Measurement Of Gamma Rays From (Lif)-Li-6 Tile As An Inner Wall Of A Neutron-Decay Detector

A neutron lifetime measurement conducted at the Japan Proton Accelerator Research Complex (J-PARC) is counting the number of electrons from neutron decays with a time projection chamber (TPC). The gamma rays produced in the TPC cause irreducible background events. To achieve the precise measurement, the inner walls of the TPC consist of Li-6-enriched lithium-fluoride ((LiF)-Li-6) tiles to suppress the amount of gamma rays. In order to estimate the amount of y rays from the (LiF)-Li-6 tile, prompt gamma ray analysis (PGA) measurements were performed using germanium detectors. We reconstructed the measured gamma-ray energy spectrum using a Monte Carlo simulation with the stripping method. Comparing the measured spectrum with a simulated one, the number of gamma rays emitted from the (LiF)-Li-6 tile was (2.3(-0.3)(+0.7)) x 10(-4) per incident neutron. This is 1.4(-0.2)(+0.5) times the value assumed for a mole fraction of the (LiF)-Li-6 tile. We concluded that the amount of gamma rays produced from the (LiF)-Li-6 tile is not more twice the originally assumed value.