- Email: [email protected]
Effect of the linseed oil surface treatment on the performance of resistive plate chambers
Nuclear Instruments and Methods in Physics Research
A 394
( 1997) 13-20
NUCLEAR INSTRUMENTS &METNODS IN PHVWCS RESEARCH SectfonA
ELSEYIER
Effect of the linseed oil surface treatment on the performance of resistive plate chambers M. Abbrescia”,
A. Colaleoa, G. Iaselli”, M. Maggi”, B. Marangelli”, S. Natali”, F. Romanoa, V. Arenab, G. Bonomib, A. Brajb, G. Gianinib, G. Liguorib, S.P. Rattib, C. Riccardib, L. Violab, P. Vitulob**
S. Nuzzoa. A. Ranieri”,
a Dipnrfimerlto d Fisk
and INFN. Buri, Itrrl~,
Received 22 July 1996; received in revised form I6 September
1996
Abstract Results on the behaviour of several bakelite Resistive Plate Chambers (RPCs) without the linseed oil the internal electrodes will be presented. Efficiency, collected charge and cluster size distributions will be the ones of a standard oiled RPC. Currents and single rate are the quantities most affected by the surface the electrodes beyond the optical/mechanical properties. A factor 4 less in currents and at least a factor 10 rate is achieved using standard oiled RPCs operated in streamer mode.
1. Introduction Over the last few years a considerable amount of work has been performed in the RPCs field [ 11, due to the need for a good operation in the LHC environment. Leading order problems are mostly solved: results on high rates capability with non-ozone-depleting gas mixtures [2] are now available and they confirm the good behaviour of these detectors. Recently, concerns have been raised about the linseed oil treatment of the electrodes which was introduced many years ago [3] to reduce the dark current of these detectors. at that time operated in streamer mode [4]. These concerns are twofold: (a) the oil treatment results in additional production time when massive production is involved, as it is the case of LHC experiments (typically several thousands square meters of active areas); (b ) the heavy ionizing background environment foreseen at LHC requires a careful study of the possible ageing properties and radiation damage of the linseed oil. In this paper we want to study systematically how the linseed oil treatment affects the performances of the RPC chambers.
* Corresponding author. Tel.: +39 382 507 268; 526 938; e-mail: [email protected]. 0 16%9002/97/$I7.00 f’?( SO1 08.9002(97
fax:
+39 382
treatment of compared to treatment of less in single
We compare the behaviour of oiled and non oiled detectors made of bakelite produced with present available technology. The ageing problem will be addressed in another paper. The radiation damage has been preliminarily studied by introducing bakelite samples into the channel of an intense neutron flux from a research reactor [5]. Typical crude linseed oil [6] is a natural organic mixture of mainly linolenic acid (SO-65%), linoleic acid ( 14-24%) and oleic acid ( 16-26%). It is commonly used, after heating refining, in special paints and coatings. In many commercial products of this type other desiccative metals are added to guarantee the drying process. Since the main interest of this paper is the effect of the linseed oil treatment, we decided to operate the chambers in streamer mode. It is well known however that these chambers can be operated in avalanche mode [4] with low gas gain and front-end amplifiers.
2. Linseed oil treatment The treatment of the RPC electrodes was done following the traditional procedure by filling the full gas volume of an assembled RPC with linseed oil. then the oil is slowly taken out. The resulting effect is the deposition of a thin layer of oil on both bakelite surfaces facing the gas volume.
Copyright @ 1997 Elsevier Science B.V. All rights reserved )00597-4
14
M. Ahhwsciu
et al. I Nucl. Instr. und Meth. in Phys. Rex A 394 (1997)
(a)
13-20
It is easy to guess that the oil might have been used to smoothen the surface of the resistive electrodes and that modern technology might produce smoother bakelite sheets which might make the treatment unnecessary. We took photographs of bakelite samples before and after the oil treatment, before and after a 1000 s exposure to a ( < 0.2 eV) flux of 3 x 10” neutrons/(cm’ s). Different situations are pictured in Fig. 1: Fig. 1(a) shows the cross section of an oiled surface from which we estimated the thickness of the oil layer to be 35-40 pm, depending on the depth of the bakelite imperfections into which the oil seeps. Figs. l(b) and (c) show, respectively, part of the surface of “non-oiled’ and “oiled” bakelite plates before irradiation. It is evident that the oil improves the smoothness of the surface. Surface imperfections seen in Fig. l(b) as white spots have diameter of few microns which mostly disappear with the oil treatment, as shown in Fig. 1(c ). The big white spots on the bottom left comer of the picture [Fig. l(c)] are the result of scratching the surface sample with a needle: in such a way we could inspect the same region after the irradiation. By visual inspection of the irradiated samples, with a 600X magnification, it is clearly seen that the appearance of the surfaces is not damaged by neutrons.
3. Experimental setup
Fig. I. (a) Cross section of an “oiled” bakelite sample showing the thickness of the linseed oil layer; (b) Surface of a “non-oiled” bakelite sample before irradiation; (c) Surface of an “oiled” bakelite sample before irradiation.
The test has been carried out using 2mm double gap RPCs, whose cross section is shown in Fig. 2. The detector is made of two gaps with common inner readout aluminum strips ( I .3 cm wide). The high voltage (hereafter HV) supply can be applied independently to each gap so that the chamber can be operated either in double gap or single gap mode, by turning off one of the supplies. The area of the detector is 50 x 50 cm’. The bakelite plates, of resistivity 0.4 x 10’ ’ 12cm. are 0.2 cm thick. The RPCs were operated in streamer mode and filled with an argon/isobutane/freon 13Bl (48%/48%/4%) gas mixture. The experimental setup (sketched in Fig. 3 ) consists of a cosmic rays telescope made of 5 horizontal RPCs, 4 of which were not oiled at the beginning. The fifth chamber is a standard (oiled) RPC used as reference detector for the various measurements. The reference chamber is one year old and has been previously used for several tests [7]. Three scintillator counters acting as triggering system define a IO x 10 cm’ trigger area. The strips have been connected at both ends to different electronics to allow simultaneously charge and time measurements. Signals from one end of the strips have been input to CAEN C205A Analog Digital Converters (ADCs), with a
PVC foil C---
graphite coating
readout
strips
--Q PVC spacer-b
Fig. 1. Cross section (not to scale) of a double gap chamber
RPC wth common strip readout.
After a month of running, RPCs I,3 and 4 were treated with linseed oil and a new set of measurements were performed to inspect all possible changes in the behaviour of the detectors. WC1
4.1. Currm ts uml sinylr RPC 2
APC 3 i?3%R WC4
Fig. 3. Sketch of the experimental setup: a telescope made of tive double gap RPCs were triggered by a threefold scmtillators coincidence. TDC’s and ADC’s electronics (not shown) have been
connected as explained in the text.
0.25 pC channel sensitivity. Signals from the other end have been discriminated and sent to LeCroy 2228A Time Digital Converters ( TDCs). This scheme allows two independent measurements of the efticiency.
4. Experimental results The experimental results are summarized in the following figures. Non-oiled RPCs will be labeled l-4 and “R” will indicate the standard oiled reference detector. When the detectors are operated in single gap mode the additional labels A and B are used to indicate the two different gaps.
rates
Fig. 4 shows the behaviour of the average dark currents of the single gaps as a function of HV before and after the oil treatment. The error bars show one standard deviation spread of the currents in the 8 single gaps used separately. After the treatment of RPCs I, 3 and 4. the values of the dark currents are. at least, a factor 4 less. For comparison the average currents of the two gaps of the reference chamber are also shown. Single rates were measured counting the ORcd signals from 8 strips. The threshold for non oiled chambers was set to -8OmV: to be conservative the threshold for the oiled case was set to -50 mV. Fig. 5(a) shows the average single rate, scaled to a square meter area. for all non oiled gaps. Fig. 5(b) shows. for comparison, the average OR rate of gaps I A/B. 3A;B and 4A:‘B after the treatment with the linseed oil. The behaviour of the reference chamber is also shown. In spite of the lower threshold setting. we see that the oil treatment decreases the rate by an order of magnitude. A significant local rate effect for the non-oiled electrodes has also been observed. Fig. 6(a) displays the rate measured on each single strip at HV = 9 kV. It may vary from I to IO kHz. The local rate, however. decreases significantly for the newly oiled RPCs ( I, 3 and 4), as shown in Fig. 6( b 1 (HV = 9 kV ), in spite of the lower threshold. The local behaviour of the oiled RPCs is much more homogeneous. In addition the two peaks observed for strip
M. Abbrescia et (11.I Nucl. Instr. and Meth. in Phys. Rex A 394 (1997)
16
13-20
L?
2
05
“E ,” 250 In x 0 In
0
All non oiled chambers
v
Chamber
6 ; 200
n
Chambers
R 1.3,4
after
oil treatment
g ; 2
Fig. 4. Current versus HV averaged as reference. Error bars correspond
on all non-oiled (open cross) and oiled gaps (full square); chamber R behaviour to one standard deviation.
is shown
gaps; threshold
-80 mV
._
,-1ooo a)
I
z 800
;
(full triangle)
700
? 600
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g 500 &400 z 200
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8000
8500
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9000
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9500
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1’1’
10000
1’
13
1.3,4 after Oil treatment
n
Reference
Chamber
7000
7500
8000
8500
9000
9500
10000 10500 HV (Volt)
Fig. 5. Average single rate (as explained in the text) versus HV at a fixed discriminating (b) Oiled gaps; threshold -50 mV. The chamber R is shown as reference.
threshold:
(a) Non-oiled
M. Ahhresciu et al. I Nucl. Instr. and Moth. in Phrs. Rex A 394
Not Oiled T’OI
( 1997) 13mXl
17
Oiled
I
4A
o
F,_,
0
2
4
6
8 Strip number
Fig. 6. (a) Comparison
of the strip rate distribution
Strip rumber
for the non-oiled gaps at HV = 9 KV and for a discriminating threshold of -80 mV; I, 3 and 4 at HV = 9 KV and for a discriminating threshold of -50 mV.
(b) Strip rate distribution for the oiled gaps of chambers
2-gap 3B and strip 7-gap 3A are probably due to the presence of a spacer. Also one strip of chamber 4 was disconnected.
Efficiencies were calculated from the ADCs information by requiring a charge signal to be present above a certain threshold. The threshold was set to a 50mV signal equivalent; this corresponds to a charge of 1OpC assuming a triangular signal, 20 ns wide at the base and readout on a 5OU load. This approach might overestimate the efficiency at low HV; however the plateau values are comparable to those obtained with other methods (i.e. TDCs and scalers). The reason for using the charge distributions in the efficiencies calculation is that not all the data were available with the TDCs information. On the other hand we are interested only in the behaviour of the chambers in the plateau region. Figs. 7(a)-(d) (Fig. 7(a), chamber 1; Fig. 7(b), chamber 2; Fig. 7(c), chamber 3; Fig. 7(d), chamber 4) show
the efficiencies for the two gaps A and B of the 4 different chambers - non-oiled case - while Fig. 7(e) shows, for comparison, the efficiency for the two gaps A and B of the reference oiled chamber. The absence of the surface treatment of the electrodes does not seem to affect the plateau efficiency of the RPCs. In some cases however (Figs. 7(a) and (d)) the knee of the plateau is shifted about 500V higher than those of the reference gaps (Fig. 7(e)). This behaviour has still to be investigated and is beyond the purpose of this paper. For further comparison. Fig. 7(f) shows the etficiency of chamber 1 after oil treatment. The plateau values are substantially unchanged by the treatment. We leave the discussion of the behaviour in the low HV region to future investigation.
4.3. Charge distributions The charge distributions have been obtained by integrating for 250 ns any analog signal induced on 14 strips in coincidence with a trigger event.
M. Abbrescia
et al. INucl. Instr. and Meth. in Phys. Rex A 394 (1997)
El00
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75
.a 2 i;
50 2.5 0
I&bG.
LLLLLLL
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75 50 25
OW
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8100
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$ 75 .!? 2 50 E 25 7
8
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a
9
HV (KV)
HV (kV)
Fig. 7. Efficiency versus HV for the non-oiled gaps (simulated threshold 2; (c) Chamber 3; (d) Chamber 4: (e) Reference Chamber; (f) Chamber
8.5
as explained in the text: -5OmV): 1 after oil treatment.
0.6
0.4
0.4
0.2
0.2
400
600
GAP 1A -
1; (b) Chamber
KV
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800 Charge
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(PC)
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GAP 28 -
800 Charge
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FL
0 1 ‘; 8 i 0.8
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KV
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0.4 0.2 0
200
400
600
GAP 48 -
800
I(
Charge
(PC)
GAP RA
Charge
(PC)
Fig. 8. Integral charge distributions. Histograms for gaps IA , 2B, 48 refer to the non-oiled case. For each value on the abscissa corresponding ordinate represents the percentage of events for which the measured charge was greater than that value.
the
hl. Abbrrsciu et (11.I Nucl. Imtr. und Mrth. in Piiys. Rex A 394 I19971 13 20
I9
HV= 8.5 KV
200
Strip number
7
y 450 HV= 8.5 KV
4co 350
-
w 330
i~
250 200 ‘50 100 r.0 0-
I
1 I1
d
5
10
15 Strip vumbe.
Fig. 9. Cluster size distributions. Histograms for gaps IA, 3A. 4A refer to the non-oiled detectors while the histogram for gap RA corresponds to one gap of the reference
chamber.
For each channel the pedestal was subtracted and the rest summed to obtain the total charge distribution collected over the area subtended by 14 strips. The results are presented as integral distributions in ‘greater-than-charge’ histograms. For each charge value on the abscissa the corresponding ordinate value represents the percentage of events with charge above that value. Fig. 8 shows, as an example, the integral distributions for the non-oiled gaps 1A, 2B, 4A and for the reference gap RA at HV = 8.5 KV.
4.4. C’htcr
six
Cluster size has been calculated via the TDC information: each discriminated strip signal arriving within 200 ns from the trigger leading edge contributed to the cluster size distribution. As for efficiency and collected charge, also the cluster size does not seem to be significantly affected by the absence of the oil treatment. Fig. 9 shows the distributions for the non oiled gaps 1A, 3A. 4A and for the reference gap RA at HV = 8.5 KV. We cannot exclude that part of the tail of large clusters is caused by cosmic ray showers. The excess of large clusters seen in gap 3A could be due to the higher cross-talk level of the readout board.
5. Conclusions Surface treatment of the internal bakelite electrodes is a necessary step to reduce the noise and the dark current of bakelite RPCs operated in streamer mode. The overall result seems to be the smoothness of the surface. However. the absence of the oiling agent does not sensitively affect other parameters such as efficiency, cluster size and charge distribution. It is not excluded however that the effect of the linseed oil (i.e. a smoother surface) could be directly obtained within the industrial process of the bakelite foil production. Recent tests [8] with chambers made with smoother surfaced bakelite compared with linseed oiled bakelite have shown similar performances. Nevertheless. the oil treatment does not degrade in any way the performance of an RPC, at least when operated in streamer mode. This paper leaves open the investigation of the ageing effects and the occasional shift of the knee in the efficiency curves. Several measurements of ageing are scheduled at the Triga Mark II Nuclear Reactor in Pavia.
References [I] (a) Proc. II Int. Workshop on the Resistwe Plate Chambers Particle
Physics
and Astrophysics.
Scientifica
in Acta (Quadern
20
M. Abbresciu et al. INucl. Imtr. and Meth. in Phys. Rex A 394 (1997)
del Dottorato) VIII (3) (1993), Univ. of Pavia; (b) Proc. III Int. Workshop on Resistive Plate Chambers and Related Detectors. Scientifica Acta (Quademi del Dottorato) XI (1) ( 1996) Univ. of Pavia. [2] M. Abbrescia et al., Scientifica Acta XI (1996) 217; A. Di Ciaccio. in: J. Lemonne, C. Vandez Velde, F. Verbeure (Ed.), IECHEP Proc., World Scientific. 1996, p. 605; A. Di Ciaccio, Scientifica Acta XI (I 996) 263; E. Cerron Zeballos et al., Nucl. Instr. and Meth. A 367 (I 995) 388; C. Bacci et al., NucI. Instr. and Meth. A 352 (1995) 552; M. Abbrescia et al., Properties of C~H~F;I based mixtures for avalanche mode operation of Resistive Plate Chambers, Nucl. Instr. and Meth., submitted. [3] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. 187 (1981 ) 377-380.
13-20
[4] P. Fonte, Scientifica Acta XI (1996) 25; R. Cardarelli. R. Santonico. V. Makeev, Scientifica Acta XI (1996) I I: 1. Duerdoth et al., Nucl. Instr. and Meth. A 348 (1994) 303; 1. Crotty et al., Nucl. Instr. and Meth. A 337 (1994)370. [5] Rapport0 finale di Sicurezza, Universiti di Pavia Laboratorio di Energia Nucleare Applicata (L.E.N.A.), 1964, Unpublished. [6] Ullmann’s Encyclopedia of Industrial Chemistry A 10 ( 1994) 227. [7] M. Abbrescia et al., Results of November 1994 Resistive Plate Chambers test beam CMS Muon Group Meeting at CERN, 1995. [8] Bari-Pavia CMS test beam I996 (work in progress).
A 394
( 1997) 13-20
NUCLEAR INSTRUMENTS &METNODS IN PHVWCS RESEARCH SectfonA
ELSEYIER
Effect of the linseed oil surface treatment on the performance of resistive plate chambers M. Abbrescia”,
A. Colaleoa, G. Iaselli”, M. Maggi”, B. Marangelli”, S. Natali”, F. Romanoa, V. Arenab, G. Bonomib, A. Brajb, G. Gianinib, G. Liguorib, S.P. Rattib, C. Riccardib, L. Violab, P. Vitulob**
S. Nuzzoa. A. Ranieri”,
a Dipnrfimerlto d Fisk
and INFN. Buri, Itrrl~,
Received 22 July 1996; received in revised form I6 September
1996
Abstract Results on the behaviour of several bakelite Resistive Plate Chambers (RPCs) without the linseed oil the internal electrodes will be presented. Efficiency, collected charge and cluster size distributions will be the ones of a standard oiled RPC. Currents and single rate are the quantities most affected by the surface the electrodes beyond the optical/mechanical properties. A factor 4 less in currents and at least a factor 10 rate is achieved using standard oiled RPCs operated in streamer mode.
1. Introduction Over the last few years a considerable amount of work has been performed in the RPCs field [ 11, due to the need for a good operation in the LHC environment. Leading order problems are mostly solved: results on high rates capability with non-ozone-depleting gas mixtures [2] are now available and they confirm the good behaviour of these detectors. Recently, concerns have been raised about the linseed oil treatment of the electrodes which was introduced many years ago [3] to reduce the dark current of these detectors. at that time operated in streamer mode [4]. These concerns are twofold: (a) the oil treatment results in additional production time when massive production is involved, as it is the case of LHC experiments (typically several thousands square meters of active areas); (b ) the heavy ionizing background environment foreseen at LHC requires a careful study of the possible ageing properties and radiation damage of the linseed oil. In this paper we want to study systematically how the linseed oil treatment affects the performances of the RPC chambers.
* Corresponding author. Tel.: +39 382 507 268; 526 938; e-mail: [email protected]. 0 16%9002/97/$I7.00 f’?( SO1 08.9002(97
fax:
+39 382
treatment of compared to treatment of less in single
We compare the behaviour of oiled and non oiled detectors made of bakelite produced with present available technology. The ageing problem will be addressed in another paper. The radiation damage has been preliminarily studied by introducing bakelite samples into the channel of an intense neutron flux from a research reactor [5]. Typical crude linseed oil [6] is a natural organic mixture of mainly linolenic acid (SO-65%), linoleic acid ( 14-24%) and oleic acid ( 16-26%). It is commonly used, after heating refining, in special paints and coatings. In many commercial products of this type other desiccative metals are added to guarantee the drying process. Since the main interest of this paper is the effect of the linseed oil treatment, we decided to operate the chambers in streamer mode. It is well known however that these chambers can be operated in avalanche mode [4] with low gas gain and front-end amplifiers.
2. Linseed oil treatment The treatment of the RPC electrodes was done following the traditional procedure by filling the full gas volume of an assembled RPC with linseed oil. then the oil is slowly taken out. The resulting effect is the deposition of a thin layer of oil on both bakelite surfaces facing the gas volume.
Copyright @ 1997 Elsevier Science B.V. All rights reserved )00597-4
14
M. Ahhwsciu
et al. I Nucl. Instr. und Meth. in Phys. Rex A 394 (1997)
(a)
13-20
It is easy to guess that the oil might have been used to smoothen the surface of the resistive electrodes and that modern technology might produce smoother bakelite sheets which might make the treatment unnecessary. We took photographs of bakelite samples before and after the oil treatment, before and after a 1000 s exposure to a ( < 0.2 eV) flux of 3 x 10” neutrons/(cm’ s). Different situations are pictured in Fig. 1: Fig. 1(a) shows the cross section of an oiled surface from which we estimated the thickness of the oil layer to be 35-40 pm, depending on the depth of the bakelite imperfections into which the oil seeps. Figs. l(b) and (c) show, respectively, part of the surface of “non-oiled’ and “oiled” bakelite plates before irradiation. It is evident that the oil improves the smoothness of the surface. Surface imperfections seen in Fig. l(b) as white spots have diameter of few microns which mostly disappear with the oil treatment, as shown in Fig. 1(c ). The big white spots on the bottom left comer of the picture [Fig. l(c)] are the result of scratching the surface sample with a needle: in such a way we could inspect the same region after the irradiation. By visual inspection of the irradiated samples, with a 600X magnification, it is clearly seen that the appearance of the surfaces is not damaged by neutrons.
3. Experimental setup
Fig. I. (a) Cross section of an “oiled” bakelite sample showing the thickness of the linseed oil layer; (b) Surface of a “non-oiled” bakelite sample before irradiation; (c) Surface of an “oiled” bakelite sample before irradiation.
The test has been carried out using 2mm double gap RPCs, whose cross section is shown in Fig. 2. The detector is made of two gaps with common inner readout aluminum strips ( I .3 cm wide). The high voltage (hereafter HV) supply can be applied independently to each gap so that the chamber can be operated either in double gap or single gap mode, by turning off one of the supplies. The area of the detector is 50 x 50 cm’. The bakelite plates, of resistivity 0.4 x 10’ ’ 12cm. are 0.2 cm thick. The RPCs were operated in streamer mode and filled with an argon/isobutane/freon 13Bl (48%/48%/4%) gas mixture. The experimental setup (sketched in Fig. 3 ) consists of a cosmic rays telescope made of 5 horizontal RPCs, 4 of which were not oiled at the beginning. The fifth chamber is a standard (oiled) RPC used as reference detector for the various measurements. The reference chamber is one year old and has been previously used for several tests [7]. Three scintillator counters acting as triggering system define a IO x 10 cm’ trigger area. The strips have been connected at both ends to different electronics to allow simultaneously charge and time measurements. Signals from one end of the strips have been input to CAEN C205A Analog Digital Converters (ADCs), with a
PVC foil C---
graphite coating
readout
strips
--Q PVC spacer-b
Fig. 1. Cross section (not to scale) of a double gap chamber
RPC wth common strip readout.
After a month of running, RPCs I,3 and 4 were treated with linseed oil and a new set of measurements were performed to inspect all possible changes in the behaviour of the detectors. WC1
4.1. Currm ts uml sinylr RPC 2
APC 3 i?3%R WC4
Fig. 3. Sketch of the experimental setup: a telescope made of tive double gap RPCs were triggered by a threefold scmtillators coincidence. TDC’s and ADC’s electronics (not shown) have been
connected as explained in the text.
0.25 pC channel sensitivity. Signals from the other end have been discriminated and sent to LeCroy 2228A Time Digital Converters ( TDCs). This scheme allows two independent measurements of the efticiency.
4. Experimental results The experimental results are summarized in the following figures. Non-oiled RPCs will be labeled l-4 and “R” will indicate the standard oiled reference detector. When the detectors are operated in single gap mode the additional labels A and B are used to indicate the two different gaps.
rates
Fig. 4 shows the behaviour of the average dark currents of the single gaps as a function of HV before and after the oil treatment. The error bars show one standard deviation spread of the currents in the 8 single gaps used separately. After the treatment of RPCs I, 3 and 4. the values of the dark currents are. at least, a factor 4 less. For comparison the average currents of the two gaps of the reference chamber are also shown. Single rates were measured counting the ORcd signals from 8 strips. The threshold for non oiled chambers was set to -8OmV: to be conservative the threshold for the oiled case was set to -50 mV. Fig. 5(a) shows the average single rate, scaled to a square meter area. for all non oiled gaps. Fig. 5(b) shows. for comparison, the average OR rate of gaps I A/B. 3A;B and 4A:‘B after the treatment with the linseed oil. The behaviour of the reference chamber is also shown. In spite of the lower threshold setting. we see that the oil treatment decreases the rate by an order of magnitude. A significant local rate effect for the non-oiled electrodes has also been observed. Fig. 6(a) displays the rate measured on each single strip at HV = 9 kV. It may vary from I to IO kHz. The local rate, however. decreases significantly for the newly oiled RPCs ( I, 3 and 4), as shown in Fig. 6( b 1 (HV = 9 kV ), in spite of the lower threshold. The local behaviour of the oiled RPCs is much more homogeneous. In addition the two peaks observed for strip
M. Abbrescia et (11.I Nucl. Instr. and Meth. in Phys. Rex A 394 (1997)
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L?
2
05
“E ,” 250 In x 0 In
0
All non oiled chambers
v
Chamber
6 ; 200
n
Chambers
R 1.3,4
after
oil treatment
g ; 2
Fig. 4. Current versus HV averaged as reference. Error bars correspond
on all non-oiled (open cross) and oiled gaps (full square); chamber R behaviour to one standard deviation.
is shown
gaps; threshold
-80 mV
._
,-1ooo a)
I
z 800
;
(full triangle)
700
? 600
W All Chambers
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100 6500
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10000
1’
13
1.3,4 after Oil treatment
n
Reference
Chamber
7000
7500
8000
8500
9000
9500
10000 10500 HV (Volt)
Fig. 5. Average single rate (as explained in the text) versus HV at a fixed discriminating (b) Oiled gaps; threshold -50 mV. The chamber R is shown as reference.
threshold:
(a) Non-oiled
M. Ahhresciu et al. I Nucl. Instr. and Moth. in Phrs. Rex A 394
Not Oiled T’OI
( 1997) 13mXl
17
Oiled
I
4A
o
F,_,
0
2
4
6
8 Strip number
Fig. 6. (a) Comparison
of the strip rate distribution
Strip rumber
for the non-oiled gaps at HV = 9 KV and for a discriminating threshold of -80 mV; I, 3 and 4 at HV = 9 KV and for a discriminating threshold of -50 mV.
(b) Strip rate distribution for the oiled gaps of chambers
2-gap 3B and strip 7-gap 3A are probably due to the presence of a spacer. Also one strip of chamber 4 was disconnected.
Efficiencies were calculated from the ADCs information by requiring a charge signal to be present above a certain threshold. The threshold was set to a 50mV signal equivalent; this corresponds to a charge of 1OpC assuming a triangular signal, 20 ns wide at the base and readout on a 5OU load. This approach might overestimate the efficiency at low HV; however the plateau values are comparable to those obtained with other methods (i.e. TDCs and scalers). The reason for using the charge distributions in the efficiencies calculation is that not all the data were available with the TDCs information. On the other hand we are interested only in the behaviour of the chambers in the plateau region. Figs. 7(a)-(d) (Fig. 7(a), chamber 1; Fig. 7(b), chamber 2; Fig. 7(c), chamber 3; Fig. 7(d), chamber 4) show
the efficiencies for the two gaps A and B of the 4 different chambers - non-oiled case - while Fig. 7(e) shows, for comparison, the efficiency for the two gaps A and B of the reference oiled chamber. The absence of the surface treatment of the electrodes does not seem to affect the plateau efficiency of the RPCs. In some cases however (Figs. 7(a) and (d)) the knee of the plateau is shifted about 500V higher than those of the reference gaps (Fig. 7(e)). This behaviour has still to be investigated and is beyond the purpose of this paper. For further comparison. Fig. 7(f) shows the etficiency of chamber 1 after oil treatment. The plateau values are substantially unchanged by the treatment. We leave the discussion of the behaviour in the low HV region to future investigation.
4.3. Charge distributions The charge distributions have been obtained by integrating for 250 ns any analog signal induced on 14 strips in coincidence with a trigger event.
M. Abbrescia
et al. INucl. Instr. and Meth. in Phys. Rex A 394 (1997)
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8.5
as explained in the text: -5OmV): 1 after oil treatment.
0.6
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Fig. 8. Integral charge distributions. Histograms for gaps IA , 2B, 48 refer to the non-oiled case. For each value on the abscissa corresponding ordinate represents the percentage of events for which the measured charge was greater than that value.
the
hl. Abbrrsciu et (11.I Nucl. Imtr. und Mrth. in Piiys. Rex A 394 I19971 13 20
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Fig. 9. Cluster size distributions. Histograms for gaps IA, 3A. 4A refer to the non-oiled detectors while the histogram for gap RA corresponds to one gap of the reference
chamber.
For each channel the pedestal was subtracted and the rest summed to obtain the total charge distribution collected over the area subtended by 14 strips. The results are presented as integral distributions in ‘greater-than-charge’ histograms. For each charge value on the abscissa the corresponding ordinate value represents the percentage of events with charge above that value. Fig. 8 shows, as an example, the integral distributions for the non-oiled gaps 1A, 2B, 4A and for the reference gap RA at HV = 8.5 KV.
4.4. C’htcr
six
Cluster size has been calculated via the TDC information: each discriminated strip signal arriving within 200 ns from the trigger leading edge contributed to the cluster size distribution. As for efficiency and collected charge, also the cluster size does not seem to be significantly affected by the absence of the oil treatment. Fig. 9 shows the distributions for the non oiled gaps 1A, 3A. 4A and for the reference gap RA at HV = 8.5 KV. We cannot exclude that part of the tail of large clusters is caused by cosmic ray showers. The excess of large clusters seen in gap 3A could be due to the higher cross-talk level of the readout board.
5. Conclusions Surface treatment of the internal bakelite electrodes is a necessary step to reduce the noise and the dark current of bakelite RPCs operated in streamer mode. The overall result seems to be the smoothness of the surface. However. the absence of the oiling agent does not sensitively affect other parameters such as efficiency, cluster size and charge distribution. It is not excluded however that the effect of the linseed oil (i.e. a smoother surface) could be directly obtained within the industrial process of the bakelite foil production. Recent tests [8] with chambers made with smoother surfaced bakelite compared with linseed oiled bakelite have shown similar performances. Nevertheless. the oil treatment does not degrade in any way the performance of an RPC, at least when operated in streamer mode. This paper leaves open the investigation of the ageing effects and the occasional shift of the knee in the efficiency curves. Several measurements of ageing are scheduled at the Triga Mark II Nuclear Reactor in Pavia.
References [I] (a) Proc. II Int. Workshop on the Resistwe Plate Chambers Particle
Physics
and Astrophysics.
Scientifica
in Acta (Quadern
20
M. Abbresciu et al. INucl. Imtr. and Meth. in Phys. Rex A 394 (1997)
del Dottorato) VIII (3) (1993), Univ. of Pavia; (b) Proc. III Int. Workshop on Resistive Plate Chambers and Related Detectors. Scientifica Acta (Quademi del Dottorato) XI (1) ( 1996) Univ. of Pavia. [2] M. Abbrescia et al., Scientifica Acta XI (1996) 217; A. Di Ciaccio. in: J. Lemonne, C. Vandez Velde, F. Verbeure (Ed.), IECHEP Proc., World Scientific. 1996, p. 605; A. Di Ciaccio, Scientifica Acta XI (I 996) 263; E. Cerron Zeballos et al., Nucl. Instr. and Meth. A 367 (I 995) 388; C. Bacci et al., NucI. Instr. and Meth. A 352 (1995) 552; M. Abbrescia et al., Properties of C~H~F;I based mixtures for avalanche mode operation of Resistive Plate Chambers, Nucl. Instr. and Meth., submitted. [3] R. Santonico, R. Cardarelli, Nucl. Instr. and Meth. 187 (1981 ) 377-380.
13-20
[4] P. Fonte, Scientifica Acta XI (1996) 25; R. Cardarelli. R. Santonico. V. Makeev, Scientifica Acta XI (1996) I I: 1. Duerdoth et al., Nucl. Instr. and Meth. A 348 (1994) 303; 1. Crotty et al., Nucl. Instr. and Meth. A 337 (1994)370. [5] Rapport0 finale di Sicurezza, Universiti di Pavia Laboratorio di Energia Nucleare Applicata (L.E.N.A.), 1964, Unpublished. [6] Ullmann’s Encyclopedia of Industrial Chemistry A 10 ( 1994) 227. [7] M. Abbrescia et al., Results of November 1994 Resistive Plate Chambers test beam CMS Muon Group Meeting at CERN, 1995. [8] Bari-Pavia CMS test beam I996 (work in progress).