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

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 48Ca, 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−2s−1 and (1.1±0.5)×10−5 cm−2s−1, respectively. It is necessary to install an additional shield to reduce the background from (n,γ) reaction to the required level.