Relationship between neutron moderator and time-dependent background for neutron time-of-flight measurement

In short-distance time-of-flight measurements, the background events increase exponentially with decreasing distance. Furthermore, the large number of background events increases the uncertainty of the measured data. The moderated neutron beam includes the time-dependent background originating from the neutron moderator, such as the 2.2 MeV gamma rays emitted by H-1 (n,& gamma;) reaction in the moderator. In this study, to develop a compact neutron resonance transmission analysis system with high accuracy, we studied the relationship between the moderator and time-dependent background based on gamma rays. Monte Carlo simulations were conducted based on PHITS and JENDL-4.0 to investigate the time distributions of neutrons and gamma rays emitted from the moderator. Ratios of the gamma-ray intensity to neutron intensity (S & gamma;/S-n ratio) were calculated for various moderators and flight distances. The moderator designs were discussed based on the obtained results for maintaining a low S & gamma;/S-n ratio.