Background studies of high energy γ rays from ( n,γ ) reactions in the CANDLES experiment

Abstract High energy γ rays with several MeV produced by ( n,γ ) reactions can be a trouble for low background measurements in the underground laboratories such as double beta decay experiments. In the CANDLES project, which aimed to observe the neutrino-less double beta decay from 48 Ca, γ rays caused by ( n,γ ) reactions were found to be the most significant background. The profile of the background was studied by measurements with a neutron source and a simulation with a validity check of neutron processes in Geant4. The observed spectrum of γ rays from ( n,γ ) reactions was well reproduced by the simulated spectra, which were originated from the surrounding rock and a detector tank made of stainless steel. The environmental neutron flux was derived by the observed event rate of γ rays from ( n,γ ) reactions using the simulation. The thermal and non-thermal neutron flux were found to be ( 1.3 ± 0.6 ) × 10 − 6  cm − 2 s − 1 and ( 1.1 ± 0.5 ) × 10 − 5  cm − 2 s − 1 , respectively. It is necessary to install an additional shield to reduce the background from ( n,γ ) reaction to the required level.