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

Lee, J. ; Hironaka, Kota ; Ito, Fumiaki*; Koizumi, Mitsuo ; Hori, Junichi*; Sano, Tadafumi*

In the short-distance time-of-flight (TOF) measurements, the background events increase exponentially with decreasing the distance, and the high background increases the uncertainty of the measurement data. The moderated neutron beam includes the time-dependent background originating from the neutron moderator, such as 2.2 MeV gamma rays emitted by H(n,) reaction in the moderator. In this paper, to achieve a compact neutron resonance transmission analysis system with high accuracy, we studied the relationship between the moderator and time-dependent background due to gamma rays. Monte Carlo simulations were carried out with the PHITS and the 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/S ratio) were calculated for various moderators and flight distances. From the obtained results, moderator designs for maintaining low S/S ratio were discussed.