Simulation of Cosmic Ray Muon Flux at Shallow Depths Underground

Geant4 has been used to estimate the rate, energy spectrum and angular distribution of cosmic ray muons at underground depths between 300 and 1000 m.w.e. The results agree well with those from the MUSIC program when the same surface spectrum is used. Care has been taken to use a parameterization of the surface spectrum which agrees well with data for the relevant energies. 1 Goals Knowledge of the underground cosmic ray muon flux is important for reactor experiments studying sin2θ13 for two main reasons. First, neutrons produced by cosmic ray muons in the rock surrounding the detector and in the scintillator itself are a source of background events. Second, the experiment would likely use cosmic ray muons passing through the detector for calibration purposes. The sites being considered for such an experiment have detectors at depths between 300 and 1000 meters water equivalent. There are some measurements of the flux from vertical muons at such shallow depths, but to our knowledge, the only experiment to have measured the integrated muon flux at a shallow depth is CHOOZ [1]. In addition, some work has been done simulating the muon flux underground using the MUSIC [2] program. Our goal in this work was to write a simple simulation in Geant4 which would give us another estimate of the muon flux at the depths of interest. As a second step the muon-induced neutron fluxes could be estimated. The muon fluxes obtained have been compared to results from the MUSIC simulation and to existing experimental results.