- Email: [email protected]
Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons
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
Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons Gianluigi Alberghi a , Lorenzo Bellagamba a , Davide Boscherini a , Alessia Bruni a , Massimo Corradi b , Alberto Mengarelli a , Matteo Negrini a ,∗, Alessandro Polini a , Marino Romano a a b
Istituto Nazionale di Fisica Nucleare (INFN) - Sezione di Bologna, Via Irnerio 46, 40126 Bologna, Italy Istituto Nazionale di Fisica Nucleare (INFN) - Sezione di Roma, P.le Aldo Moro 2, 00185 Roma, Italy
ARTICLE
INFO
Keywords: Resistive Plate Chamber (RPC) Geant4 Gamma Neutrons
ABSTRACT The sensitivity of Resistive Plate Chambers (RPCs) to neutrons and gammas is a key parameter to predict the signal rate of these chambers in a high-radiation environment, such as for the high-luminosity upgrade of the LHC. In this work the RPC sensitivity to neutral radiation is computed by means of a Geant4 simulation. The results are compatible with the available measurements and other predictions in the literature, except for neutrons with energy below 1 MeV which represent a large fraction of the radiation in the LHC environment.
1. Introduction Resistive Plate Chambers (RPCs) [1] are widely used in high energy physics experiments. Their high efficiency, fast response and low cost, make this technology one of the preferred choice to instrument large muon detector and trigger systems at collider experiments. The efficiency of RPCs to charged radiation is typically determined using muons from cosmic rays or collision data. The response to neutral radiation, such as gammas and neutrons, cannot be determined from collision data but only from dedicated experiments. The experiments at the Large Hadron Collider (LHC) operate in a large radiation background. With the increase of the luminosity of the LHC, estimates of the signal rate caused by neutral radiation originated from collisions in the beam-pipe, magnets, or from the activation of materials in the experimental area, should be assessed in order to infer the signal rate per unit area, representing one of the limitations for the proper functioning of the RPCs. In this paper we report the results of the study of the sensitivity of an ATLAS double-gap RPC chamber [2] to gammas and neutrons, obtained using a Geant4 simulation [3]. 2. Simulation setup The layout of the ATLAS double-gap RPC chambers is sketched in Fig. 1. In short, a gas volume (C2 F4 H2 ∕C4 H10 /SF6 94.7%/5.0%/0.3% mixture) is limited by two bakelite planes, kept at the constant distance of 2 mm by circular spacers placed between the two planes with 10 cm pitch. Two readout panels with copper strips are placed outside the chamber to extract the electrically induced signal. This structure
is duplicated to form a double-gap chamber, and the whole system is mechanically supported by paper honeycomb layers, electrically shielded by aluminium foils on the two outer faces. The compositions and densities of the materials are reported in Table 1. A square ATLAS RPC with 2.0 m side length has been modelled using the Geant4 (v10.03) toolkit [3]. Single particle events are simulated using the QGSP_BERT_HP physics list with default production cuts. In order to study the ionization process inside the gas gap, the step length in the gas volume is limited to 0.05 mm. If a primary ionization occurs very close to the anode, the avalanche multiplication may be insufficient to produce a detectable signal [4], so in this study the gas gap is divided in two sub-regions: a dead region adjacent to the anode and an active region adjacent to the cathode. A signal is defined by requiring the presence of at least one primary ionization cluster in the active region of the gas volume. We stress that charged particles crossing the gas gap in correspondence with a spacer will not produce a signal in this simulation, so the geometrical inefficiency is by construction included in the efficiency evaluation done in this paper. A 5.5 kV/mm electric field can be applied to each gas gap as in a real double gap RPC chamber in ATLAS, with the two outer electrodes at ground and the two inner ones at +11 kV, as shown in Fig. 1. 3. Results and discussion Single gamma and neutron events of variable energy and crossing angle with respect to the RPC are simulated. The origin points of primary particles are uniformly distributed in a 30 × 30 cm2 square area in the middle of the RPC lower face (referring to Fig. 1) and moving
∗ Corresponding author. E-mail address: [email protected] (M. Negrini).
https://doi.org/10.1016/j.nima.2019.163122 Received 29 May 2019; Received in revised form 15 October 2019; Accepted 9 November 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: G. Alberghi, L. Bellagamba, D. Boscherini et al., Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons, Nuclear Inst. and Methods in Physics Research, A (2019) 163122, https://doi.org/10.1016/j.nima.2019.163122.
G. Alberghi, L. Bellagamba, D. Boscherini et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
which is approximately 100 eV (Fig. 2). Consequently, in this work the number of primary ionization clusters is obtained by dividing by 100 eV the ionization energy deposited in the active region of the gas gap provided by Geant4. Another possible assumption is to replace the average (100 eV) with the lower limit (20 eV) of the ionization energy distribution in Fig. 1, considered as a lower threshold to produce a primary ionization, without taking into account possible electron attachment or recombination effects. The two assumptions lead to very similar results, as discussed in the following. When primary radiation is composed by low energy neutrons (E < 100 eV), the simulation produces free protons and charged nuclei with very low kinetic energy resulting from nuclear reactions with the gas inside the gap. Geant4 adds these charged nuclei to the list of particles to be tracked and, although they appear to be free and completely ionized in Geant4, in reality their presence could be an artefact of the simulation since the nuclei may be not completely ionized or still bound to the gas molecule. In this study this contribution is removed by accepting the gas ionization produced by secondary protons or nuclei only if they are generated with a kinetic energy 𝐸kin,cut > 1 keV. The results obtained for the RPC sensitivity to gamma and neutrons are shown in Fig. 3a and b, respectively. For both primary neutrons and gammas with E > 10 MeV, gap2 has a larger sensitivity than gap1; this is because secondary particles observed in gap1 have enough energy to cross also gap2, which is also sensitive to secondaries produced in the material between the 2 gas gaps. The crossing angle of the primary radiation on the chamber has a large impact on the sensitivity: in the same energy region, the computed sensitivity is larger for isotropic than for normally incident radiation, because of the larger material thickness crossed in the former case. The simulated results are compared with the sensitivity measurements available in the literature for double-gap RPCs, as reported in [8,9] (neutron sources are not monochromatic, so the experimental points are placed in the middle of the neutron energy distribution, as reported in the original papers). The measurements are in better agreement with the sensitivity computed for normal incident radiation. A possible explanation is that radiation in test-beams tends to be more collimated than environmental radiation. A rise of the sensitivity to primary neutrons is observed for E < 1 keV when their energy decreases. This is caused by neutron capture processes with subsequent 𝛾 emission, that sometimes undergo Compton scattering. The neutron capture cross section becomes so large in this energy region that this process completely dominates the sensitivity calculation. The sensitivity results depend on the simulation setup and on the sensitivity definition. The impact on the sensitivity of each assumption done in the simulation is checked by recomputing the sensitivity after variation of the corresponding simulation parameter. The result of this study for gap1 is summarized in Fig. 4a and b for photons and neutrons, respectively. The impact on the choice of the physics list is studied by comparing the results obtained with the nominal physics list (QGSP_BERT_HP) with the ones obtained using an alternative physics lists (QGSP_BIC_HP), for which the estimated neutron sensitivity for primary neutrons with energy of the order of 100 MeV is larger by approximately 50%. For a signal to be observed we require the presence of at least one ionization cluster in the active thickness of the gas gap. The impact of this assumption is checked by requiring at least 2 ionizations clusters, yielding to compatible results. The typical ionization energy for some components of the gas mixture is of the order of 20 eV. The possible assumption that the whole energy deposition by ionization in the gas gap produces free electrons (therefore neglecting possible electron attachments or electron–ion recombinations) is obtained by recomputing the number of ionizations using 20 eV instead of the nominal 100 eV obtained from HEED, has a negligible effect on the sensitivity.
Fig. 1. Schematic representation of an ATLAS double-gap RPC section, showing the main components: support structure (honeycomb), readout panel (foam), resistive plates (bakelite), gas, and spacers (polycarbonate).
Fig. 2. Ionization energy deposited in the RPC gas layer per number of ionization clusters, computed using Garfield++/HEED.
Table 1 Details of the materials used in the simulation. The composition is given in terms of molecular composition, relative number of atoms or (when applicable) weight fraction of the different components. Material
Density (g/cm3 )
Composition (weight fraction)
Honeycomb Foam Bakelite Polycarbonate Gas
0.042 0.155 1.25 1.20 0.006
C6 H10 O5 C1 H1 H(0.0574) C(0.7746) O(0.1680) C16 H14 O3 C2 H2 F4 (0.947) C4 H10 (0.050) S1 F6 (0.003)
upwards with crossing angle normal to the RPC face or isotropically distributed. In this paper, ‘‘gap1’’ is the gas gap closer to the origin of primary particles, the other is ‘‘gap2’’. The main effect driving the RPC sensitivity is the extraction of secondary charged particles from the material surrounding the gas gap, which then enter the gap and ionize the gas with very high efficiency. In this study the RPC sensitivity is defined as 𝑆 = 𝑁sig ∕𝑁0 , where 𝑁0 is the number of events that are generated and 𝑁sig is the number of events with at least one ionization cluster in the active part of the gas volume. A simulation with Garfield++/HEED [5,6] of the ATLAS RPC gas mixture show that a minimum-ionizing particle (MIP) creates, on average, approximately 17 primary ionization clusters in a 2 mm thick gas gap, consistent with the results reported in [7]. Following the efficiency evaluation in [7], we assume that the active part of the gas gap is half of the total thickness (𝑡active = 1.0 mm). The Garfield++/HEED simulation is used to determine the average ionization energy deposition necessary to produce a primary ionization cluster in the gas, 2
Please cite this article as: G. Alberghi, L. Bellagamba, D. Boscherini et al., Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons, Nuclear Inst. and Methods in Physics Research, A (2019) 163122, https://doi.org/10.1016/j.nima.2019.163122.
G. Alberghi, L. Bellagamba, D. Boscherini et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 3. Sensitivity to gammas (a) and neutrons (b) of the two gaps of an ATLAS RPC obtained from the Geant4 simulation, in case of primary radiation with normal and isotropic incidence on the RPC, as a function of the energy of the primary particle. The vertical bars on points correspond to the statistical uncertainty. Sensitivity measurements obtained from the literature are also shown [8,9].
Fig. 4. Sensitivity ratios between the predictions obtained by varying some assumptions and the nominal sensitivity, shown in Fig. 3, for primary gammas (a) and neutrons (b) with normal incidence to the RPC plane. The results are obtained by turning the electric field on (EF on), by varying the physics list (QGSP_BIC_HP), by reducing the active part of the gap (0.5 mm active), by varying the kinetic energy cut on secondary ions allowed to produce ionization (Ions 𝐸kin > 10 keV, Ions 𝐸kin > 0.1 keV), and by considering only events with at least 2 ionization clusters (2 ioniz. clusters), by setting the ionization energy to 20 eV (𝐸ion = 20 eV).
The impact on the result of the choice of 𝑡active is checked by increasing or reducing it by 0.5 mm. The results are in general stable with this variation since the secondaries producing signal in the gap usually cross the full gap. Only in the energy region between approximately 10 keV and 1 MeV, for both primary photons and neutrons, a variation of 𝑡active corresponds to a sensitivity variation of the same relative magnitude. The impact of the arbitrary choice of 𝐸kin,cut is checked by increasing and reducing its value by one order of magnitude. While this has no impact on the photon sensitivity, the estimated neutron sensitivity is largely affected by this choice in the energy range between 1 keV and 1 MeV. In detail, while lowering the value of 𝐸kin,cut has no impact, increasing it can produce a sizable reduction of the sensitivity. Since the exact value of 𝐸kin,cut is unknown, we consider our sensitivity estimate for neutrons in the range E = 1 keV–1 MeV as an upper limit affected by a large uncertainty. The impact of the electric field is determined by comparing the RPC sensitivity obtained from simulations with and without the electric field. We checked that the sensitivity results with and without electric field in the gap are compatible for the adopted nominal value 𝐸kin,cut > 1 keV, as expected, since the electric field has no impact on primary ionization. Anyway, we noticed an increase of the sensitivity to primary neutrons with energy E < 1 MeV when lowering the 𝐸kin,cut value when the electric field is present. This effect is caused by the presence of protons and nuclei produced with low kinetic energy that inside the electric field may acquire sufficient energy to further ionize the gas.
As previously discussed, this is an artefact of the simulation which is removed after setting a threshold on 𝐸kin,cut . The comparison of the results presented in this paper with a previous study of neutron sensitivity of a single gap RPC [10] show a fair agreement for primary neutrons with energy above 1 MeV but disagree below this region, where our sensitivity values decrease while the previous results become approximately constant with value close to 10−3 . In [10] the sensitivity is defined by counting the number of events in which charged particles enter in the gas gap. We checked that if in our simulation we change the sensitivity definition consistently with the one defined in [10], we reproduce a constant sensitivity. We can conclude that the different definition of the sensitivity is the source of the discrepancy in this energy region, which is particularly interesting in the estimation of the RPC rate caused by the cavern background at the LHC. 4. Conclusion The RPC sensitivity to neutral radiation is estimated by means of a Geant4 simulation. With respect to previous studies in the literature, our results are in agreement for gammas, while for neutrons with energy below 1 MeV the sensitivity computed in this work is significantly smaller due to some artefacts of the Geant4 simulation that need to be accounted for. This study is of particular interest in the assessment of the RPC operating conditions for the forthcoming high-luminosity LHC phase, when a challenging high-radiation environment will be present. 3
Please cite this article as: G. Alberghi, L. Bellagamba, D. Boscherini et al., Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons, Nuclear Inst. and Methods in Physics Research, A (2019) 163122, https://doi.org/10.1016/j.nima.2019.163122.
G. Alberghi, L. Bellagamba, D. Boscherini et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Declaration of competing interest
[3] S. Agostinelli, et al., Geant4-a simulation toolkit, Nucl. Instrum. Methods A 506 (2003) 250–303. [4] W. Riegler, et al., Detector physics and simulation of resistive plate chambers, Nucl. Instrum. Methods A 500 (2003) 144–162. [5] Garfield++, http://garfieldpp.web.cern.ch/garfieldpp/. [6] I.B. Smirnov, Modeling of ionization produced by fast charged particles in gases, Nucl. Instrum. Methods A 554 (2005) 474–493. [7] G. Aielli, et al., Response uniformity of a large size RPC, Nucl. Instrum. Methods A 456 (2000) 40–45. [8] L. Acitelli, et al., Study of the efficiency and time resolution of an RPC irradiated with photons and neutrons, Nucl. Instrum. Methods A 360 (1995) 42–47. [9] M. Abbrescia, et al., Experimental results on RPC neutron sensitivity, Nucl. Instrum. Methods A 508 (2003) 79–82. [10] M. Jamil, et al., Simulation response of single gap RPC studies for neutrons using GEANT4 Monte Carlo codes, Radiat. Meas. 43 (2008) 1554–1557.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors wish to thank the ATLAS Collaboration for the stimulating scientific environment, in particular Charlie Young, Tatsumi Koi and Rinaldo Santonico for fruitful discussions. References [1] R. Santonico, R. Cardarelli, Development of resistive plate counters, Nucl. Instrum. Methods A 187 (1981) 377–380. [2] ATLAS Collaboration, The ATLAS experiment at the CERN large hadron collider, JINST 3 (2008) S08003.
4
Please cite this article as: G. Alberghi, L. Bellagamba, D. Boscherini et al., Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons, Nuclear Inst. and Methods in Physics Research, A (2019) 163122, https://doi.org/10.1016/j.nima.2019.163122.
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons Gianluigi Alberghi a , Lorenzo Bellagamba a , Davide Boscherini a , Alessia Bruni a , Massimo Corradi b , Alberto Mengarelli a , Matteo Negrini a ,∗, Alessandro Polini a , Marino Romano a a b
Istituto Nazionale di Fisica Nucleare (INFN) - Sezione di Bologna, Via Irnerio 46, 40126 Bologna, Italy Istituto Nazionale di Fisica Nucleare (INFN) - Sezione di Roma, P.le Aldo Moro 2, 00185 Roma, Italy
ARTICLE
INFO
Keywords: Resistive Plate Chamber (RPC) Geant4 Gamma Neutrons
ABSTRACT The sensitivity of Resistive Plate Chambers (RPCs) to neutrons and gammas is a key parameter to predict the signal rate of these chambers in a high-radiation environment, such as for the high-luminosity upgrade of the LHC. In this work the RPC sensitivity to neutral radiation is computed by means of a Geant4 simulation. The results are compatible with the available measurements and other predictions in the literature, except for neutrons with energy below 1 MeV which represent a large fraction of the radiation in the LHC environment.
1. Introduction Resistive Plate Chambers (RPCs) [1] are widely used in high energy physics experiments. Their high efficiency, fast response and low cost, make this technology one of the preferred choice to instrument large muon detector and trigger systems at collider experiments. The efficiency of RPCs to charged radiation is typically determined using muons from cosmic rays or collision data. The response to neutral radiation, such as gammas and neutrons, cannot be determined from collision data but only from dedicated experiments. The experiments at the Large Hadron Collider (LHC) operate in a large radiation background. With the increase of the luminosity of the LHC, estimates of the signal rate caused by neutral radiation originated from collisions in the beam-pipe, magnets, or from the activation of materials in the experimental area, should be assessed in order to infer the signal rate per unit area, representing one of the limitations for the proper functioning of the RPCs. In this paper we report the results of the study of the sensitivity of an ATLAS double-gap RPC chamber [2] to gammas and neutrons, obtained using a Geant4 simulation [3]. 2. Simulation setup The layout of the ATLAS double-gap RPC chambers is sketched in Fig. 1. In short, a gas volume (C2 F4 H2 ∕C4 H10 /SF6 94.7%/5.0%/0.3% mixture) is limited by two bakelite planes, kept at the constant distance of 2 mm by circular spacers placed between the two planes with 10 cm pitch. Two readout panels with copper strips are placed outside the chamber to extract the electrically induced signal. This structure
is duplicated to form a double-gap chamber, and the whole system is mechanically supported by paper honeycomb layers, electrically shielded by aluminium foils on the two outer faces. The compositions and densities of the materials are reported in Table 1. A square ATLAS RPC with 2.0 m side length has been modelled using the Geant4 (v10.03) toolkit [3]. Single particle events are simulated using the QGSP_BERT_HP physics list with default production cuts. In order to study the ionization process inside the gas gap, the step length in the gas volume is limited to 0.05 mm. If a primary ionization occurs very close to the anode, the avalanche multiplication may be insufficient to produce a detectable signal [4], so in this study the gas gap is divided in two sub-regions: a dead region adjacent to the anode and an active region adjacent to the cathode. A signal is defined by requiring the presence of at least one primary ionization cluster in the active region of the gas volume. We stress that charged particles crossing the gas gap in correspondence with a spacer will not produce a signal in this simulation, so the geometrical inefficiency is by construction included in the efficiency evaluation done in this paper. A 5.5 kV/mm electric field can be applied to each gas gap as in a real double gap RPC chamber in ATLAS, with the two outer electrodes at ground and the two inner ones at +11 kV, as shown in Fig. 1. 3. Results and discussion Single gamma and neutron events of variable energy and crossing angle with respect to the RPC are simulated. The origin points of primary particles are uniformly distributed in a 30 × 30 cm2 square area in the middle of the RPC lower face (referring to Fig. 1) and moving
∗ Corresponding author. E-mail address: [email protected] (M. Negrini).
https://doi.org/10.1016/j.nima.2019.163122 Received 29 May 2019; Received in revised form 15 October 2019; Accepted 9 November 2019 Available online xxxx 0168-9002/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: G. Alberghi, L. Bellagamba, D. Boscherini et al., Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons, Nuclear Inst. and Methods in Physics Research, A (2019) 163122, https://doi.org/10.1016/j.nima.2019.163122.
G. Alberghi, L. Bellagamba, D. Boscherini et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
which is approximately 100 eV (Fig. 2). Consequently, in this work the number of primary ionization clusters is obtained by dividing by 100 eV the ionization energy deposited in the active region of the gas gap provided by Geant4. Another possible assumption is to replace the average (100 eV) with the lower limit (20 eV) of the ionization energy distribution in Fig. 1, considered as a lower threshold to produce a primary ionization, without taking into account possible electron attachment or recombination effects. The two assumptions lead to very similar results, as discussed in the following. When primary radiation is composed by low energy neutrons (E < 100 eV), the simulation produces free protons and charged nuclei with very low kinetic energy resulting from nuclear reactions with the gas inside the gap. Geant4 adds these charged nuclei to the list of particles to be tracked and, although they appear to be free and completely ionized in Geant4, in reality their presence could be an artefact of the simulation since the nuclei may be not completely ionized or still bound to the gas molecule. In this study this contribution is removed by accepting the gas ionization produced by secondary protons or nuclei only if they are generated with a kinetic energy 𝐸kin,cut > 1 keV. The results obtained for the RPC sensitivity to gamma and neutrons are shown in Fig. 3a and b, respectively. For both primary neutrons and gammas with E > 10 MeV, gap2 has a larger sensitivity than gap1; this is because secondary particles observed in gap1 have enough energy to cross also gap2, which is also sensitive to secondaries produced in the material between the 2 gas gaps. The crossing angle of the primary radiation on the chamber has a large impact on the sensitivity: in the same energy region, the computed sensitivity is larger for isotropic than for normally incident radiation, because of the larger material thickness crossed in the former case. The simulated results are compared with the sensitivity measurements available in the literature for double-gap RPCs, as reported in [8,9] (neutron sources are not monochromatic, so the experimental points are placed in the middle of the neutron energy distribution, as reported in the original papers). The measurements are in better agreement with the sensitivity computed for normal incident radiation. A possible explanation is that radiation in test-beams tends to be more collimated than environmental radiation. A rise of the sensitivity to primary neutrons is observed for E < 1 keV when their energy decreases. This is caused by neutron capture processes with subsequent 𝛾 emission, that sometimes undergo Compton scattering. The neutron capture cross section becomes so large in this energy region that this process completely dominates the sensitivity calculation. The sensitivity results depend on the simulation setup and on the sensitivity definition. The impact on the sensitivity of each assumption done in the simulation is checked by recomputing the sensitivity after variation of the corresponding simulation parameter. The result of this study for gap1 is summarized in Fig. 4a and b for photons and neutrons, respectively. The impact on the choice of the physics list is studied by comparing the results obtained with the nominal physics list (QGSP_BERT_HP) with the ones obtained using an alternative physics lists (QGSP_BIC_HP), for which the estimated neutron sensitivity for primary neutrons with energy of the order of 100 MeV is larger by approximately 50%. For a signal to be observed we require the presence of at least one ionization cluster in the active thickness of the gas gap. The impact of this assumption is checked by requiring at least 2 ionizations clusters, yielding to compatible results. The typical ionization energy for some components of the gas mixture is of the order of 20 eV. The possible assumption that the whole energy deposition by ionization in the gas gap produces free electrons (therefore neglecting possible electron attachments or electron–ion recombinations) is obtained by recomputing the number of ionizations using 20 eV instead of the nominal 100 eV obtained from HEED, has a negligible effect on the sensitivity.
Fig. 1. Schematic representation of an ATLAS double-gap RPC section, showing the main components: support structure (honeycomb), readout panel (foam), resistive plates (bakelite), gas, and spacers (polycarbonate).
Fig. 2. Ionization energy deposited in the RPC gas layer per number of ionization clusters, computed using Garfield++/HEED.
Table 1 Details of the materials used in the simulation. The composition is given in terms of molecular composition, relative number of atoms or (when applicable) weight fraction of the different components. Material
Density (g/cm3 )
Composition (weight fraction)
Honeycomb Foam Bakelite Polycarbonate Gas
0.042 0.155 1.25 1.20 0.006
C6 H10 O5 C1 H1 H(0.0574) C(0.7746) O(0.1680) C16 H14 O3 C2 H2 F4 (0.947) C4 H10 (0.050) S1 F6 (0.003)
upwards with crossing angle normal to the RPC face or isotropically distributed. In this paper, ‘‘gap1’’ is the gas gap closer to the origin of primary particles, the other is ‘‘gap2’’. The main effect driving the RPC sensitivity is the extraction of secondary charged particles from the material surrounding the gas gap, which then enter the gap and ionize the gas with very high efficiency. In this study the RPC sensitivity is defined as 𝑆 = 𝑁sig ∕𝑁0 , where 𝑁0 is the number of events that are generated and 𝑁sig is the number of events with at least one ionization cluster in the active part of the gas volume. A simulation with Garfield++/HEED [5,6] of the ATLAS RPC gas mixture show that a minimum-ionizing particle (MIP) creates, on average, approximately 17 primary ionization clusters in a 2 mm thick gas gap, consistent with the results reported in [7]. Following the efficiency evaluation in [7], we assume that the active part of the gas gap is half of the total thickness (𝑡active = 1.0 mm). The Garfield++/HEED simulation is used to determine the average ionization energy deposition necessary to produce a primary ionization cluster in the gas, 2
Please cite this article as: G. Alberghi, L. Bellagamba, D. Boscherini et al., Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons, Nuclear Inst. and Methods in Physics Research, A (2019) 163122, https://doi.org/10.1016/j.nima.2019.163122.
G. Alberghi, L. Bellagamba, D. Boscherini et al.
Nuclear Inst. and Methods in Physics Research, A xxx (xxxx) xxx
Fig. 3. Sensitivity to gammas (a) and neutrons (b) of the two gaps of an ATLAS RPC obtained from the Geant4 simulation, in case of primary radiation with normal and isotropic incidence on the RPC, as a function of the energy of the primary particle. The vertical bars on points correspond to the statistical uncertainty. Sensitivity measurements obtained from the literature are also shown [8,9].
Fig. 4. Sensitivity ratios between the predictions obtained by varying some assumptions and the nominal sensitivity, shown in Fig. 3, for primary gammas (a) and neutrons (b) with normal incidence to the RPC plane. The results are obtained by turning the electric field on (EF on), by varying the physics list (QGSP_BIC_HP), by reducing the active part of the gap (0.5 mm active), by varying the kinetic energy cut on secondary ions allowed to produce ionization (Ions 𝐸kin > 10 keV, Ions 𝐸kin > 0.1 keV), and by considering only events with at least 2 ionization clusters (2 ioniz. clusters), by setting the ionization energy to 20 eV (𝐸ion = 20 eV).
The impact on the result of the choice of 𝑡active is checked by increasing or reducing it by 0.5 mm. The results are in general stable with this variation since the secondaries producing signal in the gap usually cross the full gap. Only in the energy region between approximately 10 keV and 1 MeV, for both primary photons and neutrons, a variation of 𝑡active corresponds to a sensitivity variation of the same relative magnitude. The impact of the arbitrary choice of 𝐸kin,cut is checked by increasing and reducing its value by one order of magnitude. While this has no impact on the photon sensitivity, the estimated neutron sensitivity is largely affected by this choice in the energy range between 1 keV and 1 MeV. In detail, while lowering the value of 𝐸kin,cut has no impact, increasing it can produce a sizable reduction of the sensitivity. Since the exact value of 𝐸kin,cut is unknown, we consider our sensitivity estimate for neutrons in the range E = 1 keV–1 MeV as an upper limit affected by a large uncertainty. The impact of the electric field is determined by comparing the RPC sensitivity obtained from simulations with and without the electric field. We checked that the sensitivity results with and without electric field in the gap are compatible for the adopted nominal value 𝐸kin,cut > 1 keV, as expected, since the electric field has no impact on primary ionization. Anyway, we noticed an increase of the sensitivity to primary neutrons with energy E < 1 MeV when lowering the 𝐸kin,cut value when the electric field is present. This effect is caused by the presence of protons and nuclei produced with low kinetic energy that inside the electric field may acquire sufficient energy to further ionize the gas.
As previously discussed, this is an artefact of the simulation which is removed after setting a threshold on 𝐸kin,cut . The comparison of the results presented in this paper with a previous study of neutron sensitivity of a single gap RPC [10] show a fair agreement for primary neutrons with energy above 1 MeV but disagree below this region, where our sensitivity values decrease while the previous results become approximately constant with value close to 10−3 . In [10] the sensitivity is defined by counting the number of events in which charged particles enter in the gas gap. We checked that if in our simulation we change the sensitivity definition consistently with the one defined in [10], we reproduce a constant sensitivity. We can conclude that the different definition of the sensitivity is the source of the discrepancy in this energy region, which is particularly interesting in the estimation of the RPC rate caused by the cavern background at the LHC. 4. Conclusion The RPC sensitivity to neutral radiation is estimated by means of a Geant4 simulation. With respect to previous studies in the literature, our results are in agreement for gammas, while for neutrons with energy below 1 MeV the sensitivity computed in this work is significantly smaller due to some artefacts of the Geant4 simulation that need to be accounted for. This study is of particular interest in the assessment of the RPC operating conditions for the forthcoming high-luminosity LHC phase, when a challenging high-radiation environment will be present. 3
Please cite this article as: G. Alberghi, L. Bellagamba, D. Boscherini et al., Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons, Nuclear Inst. and Methods in Physics Research, A (2019) 163122, https://doi.org/10.1016/j.nima.2019.163122.
G. Alberghi, L. Bellagamba, D. Boscherini et al.
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Declaration of competing interest
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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The authors wish to thank the ATLAS Collaboration for the stimulating scientific environment, in particular Charlie Young, Tatsumi Koi and Rinaldo Santonico for fruitful discussions. References [1] R. Santonico, R. Cardarelli, Development of resistive plate counters, Nucl. Instrum. Methods A 187 (1981) 377–380. [2] ATLAS Collaboration, The ATLAS experiment at the CERN large hadron collider, JINST 3 (2008) S08003.
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Please cite this article as: G. Alberghi, L. Bellagamba, D. Boscherini et al., Study of a Geant4 simulation for the determination of the sensitivity of an ATLAS RPC chamber to gamma and neutrons, Nuclear Inst. and Methods in Physics Research, A (2019) 163122, https://doi.org/10.1016/j.nima.2019.163122.