- Email: [email protected]
Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background R. Guida a , B. Mandelli a ,∗, G. Rigoletti b a b
CERN, Meyrin, Switzerland Universita’ Milano-Bicocca, Italy
ARTICLE
INFO
Keywords: Resistive Plate Chamber Gas systems Greenhouse gases HFO Environmentally friendly gas mixtures
ABSTRACT Resistive Plate Chamber (RPC) detectors are widely used at the CERN LHC experiments as muon trigger thanks to their excellent time resolution. They are operated with a Freon-based gas mixture containing C2 H2 F4 and SF6 , both greenhouse gases (GHG) with a very high global warming potential (GWP). The search of new environmentally friendly gas mixtures is necessary to reduce GHG emissions and costs as well as to optimize RPC performance. Several recently available gases with low GWP have been identified as possible replacements for C2 H2 F4 and SF6 . More than 60 environmentally friendly gas mixtures have been investigated on 2 mm single-gap RPCs. The RPC detectors have been tested in laboratory conditions and at the CERN Gamma Irradiation Facility (GIF++), which provides a high energy muon beam combined with an intense gamma source allowing to simulate the background expected at HL-LHC. The performance of RPCs were studied at different gamma rates with the new environmentally friendly gases by measuring efficiency, streamer probability, rate capability, induced charge, cluster size and time resolution. To finalize the studies, the RPCs are now operated under gas recirculation with the selected new gas mixture and exposed to the intense gamma radiation of GIF++ for evaluating possible long-term aging effects, gas damage due to radiation and compatibility of LHC gas system with new gases.
1. Introduction The Resistive Plate Chamber (RPC) detectors [1] are widely employed in large-scale experiments thanks to their excellent time resolution and low production cost. At CERN, large RPC systems are used in the ATLAS, CMS and ALICE experiments for the muon trigger systems. They are suitably operated with a three components gas mixture made of C2 H2 F4 (between 90% and 95%), SF6 and iC4 H10 that allows operation in avalanche mode at high rate (∼1 kHz/cm2 ). Unfortunately C2 H2 F4 and SF6 have a global warming potential (GWP) of 1430 and 23 900 respectively, classifying them as greenhouse gases (GHGs). Given the large detector volume (about 15 m3 for each experiment) and the use of expensive GHGs, the RPC gas systems are operated with gas recirculation. About 90% of the gas mixture is recirculated inside the system while a fraction of 10% is replaced with new fresh gas.1 Despite this high recirculation factor, 87% of the CERN detectors’ GHG emission comes from the RPC systems [2]: 80% from the use of C2 H2 F4 and 7% from SF6 . Indeed even though the SF6 concentration in the ATLAS and CMS RPC gas mixture is only 0.3%, its high GWP results in a large GHG emission contribution. ∗ 1
The European Union (EU) set a F-gas (fluorinated-gas) regulation starting from January 2015 that can be summarized in the following points [3]: • Limiting the total amount of the most important F-gases that can be sold in the EU from 2015 onwards and phasing them down in steps to one-fifth of 2014 sales in 2030. • Banning the use of F-gases in many new types of equipment where less harmful alternatives are widely available. • Preventing emissions of F-gases from existing equipment by requiring checks, proper servicing and recovery of the gases at the end of the equipment’s life. Despite this recent EU F-gas regulation, GHGs would remain available for research applications but their price could raise possibly making gas detectors operation very costly. The search of new environmentally friendly gas mixtures for RPCs is therefore advisable for reducing GHG emissions and costs, as well as to optimize RPC performance and eventual aging issues.
Corresponding author. E-mail address: [email protected] (B. Mandelli). The fresh gas cannot be decreased since it is used to compensate for leaks at detector level.
https://doi.org/10.1016/j.nima.2019.04.027 Received 26 March 2019; Received in revised form 1 April 2019; Accepted 5 April 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: R. Guida, B. Mandelli and G. Rigoletti, Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.027.
R. Guida, B. Mandelli and G. Rigoletti
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx Table 1 Summary of foremost parameters obtained at the efficiency knee for standard gas mixture and two mixtures with the addition of HFO and an inert gas (He or CO2 ). The pulse charge is written as ‘‘avalanche’’/‘‘streamer’’ and 𝛥V represents the distance in V between the efficiency curve and the streamer curve at 50% efficiency [5].
2. Possible replacements of C𝟐 H𝟐 𝐅𝟒 The search for a LHC RPC ‘‘environmentally friendly’’ gas mixture has been focused on replacement of the main gas mixture component, the C2 H2 F4 (commercially known as R134a). In the Hydro-FluroCarbon (HFC) group several gases have been identified as possible candidates, as for example R245fa (GWP 1030), R32 (GWP 650) and R152a (GWP 140). In industrial applications the R134a is being substituted with fluorinated propene refrigerants called hydro-fluoro-olefins2 (HFOs), which have a GWP less than 6 [4]. With respect to the R134a, HFOs contain the same amount of fluorine atoms but one carbon more, bringing RPC to a higher working voltage. Even if several alternatives are available on the market, a suitable gas mixture replacement for the LHC RPC systems is particularly challenging to find because most of the infrastructure (i.e. high voltage systems, cables, front-end electronics) as well as the detectors themselves cannot be easily replaced. In this paper, the studies are focused on identifying new gas mixtures able to reproduce the same RPC performance observed with the current C2 H2 F4 based mixture of the LHC experiments (called standard RPC gas mixture in the following).
Gas mixture
HV (V)
Streamer prob (%)
Pulse charge (pC)
𝛥V (V)
R134a/iC4 H10 /SF6 95.2/4.5/0.3 GWP 1490
9 600
1.5
0.5/6
1000
HFO/R134a/He/iC4 H10 /SF6 37.45/37.45/20/4.5/0.6 GWP 890
10 500
1.8
0.5/6
970
HFO/R134a/CO2 /iC4 H10 /SF6 22.25/22.25/50/4.5/1.0 GWP 560
10 500
5
1.5/7.5
950
3. Characterization of RPC with different eco-friendly gas mixtures 3.1. Set-up The characterization of RPC detector with different gas mixtures has been performed in a dedicated set-up. Two High Pressure Laminate RPCs with a 2 mm gas gap, a surface of about 80 × 100 cm2 and readout strips 2.1 cm wide have been used. A set of scintillators is employed as muon trigger. The induced signals on the strips are directly acquired with a CAEN Digitizer V1730 without any amplification module. For each waveform the signals are processed to obtain pulse height, charge and timing distributions. Thanks to a gas mixing unit able to deliver 6-components gas mixtures, more than 60 different gas mixtures have been tested on RPC detectors. For each gas mixture, the RPC performance is evaluated by measuring efficiency, streamer probability, rate capability, induced charge, cluster size and time resolution. A final comparison between different selected gas mixtures is done using as key parameters the applied voltage and the streamer probability at knee efficiency, the pulse charge for avalanche and streamer, the cluster size and the voltage difference between 50% efficiency and 50% streamer probability.
Fig. 1. Efficiency (continuous line) and streamer probability (dotted lines) as a function of the HV for gas mixtures with different concentrations of SF6 , 50% CO2 , 4.5% iC4 H10 and the remaining gas is C2 H2 F4 and HFO in the same proportions.
to reach an acceptable working point but streamer probability stays higher than with the RPC standard gas mixture. To overcome this issue, it is necessary to keep a small amount of C2 H2 F4 in the gas mixture and to increase the SF6 concentration (Fig. 1). The CO2 based gas mixtures have a GWP in the range of 500–600 and they could be considered possible alternatives to the RPC standard gas mixture, even if they do not match all RPC performance requirements for the LHC experiments (Table 1).
3.2. Experimental results with HFO based gas mixtures 4. RPC operated with eco-friendly gas mixture in presence of gamma background radiation
As a first test, C2 H2 F4 was completely replaced with HFO in the RPC standard gas mixture, revealing that the use of HFO as main gas component requires operation at much higher applied voltages (more than 13 kV) with respect to R134a and signal charges are smaller. To overcome this problem, the addition of an inert gas to lower the HV working point can be envisaged [5]. The addition of about 20%– 30% of He is enough to achieve a HV working point similar to the standard gas mixture and the mixture HFO/C2 H2 F4 /He/iC4 H10 /SF6 (37.45/37.45/20/4.5/0.6) allows to obtain RPC foremost parameters very similar to the standard gas mixture (Table 1). However the use of He is not suitable at LHC experiments due to the presence of photomultipliers [6]. A good compromise can be CO2 : the addition of 10% reduces the working point by 800 V. Furthermore, even if CO2 is a quencher gas, it does not have the same effect of iC4 H10 for the photon absorption in the RPC detectors. About 40% or 50% of CO2 is necessary
The operation of RPC with selected eco-friendly gas mixtures has to be validated in LHC-like conditions, i.e. in presence of high background radiation and under gas recirculation. This can be achieved at the CERN Gamma Irradiation Facility (GIF++) [7], which provides radiation from an intense 137 Cs source of 14 TBq combined with a high energy charged particle beam (mainly muon beam with momentum up to 100 GeV/c). In this study, different absorption factors (ABSs) have been selected in order to have a gamma rate spacing from 0.7 kHz/cm2 (ABS 22000) to 55.3 kHz/cm2 (ABS 100), corresponding to about the maximum gamma rate that the RPCs will see in CMS during the HL-LHC phase [8]. RPC performance have been compared for different ABSs between the standard gas mixture and two mixtures based on the addition of HFO and CO2 : C2 H2 F4 /HFO/CO2 /iC4 H10 /SF6 (27.25/27.25/40/4.5/1) and C2 H2 F4 /HFO/CO2 /iC4 H10 /SF6 (22.25/22.25/50/4.5/1). A first comparison between the standard gas mixture and HFO-based gas mixtures has been done for the detector currents: at same efficiency, the currents of RPC operated with HFO-based gas mixtures are about 40% higher
2 There are several isomers of HFO and the most commercially available are HFO-1234yf (2,3,3,3-tetrafluoropropene) and HFO-1234ze (1,3,3,3tetrafluoropropene), with a GWP of 4 and 6 respectively. In this paper the HFO-1234ze has been used for all measurements.
2
Please cite this article as: R. Guida, B. Mandelli and G. Rigoletti, Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.027.
R. Guida, B. Mandelli and G. Rigoletti
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 2. Efficiency (continuous line) and streamer probability (dotted lines) as a function of the HV for the gas mixture C2 H2 F4 /HFO/CO2 /iC4 H10 /SF6 (27.25/27.25/40/4.5/1) at different ABSs.
Fig. 3. Avalanche (dotted line) and streamer (continuous line) charges at different ABSs for the RPC standard gas mixture and two eco-friendly gas mixtures.
with respect to the standard gas mixture. Fig. 2 shows the efficiency and streamer probability curves for the 40% CO2 gas mixture. The efficiency curves at different ABSs are completely overlapped after the correction for the resistivity of electrodes and gas density. This shows that the muon detection efficiency is not affected by the background rate. The distance between the efficiency at 50% and the streamer probability is about 830 V against the 1000 V of the standard gas mixture, i.e. the working plateau is slightly reduced with HFO-based gas mixtures. At a gamma counting rate of about 300 Hz/cm2 (ABS 220) the streamer probability is 13% for the standard gas mixture and ∼25% for the other two HFO-based gas mixtures. Another interesting parameter is the pulse charge for avalanche and streamer under high background radiation. As it was seen already in the laboratory, the avalanche charge is higher for these CO2 -based gas mixtures (about 1.7 pC) with respect to the standard gas mixture (about 1 pC) while the streamer charge is very similar for the three gas mixtures. With the increase of the radiation, both avalanche and streamer charges for the eco-friendly gas mixtures decrease while they stay almost constant for the standard gas mixture (Fig. 3). This is probably due to charge development effects inside the gas gap given by the higher number of streamers in the eco-friendly gas mixtures. Fig. 4 shows the detector counting rate as a function of detector efficiency at ABS 220 (∼41.2 kHz/cm2 ): the counting rate is almost the same for all gas mixtures and the one with 40% of CO2 reaches rates as high as the standard gas mixture, making the mixture suitable for the expected background rate of the HL-LHC Phase [8].
Fig. 4. Detector counting rate as a function of detector efficiency at ABS 220 (41.2 kHz/cm2 ) for three gas mixtures.
new impurities, and with an Ion Selective Electrode station to measure HF. Fig. 5 shows two gas chromatograms obtained by analysing the gas exiting an irradiated RPC: several impurities are created under irradiation and they can be attributed to the breaking of C2 H2 F4 and HFO. A meaningful measurement is the comparison of HF production for the two gas mixtures at detector efficiency and with the same irradiation rate (counting rate of ∼300 Hz/cm2 ): the standard gas mixture produces about 3 ppm/h of F− while the HFO-based gas mixture about 5 ppm/h.3 Considering that the HFO-based gas mixture contains 27.25% of C2 H2 F4 and 27.25% of HFO and assuming other components inert in the process, one can conclude that HFO breaks five times more easily than C2 H2 F4 . This can also be understood by the fact that HFOs have a very short atmospheric lifetime and therefore they decompose easier than C2 H2 F4 . This test highlights that if HFOs will be used in LHC experiments, the accumulation of impurities under gas recirculation will be higher with respect to C2 H2 F4 and their effect on long-term detector operation has to be studied in more detail.
4.1. Creation of impurities with HFO-based gas mixtures The operation of RPC under high gamma background radiation has to be validated also with gas recirculation since in the LHC experiments RPC gas recirculation systems are compulsory for cost and technical reasons. Nevertheless, it is well known [9,10] that under the effects of radiation and electric field, the C2 H2 F4 molecule breaks into fluorine radicals, which can accumulate under gas recirculation and could be harmful for the long-term detector operation. Furthermore in presence of water, the free fluoride ions become hydrofluoric acid (HF), which is a very reactive compound. For this reason, a measurement campaign of the return gas composition was carried out by irradiating two RPCs at the GIF++ facility with standard gas mixture and with the gas mixture C2 H2 F4 /HFO/CO2 /iC4 H10 /SF6 (27.25/27.25/40/4.5/1). Measurements were performed at different high voltages and ABSs. The gas at the exhaust of the detectors was analysed with a Gas Chromatograph coupled with a Mass Spectrometer to identify and quantify possible
3 Measurements of F− accumulated in a sampling bottle following the method described in [9].
3
Please cite this article as: R. Guida, B. Mandelli and G. Rigoletti, Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.027.
R. Guida, B. Mandelli and G. Rigoletti
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Preliminary studies on gas recirculation and production of impurities under irradiation show that the HFO breaks more easily than C2 H2 F4 creating several impurities and HF. Based on these results, the long-term operation of RPC with HFO-based gas mixture under gas recirculation and the long-term effects of these impurities have to be validated. References [1] R. Santonico, R. Cardarelli, Developments of Resistive Plate Counters, Nucl. Instrum. Methods A 187 (1981) 377. [2] M. Capeans, R. Guida, B. Mandelli, Strategies for reducing the environmental impact of gaseous detector operation at the CERN LHC experiments, Nucl. Instrum. Methods A A 845 (2017) 253–256, http://dx.doi.org/10.1016/j.nima. 2016.04.067. [3] Regulation (EU) No. 517/2014 of the European Parliament and of the Council on Fluorinated Greenhouse Gases and Repealing Regulation (EC) No. 842/2006. [4] J.S. Brown, HFOs new low global warming potential refrigerant, ASHRAE J. 51 (8) (2009) 22–29. [5] M. Capeans, R. Guida, B. Mandelli, Characterization of RPC operation with new environmental friendly mixtures for LHC application and beyond, J. Instrum. 11 (2016) C07016, http://dx.doi.org/10.1088/1748-0221/11/07/C07016. [6] J.R. Incandela, et al., The performance of photomultipliers exposed to helium, Nucl. Instrum. Methods A 269 (1988) 237–245, http://dx.doi.org/10.1016/01689002(88)90885-6. [7] R. Guida, On behalf of the EN, EP and AIDA GIF++ Collaboration, GIF++: A new CERN irradiation facility to test large-area particle detectors for the high-luminosity LHC program, PoS, ICHEP2016, 260. [8] CMS Collaboration, The Phase-2 upgrade of the CMS muon detectors, CERN-LHCC-2017-012-CMS-TDR-016. [9] R. Guida, et al., Results about HF production and bakelite analysis for the CMS Resistive Plate Chambers, Nucl. Instrum. Methods A 594 (2008) S140–S147, http://dx.doi.org/10.1016/j.nima.2008.06.009. [10] M. Capeans, I. Glushkov, R. Guida, F. Hahn, S. Haider, Optimization of a closedloop gas system for the operation of Resistive Plate Chambers at the Large Hadron Collider experiments, Nucl. Instrum. Methods A 661 (2012) S214–S221, http://dx.doi.org/10.1016/j.nima.2010.08.077.
Fig. 5. Gas chromatograms of the analysed return gas from an irradiated RPC (gamma rate ∼41.2 Hz/cm2 ) at two different voltages. Several impurities created under irradiation are visible and their concentration increases with the increase of the HV.
5. Conclusions The search for an environmentally friendly gas mixture for the RPC detectors at LHC is a challenge as it has to be compatible with all experiment requirements without changing any hardware component. The industrial alternatives to C2 H2 F4 are the HFOs, which unfortunately cannot directly substitute the C2 H2 F4 in RPC gas mixture. Few 5-components gas mixtures with a GWP lower than 600 have been identified as possible alternatives, even if they do not match all LHC RPC performance requirements of the standard gas mixture. RPCs performance has been studied at different high gamma rates (up to ∼55.3 Hz/cm2 ) with two eco-friendly gas mixtures based on HFO and CO2 : currents, streamer probability and avalanche charge are slightly higher with respect to RPC standard gas mixture and compatible with the results obtained in laboratory. For future RPC applications where high voltage systems and detectors can be designed without specific constraints, HFO based gas mixtures are promising.
4
Please cite this article as: R. Guida, B. Mandelli and G. Rigoletti, Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.027.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background R. Guida a , B. Mandelli a ,∗, G. Rigoletti b a b
CERN, Meyrin, Switzerland Universita’ Milano-Bicocca, Italy
ARTICLE
INFO
Keywords: Resistive Plate Chamber Gas systems Greenhouse gases HFO Environmentally friendly gas mixtures
ABSTRACT Resistive Plate Chamber (RPC) detectors are widely used at the CERN LHC experiments as muon trigger thanks to their excellent time resolution. They are operated with a Freon-based gas mixture containing C2 H2 F4 and SF6 , both greenhouse gases (GHG) with a very high global warming potential (GWP). The search of new environmentally friendly gas mixtures is necessary to reduce GHG emissions and costs as well as to optimize RPC performance. Several recently available gases with low GWP have been identified as possible replacements for C2 H2 F4 and SF6 . More than 60 environmentally friendly gas mixtures have been investigated on 2 mm single-gap RPCs. The RPC detectors have been tested in laboratory conditions and at the CERN Gamma Irradiation Facility (GIF++), which provides a high energy muon beam combined with an intense gamma source allowing to simulate the background expected at HL-LHC. The performance of RPCs were studied at different gamma rates with the new environmentally friendly gases by measuring efficiency, streamer probability, rate capability, induced charge, cluster size and time resolution. To finalize the studies, the RPCs are now operated under gas recirculation with the selected new gas mixture and exposed to the intense gamma radiation of GIF++ for evaluating possible long-term aging effects, gas damage due to radiation and compatibility of LHC gas system with new gases.
1. Introduction The Resistive Plate Chamber (RPC) detectors [1] are widely employed in large-scale experiments thanks to their excellent time resolution and low production cost. At CERN, large RPC systems are used in the ATLAS, CMS and ALICE experiments for the muon trigger systems. They are suitably operated with a three components gas mixture made of C2 H2 F4 (between 90% and 95%), SF6 and iC4 H10 that allows operation in avalanche mode at high rate (∼1 kHz/cm2 ). Unfortunately C2 H2 F4 and SF6 have a global warming potential (GWP) of 1430 and 23 900 respectively, classifying them as greenhouse gases (GHGs). Given the large detector volume (about 15 m3 for each experiment) and the use of expensive GHGs, the RPC gas systems are operated with gas recirculation. About 90% of the gas mixture is recirculated inside the system while a fraction of 10% is replaced with new fresh gas.1 Despite this high recirculation factor, 87% of the CERN detectors’ GHG emission comes from the RPC systems [2]: 80% from the use of C2 H2 F4 and 7% from SF6 . Indeed even though the SF6 concentration in the ATLAS and CMS RPC gas mixture is only 0.3%, its high GWP results in a large GHG emission contribution. ∗ 1
The European Union (EU) set a F-gas (fluorinated-gas) regulation starting from January 2015 that can be summarized in the following points [3]: • Limiting the total amount of the most important F-gases that can be sold in the EU from 2015 onwards and phasing them down in steps to one-fifth of 2014 sales in 2030. • Banning the use of F-gases in many new types of equipment where less harmful alternatives are widely available. • Preventing emissions of F-gases from existing equipment by requiring checks, proper servicing and recovery of the gases at the end of the equipment’s life. Despite this recent EU F-gas regulation, GHGs would remain available for research applications but their price could raise possibly making gas detectors operation very costly. The search of new environmentally friendly gas mixtures for RPCs is therefore advisable for reducing GHG emissions and costs, as well as to optimize RPC performance and eventual aging issues.
Corresponding author. E-mail address: [email protected] (B. Mandelli). The fresh gas cannot be decreased since it is used to compensate for leaks at detector level.
https://doi.org/10.1016/j.nima.2019.04.027 Received 26 March 2019; Received in revised form 1 April 2019; Accepted 5 April 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: R. Guida, B. Mandelli and G. Rigoletti, Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.027.
R. Guida, B. Mandelli and G. Rigoletti
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx Table 1 Summary of foremost parameters obtained at the efficiency knee for standard gas mixture and two mixtures with the addition of HFO and an inert gas (He or CO2 ). The pulse charge is written as ‘‘avalanche’’/‘‘streamer’’ and 𝛥V represents the distance in V between the efficiency curve and the streamer curve at 50% efficiency [5].
2. Possible replacements of C𝟐 H𝟐 𝐅𝟒 The search for a LHC RPC ‘‘environmentally friendly’’ gas mixture has been focused on replacement of the main gas mixture component, the C2 H2 F4 (commercially known as R134a). In the Hydro-FluroCarbon (HFC) group several gases have been identified as possible candidates, as for example R245fa (GWP 1030), R32 (GWP 650) and R152a (GWP 140). In industrial applications the R134a is being substituted with fluorinated propene refrigerants called hydro-fluoro-olefins2 (HFOs), which have a GWP less than 6 [4]. With respect to the R134a, HFOs contain the same amount of fluorine atoms but one carbon more, bringing RPC to a higher working voltage. Even if several alternatives are available on the market, a suitable gas mixture replacement for the LHC RPC systems is particularly challenging to find because most of the infrastructure (i.e. high voltage systems, cables, front-end electronics) as well as the detectors themselves cannot be easily replaced. In this paper, the studies are focused on identifying new gas mixtures able to reproduce the same RPC performance observed with the current C2 H2 F4 based mixture of the LHC experiments (called standard RPC gas mixture in the following).
Gas mixture
HV (V)
Streamer prob (%)
Pulse charge (pC)
𝛥V (V)
R134a/iC4 H10 /SF6 95.2/4.5/0.3 GWP 1490
9 600
1.5
0.5/6
1000
HFO/R134a/He/iC4 H10 /SF6 37.45/37.45/20/4.5/0.6 GWP 890
10 500
1.8
0.5/6
970
HFO/R134a/CO2 /iC4 H10 /SF6 22.25/22.25/50/4.5/1.0 GWP 560
10 500
5
1.5/7.5
950
3. Characterization of RPC with different eco-friendly gas mixtures 3.1. Set-up The characterization of RPC detector with different gas mixtures has been performed in a dedicated set-up. Two High Pressure Laminate RPCs with a 2 mm gas gap, a surface of about 80 × 100 cm2 and readout strips 2.1 cm wide have been used. A set of scintillators is employed as muon trigger. The induced signals on the strips are directly acquired with a CAEN Digitizer V1730 without any amplification module. For each waveform the signals are processed to obtain pulse height, charge and timing distributions. Thanks to a gas mixing unit able to deliver 6-components gas mixtures, more than 60 different gas mixtures have been tested on RPC detectors. For each gas mixture, the RPC performance is evaluated by measuring efficiency, streamer probability, rate capability, induced charge, cluster size and time resolution. A final comparison between different selected gas mixtures is done using as key parameters the applied voltage and the streamer probability at knee efficiency, the pulse charge for avalanche and streamer, the cluster size and the voltage difference between 50% efficiency and 50% streamer probability.
Fig. 1. Efficiency (continuous line) and streamer probability (dotted lines) as a function of the HV for gas mixtures with different concentrations of SF6 , 50% CO2 , 4.5% iC4 H10 and the remaining gas is C2 H2 F4 and HFO in the same proportions.
to reach an acceptable working point but streamer probability stays higher than with the RPC standard gas mixture. To overcome this issue, it is necessary to keep a small amount of C2 H2 F4 in the gas mixture and to increase the SF6 concentration (Fig. 1). The CO2 based gas mixtures have a GWP in the range of 500–600 and they could be considered possible alternatives to the RPC standard gas mixture, even if they do not match all RPC performance requirements for the LHC experiments (Table 1).
3.2. Experimental results with HFO based gas mixtures 4. RPC operated with eco-friendly gas mixture in presence of gamma background radiation
As a first test, C2 H2 F4 was completely replaced with HFO in the RPC standard gas mixture, revealing that the use of HFO as main gas component requires operation at much higher applied voltages (more than 13 kV) with respect to R134a and signal charges are smaller. To overcome this problem, the addition of an inert gas to lower the HV working point can be envisaged [5]. The addition of about 20%– 30% of He is enough to achieve a HV working point similar to the standard gas mixture and the mixture HFO/C2 H2 F4 /He/iC4 H10 /SF6 (37.45/37.45/20/4.5/0.6) allows to obtain RPC foremost parameters very similar to the standard gas mixture (Table 1). However the use of He is not suitable at LHC experiments due to the presence of photomultipliers [6]. A good compromise can be CO2 : the addition of 10% reduces the working point by 800 V. Furthermore, even if CO2 is a quencher gas, it does not have the same effect of iC4 H10 for the photon absorption in the RPC detectors. About 40% or 50% of CO2 is necessary
The operation of RPC with selected eco-friendly gas mixtures has to be validated in LHC-like conditions, i.e. in presence of high background radiation and under gas recirculation. This can be achieved at the CERN Gamma Irradiation Facility (GIF++) [7], which provides radiation from an intense 137 Cs source of 14 TBq combined with a high energy charged particle beam (mainly muon beam with momentum up to 100 GeV/c). In this study, different absorption factors (ABSs) have been selected in order to have a gamma rate spacing from 0.7 kHz/cm2 (ABS 22000) to 55.3 kHz/cm2 (ABS 100), corresponding to about the maximum gamma rate that the RPCs will see in CMS during the HL-LHC phase [8]. RPC performance have been compared for different ABSs between the standard gas mixture and two mixtures based on the addition of HFO and CO2 : C2 H2 F4 /HFO/CO2 /iC4 H10 /SF6 (27.25/27.25/40/4.5/1) and C2 H2 F4 /HFO/CO2 /iC4 H10 /SF6 (22.25/22.25/50/4.5/1). A first comparison between the standard gas mixture and HFO-based gas mixtures has been done for the detector currents: at same efficiency, the currents of RPC operated with HFO-based gas mixtures are about 40% higher
2 There are several isomers of HFO and the most commercially available are HFO-1234yf (2,3,3,3-tetrafluoropropene) and HFO-1234ze (1,3,3,3tetrafluoropropene), with a GWP of 4 and 6 respectively. In this paper the HFO-1234ze has been used for all measurements.
2
Please cite this article as: R. Guida, B. Mandelli and G. Rigoletti, Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.027.
R. Guida, B. Mandelli and G. Rigoletti
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 2. Efficiency (continuous line) and streamer probability (dotted lines) as a function of the HV for the gas mixture C2 H2 F4 /HFO/CO2 /iC4 H10 /SF6 (27.25/27.25/40/4.5/1) at different ABSs.
Fig. 3. Avalanche (dotted line) and streamer (continuous line) charges at different ABSs for the RPC standard gas mixture and two eco-friendly gas mixtures.
with respect to the standard gas mixture. Fig. 2 shows the efficiency and streamer probability curves for the 40% CO2 gas mixture. The efficiency curves at different ABSs are completely overlapped after the correction for the resistivity of electrodes and gas density. This shows that the muon detection efficiency is not affected by the background rate. The distance between the efficiency at 50% and the streamer probability is about 830 V against the 1000 V of the standard gas mixture, i.e. the working plateau is slightly reduced with HFO-based gas mixtures. At a gamma counting rate of about 300 Hz/cm2 (ABS 220) the streamer probability is 13% for the standard gas mixture and ∼25% for the other two HFO-based gas mixtures. Another interesting parameter is the pulse charge for avalanche and streamer under high background radiation. As it was seen already in the laboratory, the avalanche charge is higher for these CO2 -based gas mixtures (about 1.7 pC) with respect to the standard gas mixture (about 1 pC) while the streamer charge is very similar for the three gas mixtures. With the increase of the radiation, both avalanche and streamer charges for the eco-friendly gas mixtures decrease while they stay almost constant for the standard gas mixture (Fig. 3). This is probably due to charge development effects inside the gas gap given by the higher number of streamers in the eco-friendly gas mixtures. Fig. 4 shows the detector counting rate as a function of detector efficiency at ABS 220 (∼41.2 kHz/cm2 ): the counting rate is almost the same for all gas mixtures and the one with 40% of CO2 reaches rates as high as the standard gas mixture, making the mixture suitable for the expected background rate of the HL-LHC Phase [8].
Fig. 4. Detector counting rate as a function of detector efficiency at ABS 220 (41.2 kHz/cm2 ) for three gas mixtures.
new impurities, and with an Ion Selective Electrode station to measure HF. Fig. 5 shows two gas chromatograms obtained by analysing the gas exiting an irradiated RPC: several impurities are created under irradiation and they can be attributed to the breaking of C2 H2 F4 and HFO. A meaningful measurement is the comparison of HF production for the two gas mixtures at detector efficiency and with the same irradiation rate (counting rate of ∼300 Hz/cm2 ): the standard gas mixture produces about 3 ppm/h of F− while the HFO-based gas mixture about 5 ppm/h.3 Considering that the HFO-based gas mixture contains 27.25% of C2 H2 F4 and 27.25% of HFO and assuming other components inert in the process, one can conclude that HFO breaks five times more easily than C2 H2 F4 . This can also be understood by the fact that HFOs have a very short atmospheric lifetime and therefore they decompose easier than C2 H2 F4 . This test highlights that if HFOs will be used in LHC experiments, the accumulation of impurities under gas recirculation will be higher with respect to C2 H2 F4 and their effect on long-term detector operation has to be studied in more detail.
4.1. Creation of impurities with HFO-based gas mixtures The operation of RPC under high gamma background radiation has to be validated also with gas recirculation since in the LHC experiments RPC gas recirculation systems are compulsory for cost and technical reasons. Nevertheless, it is well known [9,10] that under the effects of radiation and electric field, the C2 H2 F4 molecule breaks into fluorine radicals, which can accumulate under gas recirculation and could be harmful for the long-term detector operation. Furthermore in presence of water, the free fluoride ions become hydrofluoric acid (HF), which is a very reactive compound. For this reason, a measurement campaign of the return gas composition was carried out by irradiating two RPCs at the GIF++ facility with standard gas mixture and with the gas mixture C2 H2 F4 /HFO/CO2 /iC4 H10 /SF6 (27.25/27.25/40/4.5/1). Measurements were performed at different high voltages and ABSs. The gas at the exhaust of the detectors was analysed with a Gas Chromatograph coupled with a Mass Spectrometer to identify and quantify possible
3 Measurements of F− accumulated in a sampling bottle following the method described in [9].
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Please cite this article as: R. Guida, B. Mandelli and G. Rigoletti, Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.027.
R. Guida, B. Mandelli and G. Rigoletti
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Preliminary studies on gas recirculation and production of impurities under irradiation show that the HFO breaks more easily than C2 H2 F4 creating several impurities and HF. Based on these results, the long-term operation of RPC with HFO-based gas mixture under gas recirculation and the long-term effects of these impurities have to be validated. References [1] R. Santonico, R. Cardarelli, Developments of Resistive Plate Counters, Nucl. Instrum. Methods A 187 (1981) 377. [2] M. Capeans, R. Guida, B. Mandelli, Strategies for reducing the environmental impact of gaseous detector operation at the CERN LHC experiments, Nucl. Instrum. Methods A A 845 (2017) 253–256, http://dx.doi.org/10.1016/j.nima. 2016.04.067. [3] Regulation (EU) No. 517/2014 of the European Parliament and of the Council on Fluorinated Greenhouse Gases and Repealing Regulation (EC) No. 842/2006. [4] J.S. Brown, HFOs new low global warming potential refrigerant, ASHRAE J. 51 (8) (2009) 22–29. [5] M. Capeans, R. Guida, B. Mandelli, Characterization of RPC operation with new environmental friendly mixtures for LHC application and beyond, J. Instrum. 11 (2016) C07016, http://dx.doi.org/10.1088/1748-0221/11/07/C07016. [6] J.R. Incandela, et al., The performance of photomultipliers exposed to helium, Nucl. Instrum. Methods A 269 (1988) 237–245, http://dx.doi.org/10.1016/01689002(88)90885-6. [7] R. Guida, On behalf of the EN, EP and AIDA GIF++ Collaboration, GIF++: A new CERN irradiation facility to test large-area particle detectors for the high-luminosity LHC program, PoS, ICHEP2016, 260. [8] CMS Collaboration, The Phase-2 upgrade of the CMS muon detectors, CERN-LHCC-2017-012-CMS-TDR-016. [9] R. Guida, et al., Results about HF production and bakelite analysis for the CMS Resistive Plate Chambers, Nucl. Instrum. Methods A 594 (2008) S140–S147, http://dx.doi.org/10.1016/j.nima.2008.06.009. [10] M. Capeans, I. Glushkov, R. Guida, F. Hahn, S. Haider, Optimization of a closedloop gas system for the operation of Resistive Plate Chambers at the Large Hadron Collider experiments, Nucl. Instrum. Methods A 661 (2012) S214–S221, http://dx.doi.org/10.1016/j.nima.2010.08.077.
Fig. 5. Gas chromatograms of the analysed return gas from an irradiated RPC (gamma rate ∼41.2 Hz/cm2 ) at two different voltages. Several impurities created under irradiation are visible and their concentration increases with the increase of the HV.
5. Conclusions The search for an environmentally friendly gas mixture for the RPC detectors at LHC is a challenge as it has to be compatible with all experiment requirements without changing any hardware component. The industrial alternatives to C2 H2 F4 are the HFOs, which unfortunately cannot directly substitute the C2 H2 F4 in RPC gas mixture. Few 5-components gas mixtures with a GWP lower than 600 have been identified as possible alternatives, even if they do not match all LHC RPC performance requirements of the standard gas mixture. RPCs performance has been studied at different high gamma rates (up to ∼55.3 Hz/cm2 ) with two eco-friendly gas mixtures based on HFO and CO2 : currents, streamer probability and avalanche charge are slightly higher with respect to RPC standard gas mixture and compatible with the results obtained in laboratory. For future RPC applications where high voltage systems and detectors can be designed without specific constraints, HFO based gas mixtures are promising.
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Please cite this article as: R. Guida, B. Mandelli and G. Rigoletti, Performance studies of RPC detectors with new environmentally friendly gas mixtures in presence of LHC-like radiation background, Nuclear Inst. and Methods in Physics Research, A (2019), https://doi.org/10.1016/j.nima.2019.04.027.