Study of the Neutron Sensitivity for the Double Gap RPC of the CMS/LHC by Using GEANT4

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1 Journal of the Korean Physical Society, Vol. 48, No. 1, January 2006, pp Study of the Neutron Sensitivity for the Double Gap RPC of the CMS/LHC by Using GEANT4 J. T. Rhee and M. Jamil Institute for Advanced Physics, Konkuk University, Seoul B. Hong, K. S. Lee, S. Park and K. S. Sim Korea Detector Laboratory, Korea University, Seoul Y. J. Kim Department of Physics, Cheju National University, Jeju Y. U. Kim Department of Physics, Chungbuk National University, Chungju S. K. Nam Department of Physics, Kangwon National University, Chunchon D. G. Koo Department of Physics, Seoul National University of Education, Seoul S. J. Lee Department of Biomedical Engineering, Seonam University, Namwon S. Y. Bahk Department of Physics, Wonkwang University, Iksan S. J. Hong Center for Neuro Science, Gachon Medical School, Inchon (Received 9 August 2005, in final form 19 October 2005) GEANT is a toolkit for simulating the passage of particles though matter. It contains a complete range of functionality, including tracking, geometry, physics models, and hits. In this article, an attempt to use a GEANT4 Monte Carlo code to watch the resistive plate chamber s (RPC) response to neutrons is presented. The simulation calculations for the RPC have been evaluated as a function of neutron energy in the range MeV 10 GeV. In order to evaluate the response of the RPC detector in the large hadron collider (LHC) background environment, we have taken into account the neutron energy spectrum expected in the compact muon solenoid (CMS) muon barrel. A hit rate due to neutrons of about 0.5 Hz/cm 2 has been estimated for a cm 2 RPC in the MB1 station. PACS numbers: Cs Keywords: Resistive plate chamber, Neutron sensitivity, Detector response, Compact muon solenoid I. INTRODUCTION Studies on the compact muon solenoid (CMS) [1, 2] and a toroidal for the large hadron collider (LHC) apparatus (ATLAS) [3] have shown that the resistive plate Corresponding Author : jtrhee@konkuk.ac.kr; Corresponding Author : mjamil@konkuk.ac.kr chambers (RPCs) and other detectors at the LHC will operate in an intense radiation background mostly made of photons, neutrons and charged hadrons. RPCs, and each detector in the CMS experiment, will work in a hostile environment rich in neutrons and gamma rays [4 6]. The maximum expected fluxes in the muon detector for neutrons and gammas are around and

2 -34- Journal of the Korean Physical Society, Vol. 48, No. 1, January 2006 cm 2 s 1, respectively, for the CMS ME1 station (endcap region), while fluxes one order of magnitude lower are expected in the MB1 station (barrel region). For evaluating the effect of this background radiation on the detector functionality, it is essential to know the RPC sensitivity for this kind of radiation. The neutron background in the energy range between 20 mev (thermal neutrons) and 1 GeV is produced from the hadronic interactions in the calorimeter and from the interactions of the primary proton beam with the beam pipe and/or the beam collimators while the gamma radiation is mainly due to neutron radiative capture and inelastic scattering. For the gamma radiation case, the energy range could be between 40 kev and 10 MeV. In order to understand how these kinds of backgrounds could affect the detector functionality, we need to know the detector sensitivity to these kinds of radiation. The motivation of our studies is to estimate the RPC configuration sensitivity response to neutrons in the range from MeV to 10 GeV. Neutrons were simulated in the RPC three-dimensional geometry by means of the GEANT4 Monte Carlo code. II. TYPES OF GEANT MONTE-CARLO CODES USED IN HEP GEANT is a system containing a detector description and simulation tools that helps physicists in studies. The GEANT3 [7] program was written to describe the passage of particles through matter. Originally designed for high-energy physics (hep) experiments, it has also found applications outside this domain in the area of medical and biological sciences, radioprotection, and astronautics. The first version was released in 1971, and the system was developed with some continuity over 20 years till the last release, GEANT3.21, in It has become a popular and widely used tool in the HEP community. In the previous RPC background simulation studies reported in [8 13] the GEANT3.21 Monte-Carlo code was used and gave reliable results. A GEANT4 simulation offers an ample set of complementary and alternative physics models based either on theory, experimental data, or parametrizations. In particular, GEANT4 provides packages specialized for modeling both electromagnetic and hadronic physics interactions. The GEANT4 electromagnetic packages handle the electromagnetic interactions of leptons, photons, and ions. Especially they include multiple scattering, ionization, bremstrahlung, positron annihilation, the photoelectric effect, Compton and Rayleigh scattering, gamma conversion, synchrotron and transition radiation, the Cernekov effect, refraction, absorption, scintilation, fluoresence, the Auger effect, and particle-induced X-ray emission (PIXE) [14]. In the present studies, we present the results concerning the following GEANT4 electromagnetic packages: Standard Energy Package Low Energy Package, based on Livermore data Libraries [15 17]. 1. GEANT4 Standard-energy Package The standard package [14] provides a variety of models based on analytical approaches to describe the interactions of electrons, positrons, photons, and charged hadrons in the energy range between 1.0 kev and 100 TeV. These models assume that the atomic electrons are quasi-free (i.e., their binding energy can be neglected for all particle interaction processes except the photoelectric effect) while the atomic nucleus is fixed (i.e., the recoil momentum can be neglected). The GEANT4 standard package mainly addresses the HEP domain. 2. GEANT4 Low-energy Package The low energy package [14,18] extends the range of accuracy of electromagnetic interactions down to energies lower than the GEANT4 standard package. This Low Energy Package approach exploits evaluated data libraries (EPDL97 [15], EEDL [16], and EADL [17]) that provide data for the calculation of cross-sections and the sampling of the final state for the modeling of photon and electron interactions with matter. The current implementation of low energy electron processes can be used down to 250 ev. This package handles ionization by hadrons and ions [19,20]. It adopts different models, depending on the energy range and the particle charge. III. RPC DETECTOR S CONFIGURATION The detector setup is described by creating a hierarchy of different elements and by specifying their positions and orientations. Using the strategies and the geometrical dimensions supplied by the CMS/RPC environment, we modeled the RPC detector setup. The RPC detector structure can be easily described to the simulation program by utilizing a variety of geometrical elements available in GEANT4. Additionally the object-oriented structure of GEANT4 allows the user to define his or her own classes. One class can be defined as a detector construction category for each detector component, such as the ground electrode, the bakelite, the gas gap, etc. These classes can be combined together to build the entire setup of the RPC. The RPC detector geometry configuration was used (two single gas gaps with central common readout strips) with the usual gas mixture of (3 % ic 4 H % C 2 H 2 F 4 ), which was simulated by using the GEANT4 Monte-Carlo code. In this present GEANT4 Monte-Carlo simulation work, a geometrical configuration with an area of cm 2 RPC was studied. The RPC detector composition in terms of materials and relative thicknesses is described in Table 1 and can be seen in Fig. 1.

3 Study of the Neutron Sensitivity for the Double J. T. Rhee et al Table 1. Double-gap RPC detector setup composition. Material Thickness (cm) Detector Components Copper (GND).005 External ground shield Polyethylene.060 Gap I material Graphite.002 Gap I material Bakelite.2 Gap I material Gas (gap I).2 Gap I material Bakelite.2 Gap I material Graphite.002 Gap I material Polyethylene.060 Gap I material Strips(copper).005 Central common read-out strips Polyethylene.060 Gap II material Graphite.002 Gap II material Bakelite.2 Gap II material Gas (gap II).2 Gap II material Bakelite.2 Gap II material Graphite.002 Gap II material Polyethylene).060 Gap II material Copper(GND).005 External ground shield 1. RPC Detector s Response to Neutron by Using a GEANT4 Monte-Carlo Code Fig. 1. Basic RPC configuration used in the simulation Two different kinds of neutron sources were chosen for this simulation study: An isotropic source of neutrons evenly distributed on the RPC chamber surface, A parallel source of neutrons perpendicularly impinging on the whole RPC s surface. For each source configuration, the sensitivity was evaluated at 29 different energies: , 10 9, 10 8, 10 7, 10 6, 10 5, 10 4, 10 3, 10 2, 10 1, 0.5, 1, 1.4, 1.5, 2, 5, 10, 25, 50, 100, 200, 250, 500, 700, 1000 MeV, 2 GeV, 5 GeV, and 10 GeV. In this simulation work, the range thresholds for secondary-particles i.e., for γ, e, and e +, production in electromagnetic processes was set to 1 µm, 1 nm, and 1 µm respectively. The geometrical setup of the double-gap RPC detector described in Table 1 has been simulated by the GEANT4 code. A primary beam of neutrons with definite energies is randomly incident into RPC, which transports all kinds of particles involved in the simulation application. Signals in the RPC detector exposed to neutrons are expected to be produced by a variety of nuclear processes (i.e., elastic scattering, inelastic scattering, radioactive capture, and so on [21]), depending on the neutron energies. The RPC detector s response, known as the RPC sensitivity, is a function of the incoming particles energies, because at different energies, different processes are responsible for the secondary-particle production. For the evaluation of RPC sensitivity to neutrons in the present cm 2 RPC chamber surface, we applied the same upper limit assumption reported in [9, 10] i.e., that each produced charged particle generates a signal in the readout strips on arriving at the gas gaps. If more than one charged particles reaches the gas gap, only the first one is assumed to produce a signal. It is important to note that in the present simulation study, only those signals due to neutrons that enter the RPC detector contribute to the neutron sensitivity. The sensitivity in our code is defined as s n = N I /N 0, where N I is the total number of charged particles arriving at any of the two gas gaps and N 0 is the total number of original primary particles impinging upon the RPC.

4 -36- Journal of the Korean Physical Society, Vol. 48, No. 1, January 2006 Table 2. RPC detector s mean sensitivities, and their variances for GEANT4 standard and Low processes. Particle s Source Energy (MeV) GEANT4 Standard GEANT4 Low ɛ σ ɛ σ Isotropic neutrons , Parallel neutrons , Statistical errors are within 1 % IV. DISCUSSION AND RESULTS The GEANT simulation program is more complex for a neutron source than for a gamma source as it implies the use of neutron-interaction cross sections for several nuclei and several energies [9]. At higher energies, it works very well; however at low energies, accurately estimating the neutron-interaction cross sections by using the GEANT3.21 Monte-Carlo code becomes rather difficult. In the case of the GEANT4 application code for the

5 Study of the Neutron Sensitivity for the Double J. T. Rhee et al Table 3. A comparison of RPC Simulation results using GEANT3 and GEANT4 for parallel and isotropic gamma sources. Particle s Source Energy(MeV) Double-gap RPC Sensitivity By G321 By G4 Standard Isotropic neutrons Parallel neutrons Table 4. Summary of the experimental and simulated neutron sensitivity results. Particles Energy (MeV) Double-gap RPC Sensitivity Experimental Results G4 Standard Results G4 Low Results Neutron 2.0 (6.3 ± 0.02) Statistical errors are within 1 % 20.0 (5.3 ± 0.5) < RPC detector, it works very effectively both for low-and high-energy neutron simulation. The RPC neutron sensitivity results that were obtained by using the GEANT4 standard and the GEANT4 low electromagnetic packages can be seen in Fig. 2. In order to estimate the problem of physics models, we applied both of these GEANT4 packages to the RPC detector configuration. The results obtained for both of these processes for an isotropic neutron sources, and for a parallel neutron source are in good agreement, except at the lowest energies (i.e, E n < 10 MeV). It is evident from Fig. 2, that for an isotropic neutron source, the sensitivity values are higher than they are a for a parallel neutron source. This dependence is due to the fact that neutrons passing isotropically through the RPC will have a larger and wider path than they will for the case of a parallel neutron source. Therefore, neutrons impinging isotropically on the RPC surface will generate more charged particle signals, which cause more signals to be detected by the strips. Hence, the isotropic neutron source s sensitivity is higher than

6 -38- Journal of the Korean Physical Society, Vol. 48, No. 1, January 2006 Table 5. RPC sensitivity estimated with CMS MB1 neutron spectrum. For RPC Area (cm 2 ) By Geant4 Standard Package By Geant4 Low Package Sensitivity Hit rate Hz/cm Fig. 2. Comparison of GEANT4 Standard and Low electromagnetic processes and measured sensitivity for the RPC set-up geometry. the parallel neutron source s sensitivity. The sensitivity values rise in the low energy region (E n < MeV), which is mostly due to γ s coming from the (n,γ) capture reaction whose cross section increases with decreasing in neutron energy as σ 1/ E n. The detector response uniformity has been studied by computing each neutron energy sensitivity fluctuation around the average. To estimate the uncertainties associated with the calculated sensitivity, we simulate 10 independent runs (i.e; m = 10) of the Monte-Carlo simulation code for each flux of neutrons at a definite energy. The final calculated sensitivity (ɛ) is given by the mean of the sensitivities (ɛ k ) calculated for each simulation run at a particular energy, and the estimate of the variance of the sensitivity σ is given by the expression [21-22] ε = 1/m m ε k, (1) k=1 σ 2 ε = 1/(m 1) m (ε k ε) 2. (2) k=1 The obtained results are presented in Table 2. Similar simulation studies for a parallel neutron source were performed by employing both the GEANT4 standard and the GEANT4 low electromagnetic packages. For the RPC configuration with a 2 mm gas gap, the present neutron sensitivity results are in agreement with the previous results [24]. A comparison between the GEANT3.21 and the GEANT4 neutron simulation results for the RPC is shown in Table 3. If the GEANT4 standard electromagnetic packages are employed, the double-gap RPC sensitivity for an isotropic neutron source is s n < for E n 1 GeV, whereas if the GEANT4 low electromagnetic packages are employed, the RPC sensitivity s n < is found for the same energy domain. Similarly, for the same RPC configuration, when using a parallel neutron source for the GEANT4 Standard electromagnetic packages, the neutron sensitivity s n < is noted for E n 1 GeV, whereas the RPC neutron sensitivity s n < is noted for the same neutron source, and the energies have been obtained using the GEANT4 low electromagnetic packages. The present GEANT4 neutron simulation results are reliable and are in agreement with the experimental results reported in Ref. 9, 11, and 12. A comparison between the simulated and the available experimental neutron-sensitivity results at low energies are presented in Table 4. V. CONCLUSIONS A simulation study was performed for neutron RPC sensitivity measurements by using the GEANT4 Monte- Carlo code. With the present GEANT4 Monte-Carlo code for the RPC neutrons in the energy range from MeV to 1 GeV were simulated. For this simulation application, all the geometrical configurations and materials of the RPC were implemented. By watching the sensitivity results obtained for the RPC both by using the GEANT4 standard and by the GEANT4 low electromagnetic packages, we showed that the GEANT4 Monte-Carlo code works very effectively both for lowand high-energy neutrons. Therefore, one can conclude that a GEANT4 simulation is a powerful instrument for understanding the physical mechanism underlying RPC operation. We hope that the present simulation studies can be an efficient instrument for evaluating the critical parameters for the design of a standard RPC to be used in the future for the construction of the CMS muon sub-detector in the LHC at CERN, Geneva. The simulation method illustrated here can be used to estimate the detector response (sensitivity) for different experimental environments by

7 Study of the Neutron Sensitivity for the Double J. T. Rhee et al Fig. 3. CMS Muon Barrel (MB1) neutron spectrum (full circles) that enters the RPC. using the various neutron-energy spectra shown in Fig. 3, we estimated the RPC sensitivity for an isotropic neutron source; the spectra of the first charged particles (e, e +, proton, and α) crossing both of the gas gaps are superimposed. We calculated the total neutron sensitivity directly from the ratio of the spectral areas; the obtained results are reported in Table 5. According to our calculated results, CMS MB1 chamber will have a hit rate due to neutrons of about 0.5 Hz/cm 2, which is in agreement with the previous results reported in Ref. 10. ACKNOWLEDGMENTS We thank M. Abbrescia from Istituto Nazionale di Fisica Nucleare (INFN) Bari, and Saverio Altieri from INFN Pavia, Italy for many interesting discussions on our results and for giving useful guidelines. This work was supported by the faculty research fund of Konkuk University in REFERENCES [1] CMS, The Muon Project, Technical Design Report, CERN/LHCC/97/32, [2] B. Hong, J. H. Kim, K. B. Lee, K. S. Lee, M. S. Ryu, S. Park, K. S. Sim and S. J. Hong, J. Korean Phys. Soc. 47, 782 (2005). [3] ATLAS, The Muon Project, Technical Design Report, CERN/LHCC/97/22, [4] S. H. Ahn, E. J. Choi, B. Hong, S. J. Hong, D. H. Kang, T. J. Kim, K. S. Lee, S. Park, W. J. Park, M. S. Ryu, H. H. Shim, K. S. Sim, S. W. Youn, Y. J. Kim, Y. U. Kim, S. K. Nam, S. R. Jung, Y. L. Lee, J. T. Rhee, S. J. Lee, D. G. Koo and S. Y. Bahk, J. Korean Phys. Soc. 41, 667 (2002). [5] S. H. Ahn, S. Y. Bahk, B. Hong, S. J. Hong, M. Ito, T. I. Kang, B. I. Kim, J. H. Kim, Y. J. Kim, Y. U. Kim, D. G. Koo, H. W. Lee, K. B. Lee, K. S. Lee, S. J. Lee, J. K. Lim, D. H. Moon, S. K. Nam, S. Park, W. J. Park, J. T. Rhee and M. S. Ryu, K. S. Sim, J. of Korean Phys. Soc. 45, 1490 (2004). [6] M. Jamil and J. T. Rhee, J. of Korean Phys. Soc. 46, 413 (2005). [7] Application Software Group - Computing and Networks Division GEANT - Detector Description and Simulation Tool CERN Program Library Long Writeup W5013 October 94, CERN Geneva, Switzerland. [8] S. Altieri et al., Nucl. Instr. Meth. Phy. A 456, 99 (2000). [9] S. P. Ratti, Resistive Plate Chambers in High Energy Physics Experiment, International Conference on High Energy Physics, ICHEP2000, Proc. of the XXXth Int. Conf in H.E.P edited by C. S. Lim and T. Yamanaka, (World Scientific Singapore, 2000), p [10] S. Altieri et al., Nucl. Instr. Meth. Phy. A 461, 57 (2001). [11] S. P. Ratti, Proc. J. of High Energy Physics, 268 (2001). [12] R. Guida, M. Abbrescia, S. Altieri, V. Baratti, O. Barnaba, G. Belli, G. Bruno, A. Colaleo, C. DeVecchi, G. Iaselli et al., Nucl. Instr. Meth. Phy. A 506, 101 (2003). [13] M. Abbrescia, S. Altieri, V. Baratti, O. Barnaba, G. Belli, G. Bruno, A. Colaleo, C. De Vecchi, R. Guida, G. Iaselliet al, Nucl. Instr. Meth. Phy. A 508, 79 (2003). [14] S. Agostinelli et al., Nucl. Instr. Meth. Phy. A 506, 250 (2003). [15] D. E. Cullen, J. H. Hubbell, and L. Kissel, The Evaluated Photon Data Library, 97 version, UCRL-50400, Vol. 6, Rev. 5. [16] D. E. Cullen, S. M. Seltzer, Tables and Graphs of Electron-Interaction Cross-Sections from 10 ev to 100 GeV Derived from the LLNL Evaluated Electron Data Library (EEDL), Z=1-100 UCRL-50400, Vol. 31. [17] S. T. Perkins, D. E. Cullen, M. H. Chen, J. H. Hubbell, J. Rathkopf, and J. Scofield, Tables and Graphs of Atomic Subshell and Relaxation Data Derived from the LLNL Evaluated Atomic Data Library (EADL), Z=1-100 UCRL-50400, Vol. 30. [18] S. Chauvie, G. Depaola, V. Ivanchenko, F. Longo, P. Nieminen, Maria Grazia Pia, Geant4 Low Energy Electromagnetic Physics Proceedings of CHEP 2001, Beijing (China), p. 337, [19] S. Giani, CERN-OPEN and INFN/AE-99/20, [20] S. Giani, CERN-OPEN and INFN/AE-99/21, [21] William R. Leo, Techniques for Nuclear and Particle Physics Experiments, (Springer-Verlag Berlin Heidelberg, 1987). [22] S. Hurtado, M. Garcia-Leon, and R. Garcia-Tenorio, Nucl. Instr. Meth. Phy. A 518, 764 (2004). [23] K. Debertin and R. G. Helmer, Gamma and X-ray Spectrometry with Semiconductors Detectors (North-Holland, Amsterdam, 1988). [24] M. Jamil, and J. T. Rhee, Simulations of Two Different Set Ups of Resistive Plate Chambers sensitivities to Neutrons Nucl. Inst. Meth. Phy. Sec. A (In Press).