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Study of the efficiency and time resolution of an RPC irradiated with photons and neutrons
NuclearInstrumentsand
& -H
Methods in Physics Research A 360 ( 1995) 42-47
__
Kl
g
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
ELSEVIER
Study of the efficiency and time resolution of an RPC irradiated with photons and neutrons L. Acitelli b, M. Angelone a, C. Bacci d, R. Cardarelli b, F. Ceradini d, G. Ciapetti ‘, A. DiCiaccio b, F. LacavaC, A. Nisati ‘, D. Orestano ‘, E. Petrol0 ‘, M. Pillon a, L. Pontecorvo ‘,*, R. Santonico b, S. Veneziano ‘, M. Verzocchi ‘, L. Zanello ’ a ENEA C.R. Froscati, 1tai.v b Universitti di Romo “Tar Vergata” and INFN, Rome, Italy ’ Universitci di Ram “LA Sapienza” and INFN, Rome. Italy d Term Universitci di Ram and INFN, Rome, Italy Abstract We present recent results on the efficiency and time resolution of an RPC operated with low gas gain under intense irradiation of photons and neutrons. These results are compared with previous measurements obtained using both high and low gain RPC. Results on the RPC sensitivity to photons and neutrons of energy around 1 MeV are also reported.
1. Introduction Resistive Plate Chambers, RPCs, have been proposed as suitable detectors for the construction of a large area muon trigger for experiments operating at the LHC, for their good time and space resolution, easiness of segmentation and low cost [ 11. RPCs are built with plates of phenolic polymers with a bulk resistivity of 10’“-lO” R cm [ 21, and are usually operated with a gas gain of about 108. In this operating condition, the main problem for the use of RPCs in an LHC experiment is their limited rate capability, due to the time needed to neutralize the charge deposited on the resistive plate after the signal formation. Measurements of the RPC efficiency as a function of the particle flux have been done by many authors in different conditions [3-l 21 and in the following we will discuss these results and compare them with new results obtained using an RPC operated with low gas gain, in the range 106-lo’, irradiated with photon and neutrons. We stress that in this measurement the RPC was working in conditions very close to those expected at LHC, where the main source of background radiation is a large flux of low energy neutrons and photons. We also studied the time resolution of the chamber as a function of the counting rate, and we compare this result with previous measurements [ 61 obtained using a high gain RPC exposed to a muon beam. In the first part of the paper we discuss the results on the dependence of the efficiency and time resolution on the * Corresponding
author.
0168-9002/95/$)9.50 @ 1995 Elsevier Science B.V. All rights reserved SSDIO168-9002(94)01219-9
counting rate, then we describe the experimental layout and our new results and finally we present a measurement of the RPC sensitivity for low energy photons and neutrons.
2. Results on efficiency vs counting rate RPCs have been used for a long time in low rate experiments. In view of their use in experiments at accelerators, measurements of the efficiency as a function of the counting rate were performed with particle beams fluxes up to 140 Hz/cm’ [ 31. These tests showed that the efficiency, the time resolution and the signal formation time deteriorated, increasing the particle flux illuminating the detector. It was also found that the measured efficiency decreased with the time elapsed from the spill start. These measurements were performed using RPC chambers with a plates bulk resistivity of about 10” fl cm. The beam size was about 3 x 3 cm’, and the spill time was 300 ms. Further tests were performed using RPCs of different resistivity, 2x 10” and 4x 10’” R cm [ 5,6], with a beam of about 10x 10 cm* and with a spill of 2.6 s. RPCs were tested at fluxes as high as 300 Hz/cm*. It was found that the rate capability depends upon the plates resistivity. Low resistivity chambers were efficient up to fluxes of about 100 Hz/cm*, while high resistivity chambers began to lose efficiency already at few tenths of Hz/cm’ (Fig. 1) . In all these measurements the RPCs were tested illuminating only a small part of the active surface, using beams of limited duty cycle. It has been argued that there could be a dependence of the RPC efficiency on the irradiated area. To study this effect, measurements have been done with pho-
in Phys. Rex A 360 (1995) 42-47
operate the RPC with a gas gain a factor of about 100 smaller than in the “standard” mode of operation. The same chamber was also tested at high intensity beam using different gas mixtures, and showed no significant degradation of the detection efficiency up to fluxes of 1000 Hz/cm* [8,12]. Similar improvements were reported also in Refs. [ 9,l I] where RPCs, illuminated over the whole surface, were operated at a low gas gain increasing their rate capabilities by a factor 20 with respect to the standard mode of operation.
low resistivity ,,
high
resistivity
rel
3
ref
7 focused
beam
3. Experimental
high
resistivity
lowresistivity j
b
low gain
L ..A_
il
,,,,,I
43
1
,,,
Fig. I. Efficiency of the RPC as a function the particle flux measured at extracted beams of 0.3 s spill (a) and 2.6 s spill (b).
tons from a nuclear reactor illuminating the whole surface of the detector [ 41. The authors found that the rate capability measured in these conditions was much smaller than the one obtained with the beam. This effect was also studied in Refs. [ 7,9], measuring the rate capabilities of small RPCs of different plates material and resistivity, irradiated with both concentrated and diffuse beams. The authors found an agreement between their results and those of the photon beam test, showing a large difference in the RPC behaviour when irradiated with concentrated or diffuse beams. A possible explanation of this effect was given by the authors in Ref. [ 71 where they suggested that this behaviour was due to lateral diffusion of the charge on the resistive plates. These results coupled to new calculations of the neutron and photon background rate for LHC experiments pointed out that a large improvement in the RPC rate capabilities was needed to safely operate an RPC muon trigger at the LHC. Two possible solutions were investigated: 1) lowering the plates resistivity and 2) reducing the gas amplification, thus reducing the charge produced in a streamer. The first method gave good results in terms of rate capabilities [ 51 but the main drawback was that the RPC operation tends to be somewhat more noisy. Moreover, the dissipated electrical power at high counting rates would become very high, due to the large current flowing through the plates to neutralize the charge produced by the streamers. The second method on the contrary, at the same time, improves the rate capabilities and decreases the power dissipated by the chamber. The basic idea is to transfer a large part of the gas amplification to the front end electronics [ 131, thus requiring high gain, high bandwidth preamplifiers. Measurements were performed using low gain RPC operated in pure freon, demonstrating the possibility to safely
layout and new results
To study this new mode of operation in conditions close to those expected for an LHC muon detector we have performed measurements using photon and neutron sources. We used a 50 x 50 cm2 double gap RPC also described in Ref. [ 121; the plates resistivity was about 4 x lO”fi cm, and the readout plane was segmented in strips of 1.2 cm pitch. Only the eight central strips were instrumented with amplifiers of 150 MHz bandwidth, x 100 voltage gain. The signals coming from the strips were individually discriminated with 100 MHz, 80 mV threshold discriminators and then ORed together to give the RPC signal. The chamber was operated as a single gap RPC powering only the gas gap close to the radioactive sources. The used gas mixture was 26% argon, 29% n-butane and 45% Freon 13B 1. The RPC was tested with cosmic rays when exposed to a known flux of photons or neutrons from a radioactive source. To define the trigger we used a cosmic ray telescope SOURCE
i d
I
0
I
I
20
Pb Shield - 1
I
I
40
I
I 60
cm
Pb Shield - 2
Fig. 2. Layout of the cosmic ray set-up and shielding configurations of the RPC (the dashed contour represents the region covered by the scintillator telescope).
I. TRACKING
L. Acifdi
44
rt al./Nuci.
lnstr. and Meth. in Phys. Rex A 360 (1995) 42-47
a)
_
4 2 L
50l1
b,
JO,,
I
3
300 I,
zon / IO0 0
; O
O.Olll
____L-. n.rmz
1 Il.003
".,U,.l
II.005
d-2 (cnf2) Fig. 3. Current (a) and rate (b) of the RPC as a function of the inverse squared distance from the source.
Fig. 4. Efficiency of the RPC as a function different positions of the source.
made of four scintillator (Fig. 2); the top one was shielded by 5 cm of lead to prevent high counting rate due to the particles coming from the radioactive sources and the bottom scintillator was shielded by 15 cm of lead in total. The noninstrumented part of the chamber was shielded by 5 cm of lead to better define the irradiated area, avoiding edge effect in the evaluation of the RPC sensitivity to photons. To compare the efficiency measured illuminating the whole detector to that measured illuminating only a small part of it, we performed measurements leaving the whole detector unshielded and measurements shielding the whole detector except for the central 10 x 10 cm*. The efficiency was measured as the ratio between the number of coincidences triggerxRPC and the number of triggers. The time resolution of the chamber was studied measuring the distribution of the delay between the trigger signal and the individual RPC strip signals. We placed a 22 mCi ‘s7Cs source (0.66 MeV photons) together with a 4.3 mCi @Co source ( 1.17 and 1.33 MeV photons) at different distances, 15 5 d 5 70 cm, from the chamber surface, and we measured the chamber efficiency under a continuous and rather uniform irradiation. In Fig. 3 we present the chamber current and counting rate as a function of the inverse squared distance of the sources. The l/d* law is fulfilled for distances down to 30 cm. For closer distances the flux is not uniform and we had to take this into account in the analysis. The efficiency as a function of the high voltage is presented in Fig. 4, for different values of the distance d of the sources. The plateau efficiency is reached at higher voltages for higher intensity since there is need to compensate for the reduction of the electric field in the gas gap due to the charge flowing through the resistive plates. The plateau extends for more than 600 V and we do not observe a reduction of the
efficiency
with increasing
of the
supply voltage for
high voltage as reported in Ref.
[Ill. The measured counting rate is defined as the ratio between the rate of the RPC signal and the area of the unshielded region; we also define an effective rate obtained unfolding the calculated photon flux and the calculated cosmic ray flux [ 141. Fig. 5 shows the flux of cosmic rays and photons for a source distance of 15 cm. In Table 1 we present the counting rates and the effective counting rates for different source distances. The difference between the two figures is relevant only for the last two measurements and is always smaller than 20%. In Fig. 6 we present the behaviour of the plateau efficiency, defined as the average over the last three points of the efficiency curve, as a function of the effective counting rate. We found a linear dependence between loss of efficiency and counting rate (effective counting rate) that we fitted by a line whose slope is 15% every 1000 Hz/cm* ( 13.1% every 1000 Hz/cm*). This figure is an improvement of a factor about 10 with respect to previous measurements performed at beams with RPC operated at high gain. We have also studied possible differences in the behaviour of the detector when irradiated over the whole area or over a small part of it. We measured the chamber efficiency by shielding with 5 cm of lead the whole detector apart from a central 10x 10 cm* hole. We placed the sources at a dis-
1 Source distances, ratestudy Table
rates and effective rates for the efficiency
vs counting
d (cm)
70
50
40
35
30
25
20
IS
R (Hz cm-*) Reff (Hz cm-*)
96 98
162 168
230 241
280 297
350 374
450 497
620 705
800 1022
L. Acitelli
et al./Nuci.
Instr. and Meth. in Phy.
Res. A 360 (1995) 42-47
45
Fig. 5. Profile of cosmic rays and of photons from the radioactive source placed at 15 cm from the chamber
tance of 23 cm from the chamber; the counting rate in this condition was 1000 Hz/cm* in the unshielded area. The measured efficiency times acceptance was 0.887 f 0.012. We repeated the measurement removing the shielding on top of the chamber. In this condition the average counting rate was 570 Hz/cm’, and the efficiency times acceptance was 0.851 f 0.014. It has to be noted however that the result of the first measurement is biased because, when the source is illuminating only the central 10x 10 cm* area of the RPC, a large fraction of the cosmic rays traverse the chamber in regions where the source Hux is shielded thus with an higher detection efficiency. The measured efficiency is the weighted average of the efficiency of the two regions (irradiated and non-irradiated region) where the weights are the fractions of cosmic rays traversing each region. These fractions can be estimated using the calculated cosmic ray flux. Assuming
0.x
0.7
Ref 4
for the non-irradiated zone the value of the efficiency measured without source we obtain a value for the efficiency in the irradiated region of 0.826 rfr 0.021. If we compare this number with that measured with the whole detector exposed to the sources, taking into account that there is a factor 2 in rate per unit surface, we can infer that the difference in efficiency is smaller than 5% and we conclude that, in this mode of operation, the effect is not as large as observed in Refs. [ 4,7]
4. Time resolution In previous measurements performed at muon beams we have shown that the time response of the detector is affected at high rates [ 61. Recent measurements [ 10,111 done at extracted beam have shown that the rate limitation are much less severe for an RPC operated at low gas gain. We have measured the time resolution and the average pulse formation time for a low gain RPC irradiated over the whole surface by a continuous flux of photons. In Fig. 7 we present the time distribution measured at different rates: the time resolution is defined as the RMS width of a Gaussian fit to these distributions. In Table 2 we present the average and the width of the time distribution; these results confirm that the signal formation time and the time resolution are very weakly influenced by the detector rate. The value of the time resolution is larger than that obtained with the same chamber on a muon beam; this is mainly due to geometrical effects introduced by the cosmic ray telescope.
5. Sensitivity
Fig. 6. Plateau efficiency of the RPC as a function of the effective rate per unit surface compared with the result of Ref. 141.
to photons and neutrons
We have measured the sensitivity of the RPC to photons of different energy. We used separately the @Co source and the 13’Cs source. The cobalt source produces in each decay I. TRACKING
L. Acitelli
46
rf al. /Nucl.
Instr. and Mrth.
in Phys. Rrs. A 360
f 1995)
42-17
160 Hz/cm*
0
_Z
1’
n
In
““-2
L
2n
““‘I,‘-,““““*‘!
30
40
50
60
-
’ 70
xn
Fig. 7. Time distribution surface.
of the RPC response for different rates per unit
two photons of 1.17 MeV and 1.33 MeV; the cesium source produces one photon of 0.66 MeV in 80% of the decays. The average sensitivity to photons has been measured as the ratio of the RPC counting rate, after subtracting the counting rate of the detector measured without source, and the number of photons traversing the unshielded area. The results are presented in Table 3 where the enors are only statistical. The systematic error due to the source activity calibration is 10%. The sensitivity to neutrons of few MeV energy has been estimated using a 24’Am-Be source, producing 2.3 x lo6 neutrons per second on the full solid angle, with a maximum energy of about 8 MeV, together with 1.6x 1O6 photons per second with a maximum energy of 4.4 MeV and 1.8 x lo9 photons per second of 59 keV. The source was enclosed in a 5 cm lead box to eliminate most of the photons, but neutrons interacting in the lead produce new photons themselves. The energy spectrum of the neutrons and photons after the lead shield has been calculated and is shown in Fig. 8. The sensitivity to neutrons has been estimated by the ratio of the RPC counting rate and the number of neutrons traversing the active surface, after subtracting the contribution to the counting rate due to noise and photons. The sensitivity to neutrons Table 2 Source distances, delay and time resolution for the timing vs counting rate study d (cm) R,r ( Hz/cm2 ) Delay (ns) Resolution (ns)
50 168 27.5 + 0. I 2.25 f 0.08
35 297 28.2 f 0.1 2.28 f 0.09
25 491 29.2 f 0. I 2.46 f 0.08
I5 1022 31.2 f0.l 2.6 k 0. I
Fig. 8. Computed flux of neutrons and photons from the 24’ Am-Be source. beyond a shield of 5 cm of lead. as a function of the energy. The distance from the source is IO cm.
of ELMeV energy is in the range (0.5-2) x lo-‘, where the uncertainty is due to the definition of the solid angle and to the subtraction of the photon and noise contribution.
6. Conclusions We have measured the efficiency for minimum ionizing particles and the time response of a low gain RPC under continuous and uniform irradiation of photon of energy around 1 MeV. In this operating condition we found that the efficiency loss due to the irradiation can be parametrized with a linear function with a slope of 13% every 1000 Hz/cm*. We do not observe any large difference in the measured efficiency if the particle flux is concentrated in a small region of the detector or if the whole surface is irradiated. The time resolution of the detector has been measured and it is about 2.5 ns; very little degradation of the time resolution and the signal formation time as a function of the counting rate has been observed. The sensitivity of the RPC to low energy neutrons and photons has been measured. We obtain a value of 0.005 for photons of 0.66 MeV energy, and 0.01 for photons of 1.3 MeV. The sensitivity to neutrons of few MeV is around 0.001; these results are in agreement with Monte Carlo calculations and with other measurements. We are now planning to test low gain RPC with new front end electronics and to test non-flammable gas mixtures.
L. Acitrlli Table 3 RPC sensitivity
Instr. and Meth. in Php.
6oCo-y sensitivity
47
sensitivity
11.0
11.2
11.4
11.6
(IO-‘)
0.907 zt 0.012
0.997 i 0.012
1.071 f0.012
1.16Ort 0.009
(lo-*)
0.38 i 0.026
0.425 zt 0.026
0.455 zt 0.027
Acknowledgements We would like to thank Prof. M. Martone and the staff of the Neutron Generator of the Laboratori di Frascati dell’ENEA for their kind hospitality and their support during the measurements. We thank G. Pagan0 and A. Rossi for their skillful help in the assembly and the installation of the detectors. We are grateful to Dr. G. Battistoni and Dr. A. Ferrari for providing us the results of their Monte Carlo simulation on the sensitivity of RPC to photons and neutrons and for the many illuminating discussions.
References
[ I]
Res. A 360 (1995) 42-47
to photons as a function of the HV
RPC supply (kV) “‘Cs-y
et al./Nucl.
F. Ceradini et al., Proc. Large Hadron Collider Workshop, CERN/90IO ECFA/90-133, eds. G. Jarlskog and D. Rein. vol. III, p. 99; R. Santonico, ibid., p. 838.
121R.
[31 141 [51 161 [71 181
191 []Ol L111 1121
[I31 1141
Santonico and R. Cardarelli. Nucl. lnstr. and Meth. 187 (1981) 377; R. Cardarelli et al., Nucl. lnstr. and Meth. A 263 ( 1988) 20. M. Bertino et al., Nucl. lnstr and Meth. A 283 ( 1989) 654. M. lori and F. Massa, Nucl. lnstr. and Meth. A 306 ( 1991) 159. C. Bacci et al., Nucl. Instr. and Meth. A 340 (1994) 466. C. Bacci et al.. Nucl. Instr. and Meth. A 345 ( 1994) 474. I. Crotty et al., Nucl. Instr. and Meth. A 329 (1993) 133. R. Cardarelli. Proc. Int. Workshop on Resistive Plate Chambers in Particle Physics and Astrophysics, eds. S. Ratti, G. Ciapetti and R. Santonico, Scientifica Acta 8 ( 1993) 159. I. Crotty et al.. Nucl. Instr. and Meth. A 337 (1994) 370. H. Faissner et al., Status report of the RD5 experiment, CERNldrdci93.49; RDS/status report January 5, 1994. I. Crotty et al., Nucl. Instr. and Meth. A 346 (1994) 107. C. Bacci et al.. Test of a Resistive Plate Chamber operating with low gas amplification at high intensity beams, Preprint ROM2 F/94/1 I 21 March 1994, submitted to NucI. Ins& and Meth. R. Cardarelli. A. DiCiaccio and R. Santonico, Nucl. Instr. and Meth. A 333 (1993) 399. M. Angelone et al., Nucl. Insu. and Meth. A 355 ( 1995) 399.
I. TRACKING
& -H
Methods in Physics Research A 360 ( 1995) 42-47
__
Kl
g
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
ELSEVIER
Study of the efficiency and time resolution of an RPC irradiated with photons and neutrons L. Acitelli b, M. Angelone a, C. Bacci d, R. Cardarelli b, F. Ceradini d, G. Ciapetti ‘, A. DiCiaccio b, F. LacavaC, A. Nisati ‘, D. Orestano ‘, E. Petrol0 ‘, M. Pillon a, L. Pontecorvo ‘,*, R. Santonico b, S. Veneziano ‘, M. Verzocchi ‘, L. Zanello ’ a ENEA C.R. Froscati, 1tai.v b Universitti di Romo “Tar Vergata” and INFN, Rome, Italy ’ Universitci di Ram “LA Sapienza” and INFN, Rome. Italy d Term Universitci di Ram and INFN, Rome, Italy Abstract We present recent results on the efficiency and time resolution of an RPC operated with low gas gain under intense irradiation of photons and neutrons. These results are compared with previous measurements obtained using both high and low gain RPC. Results on the RPC sensitivity to photons and neutrons of energy around 1 MeV are also reported.
1. Introduction Resistive Plate Chambers, RPCs, have been proposed as suitable detectors for the construction of a large area muon trigger for experiments operating at the LHC, for their good time and space resolution, easiness of segmentation and low cost [ 11. RPCs are built with plates of phenolic polymers with a bulk resistivity of 10’“-lO” R cm [ 21, and are usually operated with a gas gain of about 108. In this operating condition, the main problem for the use of RPCs in an LHC experiment is their limited rate capability, due to the time needed to neutralize the charge deposited on the resistive plate after the signal formation. Measurements of the RPC efficiency as a function of the particle flux have been done by many authors in different conditions [3-l 21 and in the following we will discuss these results and compare them with new results obtained using an RPC operated with low gas gain, in the range 106-lo’, irradiated with photon and neutrons. We stress that in this measurement the RPC was working in conditions very close to those expected at LHC, where the main source of background radiation is a large flux of low energy neutrons and photons. We also studied the time resolution of the chamber as a function of the counting rate, and we compare this result with previous measurements [ 61 obtained using a high gain RPC exposed to a muon beam. In the first part of the paper we discuss the results on the dependence of the efficiency and time resolution on the * Corresponding
author.
0168-9002/95/$)9.50 @ 1995 Elsevier Science B.V. All rights reserved SSDIO168-9002(94)01219-9
counting rate, then we describe the experimental layout and our new results and finally we present a measurement of the RPC sensitivity for low energy photons and neutrons.
2. Results on efficiency vs counting rate RPCs have been used for a long time in low rate experiments. In view of their use in experiments at accelerators, measurements of the efficiency as a function of the counting rate were performed with particle beams fluxes up to 140 Hz/cm’ [ 31. These tests showed that the efficiency, the time resolution and the signal formation time deteriorated, increasing the particle flux illuminating the detector. It was also found that the measured efficiency decreased with the time elapsed from the spill start. These measurements were performed using RPC chambers with a plates bulk resistivity of about 10” fl cm. The beam size was about 3 x 3 cm’, and the spill time was 300 ms. Further tests were performed using RPCs of different resistivity, 2x 10” and 4x 10’” R cm [ 5,6], with a beam of about 10x 10 cm* and with a spill of 2.6 s. RPCs were tested at fluxes as high as 300 Hz/cm*. It was found that the rate capability depends upon the plates resistivity. Low resistivity chambers were efficient up to fluxes of about 100 Hz/cm*, while high resistivity chambers began to lose efficiency already at few tenths of Hz/cm’ (Fig. 1) . In all these measurements the RPCs were tested illuminating only a small part of the active surface, using beams of limited duty cycle. It has been argued that there could be a dependence of the RPC efficiency on the irradiated area. To study this effect, measurements have been done with pho-
in Phys. Rex A 360 (1995) 42-47
operate the RPC with a gas gain a factor of about 100 smaller than in the “standard” mode of operation. The same chamber was also tested at high intensity beam using different gas mixtures, and showed no significant degradation of the detection efficiency up to fluxes of 1000 Hz/cm* [8,12]. Similar improvements were reported also in Refs. [ 9,l I] where RPCs, illuminated over the whole surface, were operated at a low gas gain increasing their rate capabilities by a factor 20 with respect to the standard mode of operation.
low resistivity ,,
high
resistivity
rel
3
ref
7 focused
beam
3. Experimental
high
resistivity
lowresistivity j
b
low gain
L ..A_
il
,,,,,I
43
1
,,,
Fig. I. Efficiency of the RPC as a function the particle flux measured at extracted beams of 0.3 s spill (a) and 2.6 s spill (b).
tons from a nuclear reactor illuminating the whole surface of the detector [ 41. The authors found that the rate capability measured in these conditions was much smaller than the one obtained with the beam. This effect was also studied in Refs. [ 7,9], measuring the rate capabilities of small RPCs of different plates material and resistivity, irradiated with both concentrated and diffuse beams. The authors found an agreement between their results and those of the photon beam test, showing a large difference in the RPC behaviour when irradiated with concentrated or diffuse beams. A possible explanation of this effect was given by the authors in Ref. [ 71 where they suggested that this behaviour was due to lateral diffusion of the charge on the resistive plates. These results coupled to new calculations of the neutron and photon background rate for LHC experiments pointed out that a large improvement in the RPC rate capabilities was needed to safely operate an RPC muon trigger at the LHC. Two possible solutions were investigated: 1) lowering the plates resistivity and 2) reducing the gas amplification, thus reducing the charge produced in a streamer. The first method gave good results in terms of rate capabilities [ 51 but the main drawback was that the RPC operation tends to be somewhat more noisy. Moreover, the dissipated electrical power at high counting rates would become very high, due to the large current flowing through the plates to neutralize the charge produced by the streamers. The second method on the contrary, at the same time, improves the rate capabilities and decreases the power dissipated by the chamber. The basic idea is to transfer a large part of the gas amplification to the front end electronics [ 131, thus requiring high gain, high bandwidth preamplifiers. Measurements were performed using low gain RPC operated in pure freon, demonstrating the possibility to safely
layout and new results
To study this new mode of operation in conditions close to those expected for an LHC muon detector we have performed measurements using photon and neutron sources. We used a 50 x 50 cm2 double gap RPC also described in Ref. [ 121; the plates resistivity was about 4 x lO”fi cm, and the readout plane was segmented in strips of 1.2 cm pitch. Only the eight central strips were instrumented with amplifiers of 150 MHz bandwidth, x 100 voltage gain. The signals coming from the strips were individually discriminated with 100 MHz, 80 mV threshold discriminators and then ORed together to give the RPC signal. The chamber was operated as a single gap RPC powering only the gas gap close to the radioactive sources. The used gas mixture was 26% argon, 29% n-butane and 45% Freon 13B 1. The RPC was tested with cosmic rays when exposed to a known flux of photons or neutrons from a radioactive source. To define the trigger we used a cosmic ray telescope SOURCE
i d
I
0
I
I
20
Pb Shield - 1
I
I
40
I
I 60
cm
Pb Shield - 2
Fig. 2. Layout of the cosmic ray set-up and shielding configurations of the RPC (the dashed contour represents the region covered by the scintillator telescope).
I. TRACKING
L. Acifdi
44
rt al./Nuci.
lnstr. and Meth. in Phys. Rex A 360 (1995) 42-47
a)
_
4 2 L
50l1
b,
JO,,
I
3
300 I,
zon / IO0 0
; O
O.Olll
____L-. n.rmz
1 Il.003
".,U,.l
II.005
d-2 (cnf2) Fig. 3. Current (a) and rate (b) of the RPC as a function of the inverse squared distance from the source.
Fig. 4. Efficiency of the RPC as a function different positions of the source.
made of four scintillator (Fig. 2); the top one was shielded by 5 cm of lead to prevent high counting rate due to the particles coming from the radioactive sources and the bottom scintillator was shielded by 15 cm of lead in total. The noninstrumented part of the chamber was shielded by 5 cm of lead to better define the irradiated area, avoiding edge effect in the evaluation of the RPC sensitivity to photons. To compare the efficiency measured illuminating the whole detector to that measured illuminating only a small part of it, we performed measurements leaving the whole detector unshielded and measurements shielding the whole detector except for the central 10 x 10 cm*. The efficiency was measured as the ratio between the number of coincidences triggerxRPC and the number of triggers. The time resolution of the chamber was studied measuring the distribution of the delay between the trigger signal and the individual RPC strip signals. We placed a 22 mCi ‘s7Cs source (0.66 MeV photons) together with a 4.3 mCi @Co source ( 1.17 and 1.33 MeV photons) at different distances, 15 5 d 5 70 cm, from the chamber surface, and we measured the chamber efficiency under a continuous and rather uniform irradiation. In Fig. 3 we present the chamber current and counting rate as a function of the inverse squared distance of the sources. The l/d* law is fulfilled for distances down to 30 cm. For closer distances the flux is not uniform and we had to take this into account in the analysis. The efficiency as a function of the high voltage is presented in Fig. 4, for different values of the distance d of the sources. The plateau efficiency is reached at higher voltages for higher intensity since there is need to compensate for the reduction of the electric field in the gas gap due to the charge flowing through the resistive plates. The plateau extends for more than 600 V and we do not observe a reduction of the
efficiency
with increasing
of the
supply voltage for
high voltage as reported in Ref.
[Ill. The measured counting rate is defined as the ratio between the rate of the RPC signal and the area of the unshielded region; we also define an effective rate obtained unfolding the calculated photon flux and the calculated cosmic ray flux [ 141. Fig. 5 shows the flux of cosmic rays and photons for a source distance of 15 cm. In Table 1 we present the counting rates and the effective counting rates for different source distances. The difference between the two figures is relevant only for the last two measurements and is always smaller than 20%. In Fig. 6 we present the behaviour of the plateau efficiency, defined as the average over the last three points of the efficiency curve, as a function of the effective counting rate. We found a linear dependence between loss of efficiency and counting rate (effective counting rate) that we fitted by a line whose slope is 15% every 1000 Hz/cm* ( 13.1% every 1000 Hz/cm*). This figure is an improvement of a factor about 10 with respect to previous measurements performed at beams with RPC operated at high gain. We have also studied possible differences in the behaviour of the detector when irradiated over the whole area or over a small part of it. We measured the chamber efficiency by shielding with 5 cm of lead the whole detector apart from a central 10x 10 cm* hole. We placed the sources at a dis-
1 Source distances, ratestudy Table
rates and effective rates for the efficiency
vs counting
d (cm)
70
50
40
35
30
25
20
IS
R (Hz cm-*) Reff (Hz cm-*)
96 98
162 168
230 241
280 297
350 374
450 497
620 705
800 1022
L. Acitelli
et al./Nuci.
Instr. and Meth. in Phy.
Res. A 360 (1995) 42-47
45
Fig. 5. Profile of cosmic rays and of photons from the radioactive source placed at 15 cm from the chamber
tance of 23 cm from the chamber; the counting rate in this condition was 1000 Hz/cm* in the unshielded area. The measured efficiency times acceptance was 0.887 f 0.012. We repeated the measurement removing the shielding on top of the chamber. In this condition the average counting rate was 570 Hz/cm’, and the efficiency times acceptance was 0.851 f 0.014. It has to be noted however that the result of the first measurement is biased because, when the source is illuminating only the central 10x 10 cm* area of the RPC, a large fraction of the cosmic rays traverse the chamber in regions where the source Hux is shielded thus with an higher detection efficiency. The measured efficiency is the weighted average of the efficiency of the two regions (irradiated and non-irradiated region) where the weights are the fractions of cosmic rays traversing each region. These fractions can be estimated using the calculated cosmic ray flux. Assuming
0.x
0.7
Ref 4
for the non-irradiated zone the value of the efficiency measured without source we obtain a value for the efficiency in the irradiated region of 0.826 rfr 0.021. If we compare this number with that measured with the whole detector exposed to the sources, taking into account that there is a factor 2 in rate per unit surface, we can infer that the difference in efficiency is smaller than 5% and we conclude that, in this mode of operation, the effect is not as large as observed in Refs. [ 4,7]
4. Time resolution In previous measurements performed at muon beams we have shown that the time response of the detector is affected at high rates [ 61. Recent measurements [ 10,111 done at extracted beam have shown that the rate limitation are much less severe for an RPC operated at low gas gain. We have measured the time resolution and the average pulse formation time for a low gain RPC irradiated over the whole surface by a continuous flux of photons. In Fig. 7 we present the time distribution measured at different rates: the time resolution is defined as the RMS width of a Gaussian fit to these distributions. In Table 2 we present the average and the width of the time distribution; these results confirm that the signal formation time and the time resolution are very weakly influenced by the detector rate. The value of the time resolution is larger than that obtained with the same chamber on a muon beam; this is mainly due to geometrical effects introduced by the cosmic ray telescope.
5. Sensitivity
Fig. 6. Plateau efficiency of the RPC as a function of the effective rate per unit surface compared with the result of Ref. 141.
to photons and neutrons
We have measured the sensitivity of the RPC to photons of different energy. We used separately the @Co source and the 13’Cs source. The cobalt source produces in each decay I. TRACKING
L. Acitelli
46
rf al. /Nucl.
Instr. and Mrth.
in Phys. Rrs. A 360
f 1995)
42-17
160 Hz/cm*
0
_Z
1’
n
In
““-2
L
2n
““‘I,‘-,““““*‘!
30
40
50
60
-
’ 70
xn
Fig. 7. Time distribution surface.
of the RPC response for different rates per unit
two photons of 1.17 MeV and 1.33 MeV; the cesium source produces one photon of 0.66 MeV in 80% of the decays. The average sensitivity to photons has been measured as the ratio of the RPC counting rate, after subtracting the counting rate of the detector measured without source, and the number of photons traversing the unshielded area. The results are presented in Table 3 where the enors are only statistical. The systematic error due to the source activity calibration is 10%. The sensitivity to neutrons of few MeV energy has been estimated using a 24’Am-Be source, producing 2.3 x lo6 neutrons per second on the full solid angle, with a maximum energy of about 8 MeV, together with 1.6x 1O6 photons per second with a maximum energy of 4.4 MeV and 1.8 x lo9 photons per second of 59 keV. The source was enclosed in a 5 cm lead box to eliminate most of the photons, but neutrons interacting in the lead produce new photons themselves. The energy spectrum of the neutrons and photons after the lead shield has been calculated and is shown in Fig. 8. The sensitivity to neutrons has been estimated by the ratio of the RPC counting rate and the number of neutrons traversing the active surface, after subtracting the contribution to the counting rate due to noise and photons. The sensitivity to neutrons Table 2 Source distances, delay and time resolution for the timing vs counting rate study d (cm) R,r ( Hz/cm2 ) Delay (ns) Resolution (ns)
50 168 27.5 + 0. I 2.25 f 0.08
35 297 28.2 f 0.1 2.28 f 0.09
25 491 29.2 f 0. I 2.46 f 0.08
I5 1022 31.2 f0.l 2.6 k 0. I
Fig. 8. Computed flux of neutrons and photons from the 24’ Am-Be source. beyond a shield of 5 cm of lead. as a function of the energy. The distance from the source is IO cm.
of ELMeV energy is in the range (0.5-2) x lo-‘, where the uncertainty is due to the definition of the solid angle and to the subtraction of the photon and noise contribution.
6. Conclusions We have measured the efficiency for minimum ionizing particles and the time response of a low gain RPC under continuous and uniform irradiation of photon of energy around 1 MeV. In this operating condition we found that the efficiency loss due to the irradiation can be parametrized with a linear function with a slope of 13% every 1000 Hz/cm*. We do not observe any large difference in the measured efficiency if the particle flux is concentrated in a small region of the detector or if the whole surface is irradiated. The time resolution of the detector has been measured and it is about 2.5 ns; very little degradation of the time resolution and the signal formation time as a function of the counting rate has been observed. The sensitivity of the RPC to low energy neutrons and photons has been measured. We obtain a value of 0.005 for photons of 0.66 MeV energy, and 0.01 for photons of 1.3 MeV. The sensitivity to neutrons of few MeV is around 0.001; these results are in agreement with Monte Carlo calculations and with other measurements. We are now planning to test low gain RPC with new front end electronics and to test non-flammable gas mixtures.
L. Acitrlli Table 3 RPC sensitivity
Instr. and Meth. in Php.
6oCo-y sensitivity
47
sensitivity
11.0
11.2
11.4
11.6
(IO-‘)
0.907 zt 0.012
0.997 i 0.012
1.071 f0.012
1.16Ort 0.009
(lo-*)
0.38 i 0.026
0.425 zt 0.026
0.455 zt 0.027
Acknowledgements We would like to thank Prof. M. Martone and the staff of the Neutron Generator of the Laboratori di Frascati dell’ENEA for their kind hospitality and their support during the measurements. We thank G. Pagan0 and A. Rossi for their skillful help in the assembly and the installation of the detectors. We are grateful to Dr. G. Battistoni and Dr. A. Ferrari for providing us the results of their Monte Carlo simulation on the sensitivity of RPC to photons and neutrons and for the many illuminating discussions.
References
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