Cosmic Ray Induced Neutron Production in a Lead Target*

Experiments searching for rare events underground need to exhibit radioactive background as low as possible. One of the sources of the background for such experiments are cosmic ray muons and cosmic ray muon-induced neutron production. However, simulations of cosmic ray muon-induced neutron production needs to be compared to underground measurements
Reported here is a comparison the 2002 NMDS-II cosmic ray muon-induced neutron data sets collected at the Center for Underground Physics in Pyhasalmi (CUPP) with Geant4 simulations. The data was collected at two depths, 583 m.w.e. collected for 6504 hours and for 1440 hours at 1166 m.w.e. The detector consists of a 30 cm cube Pb-target surrounded by 60 He-3 tubes. The He-3 detectors serve to measure the neutron event multiplicity. The He-3 detectors were calibrated using a Cf-252 neutron source and found to have a neutron detection efficiency of 23.2±1.2. During the course of data collection the highest neutron multiplicity event, having 54 observed neutrons, was measured at 583 m.w.e. and corresponds to ∼ 233±12 produced neutrons when corrected for detection efficiency.
Both the Monte Carlo simulations and the data sets at both depths are consistent with a 2-parameter power law fit, k × (n^p). Each neutron multiplicity distribution is consistent to within the measuring error that the same power law slope parameter p describes each neutron multiplicity distribution.
For the simulation at 583 m.w.e., p=2.37±0.01 with χ^2/DoF = 1.24 and for the data collected at 583 m.w.e, p=2.36 ± 0.10 with χ^2/DoF = 0.76. At 1166 m.w.e., p=2.31±0.01 for the simulation with χ^2/DoF = 1.24., and for the data collected at 1166 m.w.e. with only 6 detected events above observed multiplicity 5, yields p=2.50 ± 0.35 found using a Maximum Likelihood Estimation method.