increasing voltage drop across the resistive plates. We have ignored this effect except to note that various plots will be labelled with voltage relative to the knee of the efficiency plateau so that comparisons can be made. For the second series of tests we used a gas mixture of 95% C,F,H (also ecologically friendly) and 5% iso-C,H,,bubbled through a water reservoir at 10 C in order to add 1% water vapour. For these tests the working voltage was more constant.
3. Measurement technique There are various types of measurements made with this chamber. We have measured the charge spectrum of the fast signal derived from the cathode pad. We have measured the time spectrum with the threshold of the discriminator set at 1OfC. We also measured the total charge produced by the avalanche or streamer. For this measurement we connected an oscilloscope directly to the anode, using the high impedance of the oscilloscope to connect the anode to ground potential. A typical oscilloscope display is shown in Fig. 2. The step is due to the movement of the electrons towards the anode; the long ramp is due to the drift of the positive ions toward the cathode. Eventually the voltage levels off when all the ions have been collected. The total voltage deviation can be converted to charge, as we have measured the total capacitance of the anode to ground. In this case it is 0.7 nF. We are interested in the ratio of fast charge/total charge, i.e. the size of the initial step to the total deviation. This is clearly visible in Fig. 2; however we are also interested in small avalanches, where the total deviation due to the collection of positive ions is several mV. Obviously, the sensitivity and noise of the system would make it difficult to measure the step directly. We. thus, also monitored the fast signal measured by a fast current amplifier attached to the cathode, having first calibrated
E. Cerrorl Zehallos et al. iNuc1. Imtr. and Meth. in Phy. Vertical scale
5 mV / div
Horizontal scale
5 ns I div
Rex A 396 (1997) 93-102
95
only the electron signal that produces the observed fast signal of the avalanche. Another factor that we have to take into account is the coupling between the cathode and anode. A positive going signal is generated on the cathode and negative on the anode; thus, any capacitive coupling between the two plates will lead to a reduction of the signal. This reduction will depend on how each electrode is decoupled to ground. Additionally there is capacitive coupling between the electrode and ground which will allow part of the signal to be lost rather than fed into the amplifier. Thus, we cross-check our calibration by (a) knowing the gain of the amplifier attached to the cathode and (b) measuring the change of the mean of the charge spectrum obtained from the cathode when the anode is either coupled to ground through a 50 R or I M!ZI resistor. Even after this calibration procedure the uncertainty in the exact charge we estimate to be - 20%.
Fig. 2. Typical avalanche signal observed by an oscilloscope directly attached to the anode with a I MR input impedance.
4. Results vertical scale
5 mV/div
Horizontal scale
lCOIlS/diV
Fig. 3. Avalanche signal on expanded time scale. We attribute the first fast signal to the movement of the electrons. the fast ramp to the movement of the negative ions and the slow ramp (almost flat on this time scale) to the drift of the positive ions.
the output of this amplifier to the step observed for large avalanches. However, it should be noted that this step is not always a clean step for calibration. An example is shown in Fig. 3. Here one can observe the fast signal due to the electrons, a fast ramp that could be caused by the drift of negative ions (since the gas is electronegative), finally, followed by the slow ramp as the positive ions drift to the cathode. On a slow time scale (shown in Fig. 2) one cannot distinguish between the ramp of the negative ions and the fast electron signal; however it is
In Fig. 4 we show the efficiency plot of the RPC with the two gases under test. We also show the probability of producing a streamer. In the upper plot we show two curves; one is for ‘prompt’ streamer and the other for ‘any’ streamer. By ‘prompt’ streamer we mean that there has been more than 5 pC of charge within 50 ns of the original avalanche. The probability for ‘any’ streamer is obtained by inspecting the output from the RPC for some microseconds with the oscilloscope. The lower plot refers to the C,F,H gas mixture. With this gas, the streamer was always ‘prompt’ it does not have this characteristic delay between the avalanche and streamer as first observed by Cardarelli et al. [S]. Also, as discussed more fully below, C?F,H is more ‘quenching’ than C,F,H,; thus, any streamer produced is less violent and the transition between avalanche and streamer happens for a higher avalanche charge. Thus, we do not distinguish between ‘prompt’ and ‘any’ streamer for the lower plot. However, we also plot the occurrence of a signal > 20 pC within our 50 ns gate; this is a mixture of streamers and large avalanches. In Fig. 5 we show the ratio of ‘fast signal/total signal’ plotted as a function of fast charge. Ideally, this should be 1jrrD (where c( is the Townsend coefficient and D the size of the gas gap) if the attachment coefficient is zero. This is shown as a dotted line superimposed on the figure. Obviously, this ratio is well below this l/stD value for small avalanches. We assume the cause is that the ‘electronegative’ freons have a non-zero attachment coefficient. When the attachment coefficient is non-zero, we expect a ratio of fast/total signal to be (x’/Y).(l,/r’D) where z’ = c! - q and ‘1 is the attachment coefficient. There is a clear rise of the ratio of fast/total for increasing avalanche size: we attribute this to space charge effects. The
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Streamers
11.0
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High \bltage [kVl Fig. 4. Efficiency versus voltage for the two gas mixtures. We also show, the probability of a streamer.
electric field experienced by the electrons at the head of the avalanche is affected by the positive ions produced during the growth of the avalanche. These positive ions reduce the electric field and thus limit the growth of the avalanche. This will effectively move the centre of gravity of the electrons in the avalanche away from the anode, thus leading to an increase of this ratio of fast/total charge. As a comparison, we also include a measurement of this ratio for freon 13Bl that we have made previously [9]. Even though the systematic uncertainty is different between the two measurements, it is instructive to see an increase of the ratio of fast/total charge. It should be noted that only events producing single avalanches in the chamber are used for this measurement. Events that have
an after-pulse or streamer have been eliminated; thus the large number of data points corresponding to a fast signal in the range lo-20 pC with the C,F,H gas mixture indicates that large and clean avalanches can be produced with this mixture (i.e. no transition to streamers or after-pulses). In Fig. 6 we show the total charge produced by a single through-going cosmic ray as a function of voltage for the two gases. It is clear that for the C,F,H, mixture, the total charge is dominated by the production of streamers above the knee of the efficiency plateau. The lower probability of streamer and the lower intensity of the streamer discharge with the C2F,H gas mixture leads to a large reduction in the amount of charge in the gas gap.
E. Cerron Zehallos et al. / Nucl. Instv. and Meth. in PhJs. Rex A 396 (19Y7) Y3Sl0-7
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-g
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Fig. 5. Ratio of fast signal to total signal plotted as a function of fast charge for the two gas mixtures. We also show the theoretical of I/ND. Additionally, we show a measurement made previously with a mixture containing CBrF, [9].
u” _a A gJ00 5 3 c-
r 1 -
value
knee of efficiency plateau: C2F4H2
V
knee of efficiency
11
10.5
11.5 High bItage
[kV]
Fig. 6. Total charge produced in the gas gap by a single through-going gas mixtures under test. The right-hand axis is marked with power 100 Hz/cm’.
cosmic ray plotted as a function of applied voltage for the two dissipated in the gas gap per square metre assuming a flux of
Knowing
bility, knowing the amount of charge produced and the resistivity of the plates. We show, in Fig. 7, the rate necessary to produce a 200 V drop, assuming a resistivity of 4 x 10” Q cm and a plate thickness of 2 mm for both electrodes. Obviously, the rapid change in efficiency with voltage as shown in Fig. 4 would make a 200 V drop excessive unless one is working above the knee of the efficiency plateau. The high fraction and large charge produced by streamers in the C,F,H, gas mixtures
the
total
charge,
we can
in the gas gap assuming
calculate
the
power
a flux of 100 Hz/cm’ over a 1 m2 area. This is marked on the right-hand axis. Although 1 W/m’ does not seem a large source of power, it will heat the gas contained in this 2 mm thick volume by over 300°C in 1 h. Since the resistive plates are not good thermal conductors, some thought has to be given to the extraction of the heat even at this relatively low flux of 100 Hz/cm’. One can also estimate the rate capadissipation
Particle flux corresponding to 2(x) V drop
Knee of efficiency plateau
I
11.5
12
6
12.5
High Voltage [kV] Fig. 7. Rate needed to produce a 200 V drop across the resistive plates assuming a resistivity of 4 x 1O”‘Rcm and a plate thickness of 2 mm for each electrode.
would result in a rate capability - 50 Hz/cm’. However, the C,F,H mixture is capable of being operated at - 500 Hz/cm2. In Fig. 8 we show the time spectrum obtained with a discriminator set with a -10 fC threshold for the C,F,H gas mixture for four voltages (11.3, 11.5, 11.9 and 12.3 kV). In this figure we also show the Full Width at Base (FWAB), which is a window that contains 99% of all the events. It is clear that for optimal timing one has to work with the voltage set well above the knee of the efficiency plateau. Similar results were obtained with the C2F,H2 gas mixture (i.e. one has to work at 400 V above the knee of the efficiency plateau to obtain a FWHM of 4 ns). In Fig. 9 we show the time walk with voltage. This large time walk ( - 2.5 ns/lOO V) will be of concern in critical timing applications. In Fig. 10 we show the charge spectrum at four voltages 11.3, 11.5, 11.9 and 12.3 kV (i.e. 200V below the knee, the knee, 400 and 800 V above) for the C,F,H gas mixture. Similar results were obtained with the C,F,H, gas mixture. The principle effect is the change from the exponential shape for low voltages to a Landau shaped distribution at the knee and then to the apparent Gaussian shape 800 V above the ‘knee’ voltage; however, it should be noted that the histogram corresponding to 12.3 kV only contains 10% of all events. There is a long tail containing the other 90%; thus, in reality it is not Gaussian. The change from exponential to the Landau shape is due to this strong limitation in the growth of large avalanches due to space-charge effects.
5. Discussion Our initial interest was to understand the differences in the shape of the charge spectra obtained by various groups working with a 2 mm gap RPC with C2F,H,. We find that the shape changes as one varies the applied voltage. Thus, the different shapes presented by various groups is just a reflection of operating the RPC at a different gas gain. The mechanism that we believe is responsible for this change in shape is the space charge of the positive ions created as the avalanche grows. This could also explain our measured rise in this ratio fast/total signal. Reducing the voltage produces an exponentialshaped charge spectrum. It is very difficult to work with this exponential-shaped spectrum since it requires an ever lower discriminator threshold. In order to move away from this exponential-shaped charge spectrum, we are forced to operate the RPC in a mode where the gas gain of large avalanches is limited by space charge (luckily there exists this space-charge limitation; otherwise these avalanches would all grow to a size that would trigger a transition to a streamer). In this mode of space-charge-limited avalanches, we do not expect an increase in the window of safe operating voltage (range of voltage at full efficiency with - 0% probability of streamer) by increasing the gas gap (to 3 mm, for example). In fact, one may find a reduction in the window of safe operation, since the density of positive ions in an avalanche for a 3 mm gap RPC will be less than a 2 mm gap RPC. It is the density of ions that give rise to
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efficiency plateau
400 V above knee
of efficiency plateau
800 V above knee of efficiency plateau
Fig. 8. Time spectra measured with the C2F,H gas mixture for four voltages. The full width half maximum (FWHM) is shown and the window
necessary
to contain
99% of the total events. (FWAB).
space-charge effects, which plays the all-important role of limiting the avalanche growth. C2F,H is a more suitable gas than C,F,H,; it is denser and has one less hydrogen per molecule - which could be important in environments where there is a high background of neutrons. It is more quenching, thus the transition to streamer occurs at larger avalanche size. Additionally, any streamer produced has a much lower total charge. However, it appears to have a larger attachment coefficient, thus working at very low gas gains would be difficult. For an effective mechanism to limit the growth of large avalanches, the gas gain has to vary rapidly with a change in electric field, otherwise the gas gain will not be modified by space charge of the positive ions. (It
should be noted that the efficiency of the C2F,H gas mixture does rise more quickly with voltage than the C2FSHL mixture, thus the gas gain does vary more rapidly with voltage). However, the rapid change of gain with a small change in the electric field has adverse side effects; the gas gain will vary rapidly with a small change in the gap dimension; the probability of streamers will increase rapidly with applied voltage (once the threshold for streamer production has been passed). One may enhance the window of safe operation if one can find a gas mixture that allows one to operate the RPC at a low applied voltage. It is the interplay between the field due to space charge and the electric field across the gas gap that leads to the reduction in the growth of the avalanche. However, a gas mixture that operates at a lower electric field
00
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Knee of efficiency plateau C2F4H2
F
Knee of efficiency plateau C2F5H
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Fig. 9. Time walk with voltage
for two gas mixtures.
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may also have a slower drift velocity of the electrons; thus the time resolution could be degraded. The time resolution together with our measurement of time walk indicates that the use of a single-gap RPC may have marginal timing properties for LHC experiments. The fact that the streamers are delayed for the C,F,H, allows one to operate slightly above the threshold for streamer production. One can use the preceding (and clean) avalanche signal to generate the trigger signal. The large cluster of hit strips associated with the streamer occurs later. Nonetheless, there may be problems due to the large amount of produced charge (i.e. limited rate capability) and the extra noise in the system. It is instructive to question why this gas (C,F,H,) has delayed streamer production, while others gas mixtures do not. The transition between avalanche and streamer is initiated by the high electric field around the cloud of positive ions [lo]. This high field can directly ionise the gas triggering the production of a streamer. The field around the positive ions is partially negated by the electrons at the head of the avalanche; however. these electrons disappear when they enter the anode plate; this leads to a sudden increase in field around the positive ions. Since these gases are electronegative there are also negative ions generated in the avalanche which are interspersed among the positive ions. These will slowly drift out towards the anode leading to a further increase in field around the positive ions. On reaching the anode, the extra electron carried by the negative ions should jump across onto the anode (and disappear) allowing the neutralised ion to drift off. However, it is not clear how instantaneous this process is and how long the negative ions may sit on the surface of the anode. Thus, we have two processes which depend on the species of negative ions (thus, can be gas dependent): the drift speed of the negative ions and the speed that the negative ions are discharged on reaching the anode. As shown in Fig. 3, we have a signal that we associate with the drift of the negative ions with the C2F,H2 gas. Finally. we show some typical oscilloscope traces of streamers in the two gases. For this measurement we directly connect the anode to the oscilloscope: the input of the oscilloscope is terminated by 50 R. For the C2FdH2 mixture shown in Fig. 11 we are operating at 300 V above the knee of the efficiency plateau; Fig. 12 shows streamers in C,F,H at a voltage 1400 V above the knee of the efficiency plateau. It is clear that the streamers in C,F,H are more heavily quenched. The LHC experiments plan on building many thousand of square metres of RPC, with each chamber being of some square metres in size. One of the critical parameters for the mass production of these chambers will be the tolerance of the gas gap. A change in the dimension of this gap causes a change in electric field. Thus. if ‘streamer-free’ and high-eficiency operation can be achieved for a 200 V range with an applied voltage of
Avalanche
, (pre-cursor)
101
Venical scale
50 mV/div
Horizontal scale
100 ns/div
Fig. 11. Typical streamer signals with CzFIHz gas mixture. The anode was directly connected to the oscilloscope with input termination of 50 R. Voltage set at 300 V above ‘knee’ of efficiency plateau.
I
I
I
1
vertical scale
50 mV/div
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100 11s/ div
/
1
/
I
J
Fig. 13. Typical streamer signals with CLFsH gas mixture. The anode was directly connected to the oscilloscope with input termination of SO!2 Voltage set at 14OOV above ‘knee’ of efficiency plateau.
10 kV, this corresponds to change of 40 Fern in the 2 mm gap (i.e. the chambers have to be constructed with a tolerance of 0 - 12 pm). However, there is a correction to be made since a wider gap indeed has a lower field, but the avalanche has some extra distance to develop. We can estimate this correction since Ammosov et al. [4]
compare the operating voltage of a 2 and 3.5 mm chamber filled with 97”/;, C2F,H, and 3% iso-C,H,,. They report that the operating voltage increases from 8 to 13 kV for this increase of gap width; thus, the required tolerance is 60 urn (i.e. 0 - 60/,/12 = 17 pm). This is still a very tight tolerance!
cost of C7FSH is a factor - 3 more than CIFIHL; this could also be a critical constraint for large systems.
References [l]
6. Conclusions We have tested a 2 mm gap RPC with two types of freon. For both we find an increase in fast/total charge for increasing avalanche magnitude; this leads us to speculate that the growth of large avalanches is limited by space charge. We believe that this mechanism is also responsible for the change in shape of the charge spectrum. The shape changes from an exponential at low gas gain to a ‘Landau shaped’ spectrum at high gas gain. We find that the gas C2F5H has a larger voltage window of ‘safe operation’ (full efficiency with negligible streamers) and thus is a more suitable gas for use in narrow gap RPCs. Additionally we find that any streamers produced in C?F,H are ‘prompt’ but less violent than in C2FJHI. We have measured a large time-walk with voltage, which could limit the time resolution for large RPCs systems. Additionally, one needs to operate above the knee of the efficiency plateau to obtain a time resolution acceptable for LHC applications; however, at this elevated voltage the rate capability will be in the order of tens of Hz/cm’ for the CZFSH2 gas mixture and - 100 Hz /cm’ for the C2F5H gas mixture. Finally, the
[2] [3]
[4]
[S]
[6] 177 [8] [9] [IO]
ATLAS Technical proposal. CERN,,LHCC.‘94-43. LHCCiPZ ( 1994). The CMS technical proposal, CERN;‘LHCC/94-38. LHCC’PI (1994). M. Abbrescia et al., Performances of a Resistive Plate Chamber operated in avalanche mode under 137Cs irradiation. CMS note 97;003. Nucl. Instr-. and Meth. A.. submitted. V.V. Ammosov. V.A. Gapienko. V.A. Sen’koo. E.A. Usenko, I.L. Vasiliev. V.A. Zaets, Comparison between 1 mm and 3.5 mm gap RPCs operated at low gas gain. September 1996. Protvino. Russia. Unpublished Report. M.G. Alviggi. P. Bagnaia, P. Camarri. R. Cardarelli. P. Creti, R. de Asmundis. A. Di Ciaccio. E. Gorini, M. Primavera. L. Pontecorvo. P. Oberson. R. Santonico, S. Veneziano, Resistive Plate Chambers in ATLAS. ATLAS Internal Note. MUON-NO-131, 1 October, 1996. E. Cerron Zeballos et al. Nucl. Instr. and Meth. A 381 (1996) 569. 1. Crotty et al.. Nucl. Instr. Meth. A 360 (1995) 511. R. Cardarelli. V. Makeev. R. Santonico, Nucl. Instr. and Meth. A 382 (1996) 470. E. Cerron Zeballos et al.. Nucl. Instr. and Meth. A 373 (1996) 35. P. Fonte. A Model of Avalanche to Streamer Transition in PPC/RPC Detectors, talk given at 3rd Int. Workshop on Resistive Plate Chambers, Pavia, October 1995. Scientifica Acta XI (1996) 25.