Gamma background simulation for the RPC Endcap and Barrel regions of CMS/LHC using GEANT4 We present some results obtained by the CMS Resistive Plate Chamber Sensitivity to Gamma rays which were simulated by Geant4 Monte Carlo Code. The calculations have been performed as a function of gamma energy in the range MeV. In order to evaluate the response of detector in the large hadron collider (LHC) background environment, gamma spectrum expected in the CMS Muon Endcap, and Barrel region were taken into account. A hit rate of about Hz/cm 2, 42.0 Hz/cm 2, 29.0 Hz/cm 2, and 28.0 Hz/cm 2 in the ME1, ME2, ME3, and ME4 was observed respectively, due to an isotropic gamma source using Geant4 Standard electromagnetic packages for a double-gap RPC. While for the same gamma source and Geant4 packages a hit rate of about 0.4 Hz/cm 2, Hz/cm 2 was measured for the MB1 and MB4 station respectively. Similar characteristics of gamma hit rates have been observed for Geant4 Low Electromagnetic packages. J. T. Rhee, M. Jamil IAP, Konkuk-University, Seoul , Korea.

contents  Motivation of the RPC simulation studies  Radiation Environment as Background  Sources of Backgrounds in Muon stations  Detector - Resistive Plate Chamber  GEANT4 Toolkit  RPC gamma background studies  RPC Detectors Response as gamma sensitivity  Comparison of simulation results with Geant3/ Experimental Results  RPC gamma background hit rates for CMS endcap and barrel Regions  Results of RPC gamma background hit rates  Summary of Simulation Results

Motivation of the RPC simulation studies  The Resistive Plate Chamber (RPCs) play a significant role in the muon triggers of experiments designed at the future high energy colliders (here referred to LHC).  The muon detector system of CMS consists of three sub-detectors, the barrel Drift Tube chambers (DTs), the endcap Cathode Strip Chambers (CSCs) and the Resistive Plate Chambers (RPCs).  At Large Hadron Collider (LHC), the beam provide an energy of 20 interaction per bun ch crossing at beam frequency of 40 MHz.  The environment will be heavily contaminated by gamma and neutron backgrounds whose rate could vary in different regions of detectors from 1 to ~ 1 KHz/cm 2.  The overall doses during 10 years exposure may vary in the range 1 Gy (for barrel region), and 100 Gy (for endcap region).  These kinds of background radiation environment represent a danger to all exposed detectors and electronic components. Therefore it is important to constantly monitor their radiation level.  In the CMS muon system, RPC chambers are located in the barrel and in the endcap regions. In the barrel region, the RPC strips run parallel to the beam and in the endcap they are in radial direction  In order to understand how these kinds of background 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 hit rate for the gamma background expected in the CMS muon endcap, and barrel regions.  Gamma rays were simulated in the RPC double-gap tri-dimensional geometry by means of GEANT4 Monte Carlo code.

Radiation Environment as Background  The Muon system has three purposes: Identification, Trigger and Momentum measurement of muons.  Due to their large detector elements, the chambers of the muon system are very sensitive to background particles in the LHC environment  The muon system receives background hits from secondary muons resulting from pion and kaon decays as well as from punch-through hadrons and from low energy electrons originating after slow neutron capture by nuclei with subsequent photon emission.  Background which affect the chamber can be classified as follows: Low energy radiative electrons following slow neutron capture near or inside the muon chambers. These kind of neutrons originate from hadronic cascades starting somewhere in the detector or in accelerator components. Charged hadrons from hadronic cascades: backsplash from HF and albedo and leakage from HE and the Collimeter shielding. Decay muons coming mostly from the π/K decay inside the central cavity. Muons and other particles created in the accelerator tunnel after beam losses.

Sources of Backgrounds in Muon stations Muons and charged hadrons traversing a muon station will generate a hit with almost 100% probability.  These particles originate mainly from calorimeter punch- through (in ME1/1, and MB1), but for the forward muon system also from leakage out of the calorimeter edge at η = 3.0 and from interactions in the beam pipe.  Four major sources of background hits: Random hits induced by neutrons/gammas. Punch-through and pi/K in-flight decays. Tunnel muons. e/m debris associated with energetic muons going through matter, e.g., calorimeter, iron disks etc.  The CMS muon trigger system based on the Resistive plate chambers(RPCs) will have to fulfill two basic requirements, namely: Efficient detection of high transverse momentum (p T ) muons High rejection of uninteresting events The achievement of the former goal mainly depends on the efficiency and timing properties of RPCs.

Resistive Plate Chamber Detector  The resistive plate chamber (RPC) was first developed by R. Santonico and his group in the late 1970's.  The RPC is a very successful wireless detector and is used in high energy physics experiments because of its high efficiency and fast response.  The RPC consist of two parallel plates, made out of phenolic resin known as bakelite, with a bulk resistivity Ωcm, separated by a gas gap of few millimeters.  The whole structure is made gas tight. The outer surfaces of resistive material are coated with conductive graphite paint to form the HV and ground electrodes. The read-out is performed by means of aluminum or copper strips separated from the graphite coating by an insulating PET film.  A high-energy primary particle passing through the detector ionizes some atoms of the gas or in the internal electrode layer.  The detector efficiency is the sum of the contribution of the gas ionization, and the ionization in the electrode layer closer to the gas gaps.  A DC voltage difference generating an electric field of about 5 kV/mm (depending also on the gas mixture) applied to the electrodes, which accelerates the ions and electrons creating an avalanche of charge.  The induced signal on the strips is the average of possible avalanches from both of the gas gaps. The simulation calculations of the RPC detector have been evaluated as a function of gamma’s energy in the range 0.005MeV to 10 GeV. RPC set up configuration : 20cm X 20cm (example) 2 Gas Gaps with a gas mixture of (3% iC 4 H % C 2 H 10 F 4 ) was used. Both the gas gaps were set as sensitive area.

GEANT4 Toolkit  As the scale and the complexity of high energy physics experiments increase, simulation studies require more and more care and become essential to design and optimize the detectors, to develop and test the reconstruction and analysis programs, and to interpret the experimental data. GEANT is a system of detector description and simulation tools that help physicists in such studies.  Geant4 is a toolkit for the simulation of the passage of particles through matter.  The principal applications of GEANT in High Energy Physics are, the transport of particles (i.e tracking) through an experimental setup for simulation of detector response.  An abundant set of Physics Processes handle the diverse interactions of particles with matter across a wide energy range, as required by Geant4 multi-disciplinary nature; for many physics processes a choice of different models is available.  The silent features of GEANT system allow us to: * Describe an experimental setup by a structure of geometrical volumes. * Accept events simulated by Monte Carlo generators. * transport particles through the various regions of setup, taking into account geometrical volume boundaries and physical effects according to the nature of particle themselves, their interactions with matter and themselves. * recode particle trajectories and the response of the sensitive detectors. * visualize the detectors and the particle trajectories.

 RPCs, and each detector in the CMS experiment, will work in a hostile environment rich of neutron and gamma rays. Calorimeters are essentially the only devices that can measure the energy of jets.  In order to understand how these kinds of background could affect the detector functionality, we need to know the sensitivities of these kinds of radiation.  Gamma rays were simulated in the RPC double-gap 3-dimensional geometry by means of GEANT4 Monte Carlo code. We present the results concerning the following Geant4 electromagnetic Packages: * Standard Package; * Low Energy Package, based on Livermore data Libraries.  Two Different kinds of gamma sources were chosen for this simulation studies: (i) An isotropic source of gammas and (ii) A parallel source of gammas  The range threshold for secondary particles (i.e., for γ, e−, and e+) production in electromagnetic processes was set to 1 μm, 1nm, and 1μm was applied respectively. RPC gamma background studies

RPC Detectors Response as gamma sensitivity * The sensitivity in the code is defined as: Sens =N I /N 0, where N I is the number of charged particles and N 0 is the number of original primary particles impinging on RPC. * By employing Geant4 Standard electromagnetic Packages the double-gap RPC sensitivity for an isotropic gamma source is s γ <5.873×10 −2 for E γ ≤ 1 GeV, * while applying Geant4 Low electromagnetic Packages the RPC sensitivity, s γ < 5.292×10 −2 has been found for the same energy domain.  A Comparison of available experimental and simulated gamma sensitivity results.

Comparison of simulation results with Geant3/ Experimental Results * A comparison of RPC Simulation results using GEANT3 and GEANT4for parallel and isotropic gamma source.  Summary of the experimental and simulated gamma sensitivity results.

RPC gamma background hit rates for CMS endcap and barrel Regions For simulating the endcap region of the RPC for gamma rays, we applied our preliminary results to Fig. a of the CMS muon TDR (CERN/LHCC/97/32 which shows the particle spectra in CSC gas layers of ME1/1, ME2/1 and ME4/2.) The assumption that we could employ our simulation results shown in Table 5 to these regions is based on the fact that the CSC and the RPC in the endcap area of CMS have same locations. Fig. a, b

The assumption that we could employ our simulation results shown in Table 5 to these regions is based on the fact that the CSC and the RPC in the endcap area of CMS have same locations. Similarly for barrel area hit rate estimation we applied the simulation results to Fig. b, which shows gamma background spectra in the drift tube gas layers of MB1 and MB4. Particles hit rate can be found by employing same number of particles impinging in the code and getting the sensitivities values bin by bin of the Fig. a and Fig. b. *After summing up all those sensitivities results and dividing them by the total number of gamma’s, gives the average sensitivities in those regions i.e., ME1, ME2, ME3, ME4, and MB1, and MB4 for CMS endcap, and barrel stations. Finally after applying those results to (Fig of the the CMS muon TDR) gives the total hit rate of gamma’s in those respective regions. RPC gamma background hit rates for CMS endcap and barrel Regions

* CMS/RPC endcap region gamma sensitivity, and background hit rate results. * CMS/RPC barrel region gamma sensitivity, and background hit rate results. Results of RPC gamma background hit rates *We calculated total gamma sensitivity directly from the ration of spectral areas; the obtained results are reported in Table below. *According to these numerical values, we can estimate that the CMS endcap ME1, ME2, ME3, and ME4 will have a hit rate due to an isotropic gamma source (using Geant4 Standard E/M package) of about 131.0Hz/cm 2, 42.0 Hz/cm 2, 29.0 Hz/cm 2 and 28.0 Hz/cm 2 respectively. *whereas the corresponding rate for a same source using Geant4 Low electromagnetic package would be about Hz/cm 2, 45.0 Hz/cm 2, 35.0 Hz/cm 2 and 33.0 Hz/cm 2 respectively. In case of isotropic gamma source using Geant4 Standard electromagnetic package for the region of CMS MB1, and MB4, the hit rate would be about 0.4 Hz/cm 2, Hz/cm 2, while for same source and region using Geant4 Low electromagnetic package, the hit rate could be 0.5 Hz/cm 2, 1.7 Hz/cm 2 respectively.

Summary of Simulation Results Comparison of Geant4 Standard and Low Electromagnetic gamma for Isotropic sources results with their statistical errors.

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