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Analysis of compact and sealed RPCs feasibility

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2013 JINST 8 P03012

(http://iopscience.iop.org/1748-0221/8/03/P03012)

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2013 JINST 8 P03012

PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALAB

RECEIVED: August 20, 2012ACCEPTED: February 4, 2013PUBLISHED: March 18, 2013

SPECIAL ISSUE ON RESISTIVE PLATE CHAMBERS AND RELATED DETECTORS RPC2012

Analysis of compact and sealed RPCs feasibility

M. Morales,a,1 J.L. Rodrguez-Sanchezb and J.A. Garzona

aLabCAF, F. Fisica, Univ. Santiago de Compostela,Santiago de Compostela, Spain

bDepartment of Particle Physics, Univ. Santiago de Compostela,Santiago de Compostela, Spain

E-mail: miguel.morales@usc.es

ABSTRACT: In this article, the feasibility of developing compact and portable float glass sealedResistive Plate Chambers, sRPCs, is analyzed. For this purpose, several small (80 cm2) sealedchambers have been constructed using inexpensive materials like windows float glass, copper tapeand nylon fishing line. For the sake of simplicity, during this first development stage, only R134ahas been used as ionizing gas.

In order to distinguish gas leakage from internal gas degradation, a couple of sRPCs weretested inside a box with flowing gas: one with R134a and another with N2. Prompt charge, signalrising slope and operational current were used to assess chambers performance degradation during atwo-week period. Regarding these variables, small leakages were spotted as the main reason for theperformance degradation observed after about one week of steady operation at the sRPC workingin N2 environment. The sRPC working in an R134a environment did not show any significativedegradation during the whole test. A discussion on merits and limitations of the proposed designis provided.

KEYWORDS: Detector design and construction technologies and materials; Resistive-plate cham-bers; Gaseous detectors; Data acquisition circuits

1Corresponding author.

c 2013 IOP Publishing Ltd and Sissa Medialab srl doi:10.1088/1748-0221/8/03/P03012

2013 JINST 8 P03012

Contents

1 Motivation: small and portable RPCs 1

2 Experimental setup 1

3 Results and discussion 3

4 Summary and conclusions 7

1 Motivation: small and portable RPCs

Resistive Plate Chambers (RPCs) belong to the family of gas ionization detectors, perhaps thebroader group of devices used to measure ionization radiation. In some detectors of the family, likethe Geiger Muller counters, a sealed volume is filled with an ionizing gas mixture, being able tosustain approximate constant performances for a long time. Other detectors, like drift chambers,usually have large sizes and require an external gas system to maintain proper operation.

Timing RPC (tRPCs) characteristically provide an outstanding time resolution, usually below100 ps, by using several very narrow gaps, working in avalanche mode and incorporating highfrequency preamplifiers. RPCs usually run reasonably well while gas keeps flowing through thechamber. However, relying on bulky gas systems [1] is not always possible or simple and the needarises for recycling gas back into the system [2]. Besides, the lack of portability of such gas systemsdoes not allow RPCs to be used in relatively small setups.

This work has focused mainly in the development of small timing RPCs able to operate, asGeiger-Muller counters, without the need for an external gas flowing system. Such a detector mightbe of interest in small experimental setups needing to take data for short periods of time. Table 1shows a rough comparison between a regular small Geiger counter and a small one-gap RPC cell.

At this first stage, our main goal has been exploring the feasibility of such small and au-tonomous detectors, starting with a very simple and compact design and analyzing their behaviorin a short period of time (two weeks). Such devices can be very easily built in a few hours withnon-expensive materials.

2 Experimental setup

Figure 1a shows a picture of a sealed RPC inside a test box. The resistive electrodes were made of(160502) mm3 float glass with a resistivity of = 51012 cm. Gas gaps were defined by0.3 mm nylon fishing line spacers and sealed with Teflon band and epoxy glue (Araldite-Standard).High voltage was applied by means of 150 40 mm2 copper tape electrodes as it is shown infigure 1. The signals were picked up at both sides of that electrode. Electrodes of the same sizewere used to ground the external resistive plates at both faces of the detector. The chambers were

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Table 1: Geiger Muller counter and one-gap sRPC gas chamber comparative.

sRPC Geiger MullerUsual sizes 40 40 0.3 mm3 28 20 mm3Gas Volume (V) 480 mm3 560 mm3Inner surface (S) 3250 mm2 434 mm2S/V ratio 6.8 mm1 1 mm1Gas mixture R134a/SF6/Isobutane Neon/Argon + Halogen gasWorking mode Avalanche Geiger-MullerUsual Voltage 3000 V 550 VElectric field constant 1/r

150 mm 40 m

m

(a) First prototype of a two-gap sRPC in a test box.

HighVoltage

R134a / N2

sRPC

Al boxGas input Gas output

Ground

Epoxy glue

Glass

Cu electrodes

2 nF

2 nF1 M

100 kTeflon tape

(b) Electrical layout of a sRPC inside the gas box

Figure 1: Two-gap sRPC

filled with R134a gas. The basic layout of a detector and the electrical layout used in our tests isshown in figure 1b.

Two sealed chambers, installed inside a gas-tight box, were operated in avalanche mode withtwo different gas environments: one with flowing N2, in order to identify any leakage and a possibleexternal contamination, and another with flowing R134a, in order to compensate gas leakage andto highlight any inner gas degradation. An identical unsealed RPC was built and used as referencechamber for verifying the results obtained with the sealed RPCs.

In order to analyze the behavior of the sealed chambers, the experimental setup shown in fig-ure 2 was constructed, allowing to simultaneously measure operational current, event rates, signalwaveforms and temperature. As ionizing radiation a 22Na gamma source was used. A decayingnuclide produces an e+ which, after its annihilation with an e of the medium, yields two gammasgoing in opposite directions. These radioactive source was placed between the RPC under test andan external fast scintillator arrangement consisting of a 123 cm2 BC422 Bicron scintillator, readout at both sides by two H6533 Hamamatsu fast photomultipliers (PMs). The distances to bothdetectors were big enough ( 20 cm) to avoid any counting saturation. With this setup, signalscoming from the same annihilation being in coincidence with both detectors could be selected.

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sRPC

BC422 Scint.PM1

Oscilloscope TEK TDS7104 Digitized

WaveformsTrigger

PT100 Sensor

Camberra2071A

Counter

CAEN2255A

Dual Timer

File1

ADCMCP3424

Arduino MegaAtmega1280

DAQ Board I, T

Rates

Bridge Amplifier

22Na

PM2

Temperature, T

CAEN N471a

P. Supply

CAENN625

FI/FO

WaveformFile2

CAEN N840

O. Disc

CAEN N455

Quad. Coinc.

BGM1013

INA118IntensityMonitor, I

sRPC

sRPC + PMs

Labjack U3-HV

DAQ Board

Figure 2: Acquisition setup.

Signal readout was carried out with a charge-sensitive preamplifier based on the PhillipsBGM1013 integrated preamplifier, providing a gain of 35.5 dB at 1 GHz. The amplified signalwas split with a CAEN N454 Fan In/Fan Out module. One of the output waveforms was digitizedwith a 1 GHz bandwidth TDS7104 Tektronix oscilloscope and then stored in a file for its offlineanalysis. The second signal was used in coincidence with the scintillator output to provide a coin-cidence trigger with gammas produced by the radioactive source. All the rates of the detector andboth PMs were measured by a Canberra 2071A counter and stored in a file by means of an LabjackU3-HV acquisition board.

High voltage was provided by a CAEN N471A high voltage power supply. The operationalcurrent was monitorized by the built-in analog socket that, according to the manufacturer technicalspecifications, provides an accuracy of 2%10 nA. In order to improve the resolution of theoperational current monitoring, an electronic add-on was used. It consisted of a bridge amplifier,INA118, and a high resolution ADC, MCP3424; the devices were configured as shown in figure 2.In this way, calibrating the system with the help of a 0.1 nA accurate Philips PM2525 amperemeter,and taking 15 samples per second, the current accuracy was enhanced up to 0.2 nA.

Using the described experimental setup, our detectors were operated in a controlled tempera-ture environment, with the capability of monitoring the main working variables. It is worth notingthat no atmospheric pressure correction has been made in the analysis of the sealed RPCs data.As these are closed systems, the mass width of the gas stays constant regardless of the externalpressure. As a consequence, no large variations in the counting are expected. The main variablesinforming about the properties of the detectors that have been analyzed are: signal amplitude andrising slope, prompt charge and time resolution.

3 Results and discussion

The behavior of the sealed RPCs has been analyzed and compared to the behaviour of the referencechamber. The first variable analyzed is the operational current. It is well known that it gives a good

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Current[nA]

0.0

0.2

0.4

0.6

0.8

1.0

Ve[kV]

2.7 2.8 2.9 3.0 3.1 3.2

R134a

R134a/SF6/Isobutane (85/10/5)

(a) Operational current dependence with the effectiveelectric field for the reference RPC with both R134a andR134a/SF6/Ibutane (85/10/5) gas mixture.

Cu

rren

t[n

A]-

O

set

0.0

0.2

0.4

0.6

0.8

1.0

Ve[kV]

2.6 2.7 2.8 2.9 3.0 3.1

Boxed tRPC

Day 1

Day 10

Day 20

(b) Operational current trend of the reference RPC, insidea box filled with R134a, with the effective electric fieldafter several days of steady operation.

Figure 3: Analysis of the operational current behavior with both R134a and standard gas mixturewith the boxed sRPCs.

insight on the intensity of the gas drifting [4], as long as it is related with both multiplication andattachment coefficients of the gas [5]. Therefore, the gas condition is expected to be the main wear-ing out factor. In such small RPCs, a current accuracy better than 1 nA is required to provide someinsight of the processes happening in the gas in short time (a few days) periods. For this purpose theeffective voltage Ve f is used. It is the actual voltage existing in the gas gap, i.e. the external powersupply voltage minus the drop of voltage at the resistive plates. Analyzing the operational cur-rent at different applied voltages, our setup allows to detect any possible gas degradation studyingthe evolution of the monitored current. Figure 3a shows the difference in the current, at differentvoltage Ve f , for two well known gases mixtures: R134a and R134a/SF6/Isobutane (85/10/5) [6].Figure 3b shows the evolution of the current of the reference RPCs, placed inside a closed gas box,along a period of three weeks; the observed changes in the current may be due to minor leakages.

Figure 4 show the evolution of the measured rates in coincidence for all the analyzed cham-bers; figure insets show the corresponding evolution of the external daily averaged temperature.Regarding the reference RPC, figure 4a shows that it reaches the stability from the very beginning,with a rate of 6.4 0.2 Hz; almost two times the one of sealed RPCs. The temperature stayed sta-ble by 0.5C. Figure 4b shows the behavior of the sRPC inside the box with flowing R134a. The1 nA current fluctuations observed during the first days may be due to the 3C uncontrolledchanges in the temperature. This is a very well known effect that has been thoroughly studiedin [3]. After the fifth day both, rate and current, stayed stable for the rest of the experiment.

Finally, figure 4c shows that the sRPC operated in the box with flowing N2 stayed stableduring the first 6-7 days. Later, the measured current started to grow steadily while the rate startedto decrease after the 12th day.

It is interesting to realize that, from the very beginning sealed chambers provide both lowerrates and larger currents than the reference RPC. This effect may be caused by the electrical prop-

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Rate(H

z)

2

3

4

5

6

7

8

9

10

11

Curre

nt(nA)

0

1

2

3

4

5

6

7

Time(days)0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

RateCurrent T(

C)

202122232425

Time(days)2 4 6 8 10 12 14

(a) Reference RPC.

Rate

(Hz)

2

3

4

5

6

7

8

9

10

11

Curre

nt(n

A)0

1

2

3

4

5

6

7

Time (days)1 2 3 4 5 6 7 8 9 10 11 12 13 14

Rate Current

T (C

)

202122232425

Time (days)2 4 6 8 10 12 14

(b) sRPC working inside a box with R134a.

Rate(H

z)

2

3

4

5

6

7

8

9

10

11

Curre

nt(nA)

0

1

2

3

4

5

6

7

Time(days)1 2 3 4 5 6 7 8 9 10 11 12 13 14

Rate Current T(

C)

202122232425

Time(days)2 4 6 8 10 12 14

(c) sRPC working inside a box with N2.

Figure 4: In this figures main frame, rate and the operational current for the different RPCs setupsalong two week time are shown where the insets show the laboratory temperature as reference.

erties of the Araldite-glue that may shunt electrical charges between HV and ground.

Figure 5 shows the prompt charge, defined as the amplitude of the amplified and digitized

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Prompt charge [ADC units]0 500 1000 1500 2000 2500

Prob

abilit

y

-310

-210

-110 Day 1 (1532 events)Day 7 (1543 events)Day 14 (1628 events)

(a) Reference RPC.1/v(dv/dt)[1/ns]

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Prob

abilit

y

0.01

0.02

0.03

0.04

0.05

0.06

0.07Day 1 (1502 events)Day 7 (1445 events)Day 14 (1640 events)

(b) Reference RPC.

Prompt charge [ADC units]0 500 1000 1500 2000 2500

Prob

abilit

y

-310

-210

-110 Day 1 (2732 events)Day 7 (2918 events)Day 14 (2133 events)

(c) sRPC inside a box with R134a.1/v(dv/dt)[1/ns]

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Prob

abilit

y

0

0.02

0.04

0.06

0.08

0.1Day 1 (2495 events)Day 7 (2706 events)Day 14 (1627 events)

(d) sRPC inside a box with R134a.

Prompt charge [ADC units]0 500 1000 1500 2000 2500

Prob

abilit

y

-310

-210

-110 Day 1 (2732 events)Day 7 (2118 events)Day 14 (2370 events)

(e) sRPC inside a box with N2.1/v(dv/dt)[1/ns]

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Prob

abilit

y

0

0.02

0.04

0.06

0.08

0.1 Day 1 (2502 events)Day 7 (1745 events)Day 14 (2137 events)

(f) sRPC inside a box with N2.

Figure 5: Distributions of both the prompt charge and slopes for all the RPCs analyzed in a 7 dayspace over the experiment time. Differences in the number of entries are a consequence of changesin the measuring time and the oscilloscope working mode.

signal, of the three detectors during periods of two weeks. Figure 5a shows how, in the referenceRPC, the charge stays constant during the whole period, as it was expected. Figure 5c shows how,at the sRPC in R134a, a significative amount of larger charges do appear from the very beginningto, later, decrease slightly with time; this effect might be a consequence of the decrease of thetemperature in 2C during the data taking period. However, figure 5e shows how, at the sRPCworking in N2, there is shift towards higher charges with time.

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Low

prom

pt ch

arge

ratio

0.4

0.5

0.6

0.7

0.8

0.9

1

Time (days)1 2 3 4 5 6 7 8 9 10 11 12 13 14

Standard RPCsRPC inside R134a box sRPC inside N2 box

Figure 6: Low prompt charge ratio for reference RPC and sRPC immersed two gas environments,R134a and Nitrogen.

As, in this case, the temperature stayed within 1C during the whole period, the observedbehavior might be a consequence of the degradation of the detector due to some gas contamination.This effect is the same already shown in figure 4c for operational current. Figures 5b, 5d and 5fshow the distribution of the rising slopes, measured between the 10% and 90% of the amplitudefor the respective detectors. In both, the reference RPC and the sRPC working in R134a gasenvironment, the slopes show the same behavior through time. By contrast, the sRPC working inN2 environment shows an increase in fast signals with time. The greater abundance of large chargesobserved in both sRPCs might be related to the higher currents already reported for those chambers.

Figure 6 summarizes the behavior of the sRPCs already discussed in the previous paragraphscompared to the reference chamber. It shows the evolution in time of the ratio between low chargesand all the prompt charges. The threshold between low and high charges was set at 1000 ADCunits. It can be seen how the ratio stays constant for the reference RPC, and decreases slightlyfor the sRPC working in the R134a gas box, fact that can be explained as due to changes in thetemperature. On the other hand, the behavior of the sRPC working in a N2 environment, on theother side, stays constant during about 6-7 days, starting then a steady decrease, possibly due to gascontamination. The smaller ratios observed in both sRPCs might be due to the same effect alreadyobserved in the prompt charges distributions.

4 Summary and conclusions

Resistive Plate Chambers are very useful detectors whenever good time resolution is needed forthe counting of charged particles. However, usually they require very bulky gas systems and, asconsequence, their use is often limited to big experimental setups. The use of sealed RPCs, sRPCs,might be an acceptable alternative in small experiments, or at places with difficult access, acquiringdata during short periods of time (a few days). They might also be very appropriate for triggeringin low rate experiments.

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Several compact, two-gap sRPCs (with a gas volume of 2.4 cm3) have been built with verycommon materials and their properties have been compared with those of a reference open RPC.Namely, the behavior in time of their operational current, working rate and prompt charge distri-butions has been analyzed. The sRPCs behaved smoothly during about 6-7 days before showingany appreciable change. However, from the very beginning, the measured current and the observedprompt charge distributions differed from the figures supplied by the reference chamber. This effectmight be caused by the electrical conductivity of the glue used in their manufacture.

The sRPC working inside an R134a environment did not show any changes during a two weekperiod, indicating that the gas did not suffer any significant aging during that time. Concerningthe properties of a sRPC working within a N2 environment, it started to degrade after about oneweek time, indicating the presence of small leaks. This might originate in the difficulties to achievea good fixing between the used glue and the glass. This issue might be corrected in the futureperforming a specific treatment of the glass before the gluing process.

Results given in this article are very encouraging because they prove the feasibility of veryeasy developing small and compact RPCs able to show a steady operation for periods longer thanone week. This kind of one-way detectors may offer a very useful alternative whenever a suddenneed of a good time resolution detector is needed. More work is still needed in order to analyze theperformance stability of the detectors, namely the efficiency and the time resolution, using severaldifferent designs.

Acknowledgments

We thank the LIP-Coimbra members: Luis Lopes, Alberto Blanco and Paulo Fonte for sharing withus their invaluable knowledge on RPCs detectors and Georgy Kornakov, from the LabCAF, for hisvery useful technical support and interesting assesments.

References

[1] S. Kalmani, N. Mondal, B. Satyanarayana, P. Verma and A. Joshi, On-line gas mixing andmulti-channel distribution system, Nucl. Instrum. Meth. A 602 (2009) 845.

[2] D.M. Rossi and H. Simon, A closed-circuit gas recycling system for RPC detectors, Nucl. Instrum.Meth. A 661 (2012) S230.

[3] D. Gonzalez-Diaz et al., The effect of temperature on the rate capability of glass timing RPCs, Nucl.Instrum. Meth. A 555 (2005) 72.

[4] S. Ramo, Currents Induced by Electron Motion, Proc. IRE 27 (1939) 584-585.

[5] W. Riegler, C. Lippmann and R. Veenhof, Detector physics and simulation of resistive plate chambers,Nucl. Instrum. Meth. A 500 (2003) 144.

[6] L. Lopes, P. Fonte and A. Mangiarotti, Systematic study of gas mixtures for timing RPCs, Nucl.Instrum. Meth. A 661 (2012) S194.

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Motivation: small and portable RPCsExperimental setupResults and discussionSummary and conclusions