Studies on optimization of moderator thickness for BF3 detectors used for monitoring of fissile material

Monitoring of fissile material content requires the energy independent response due to inherent moderation in waste matrix present inside the waste drum. The detector assembly consists of three BF3 detectors each being 100 cm long and covered with 3.7 cm thick HDPE moderators arranged in a linear geometry. The response of the detector assembly as measured with neutron sources of different energies was observed to be consistent within acceptable limits. Response of the detector assembly for 252Cf source which has a fission neutron spectrum similar to that of fissile material was obtained and compared with FLUKA based simulation. The sensitivity of the system as evaluated using FLUKA simulation for a 252Cf neutron source was found to be in good agreement with the experimental values obtained. This study indicates that the optimum moderator thickness for effective monitoring of fissile material using BF3 neutron detector assembly is 6 cm. (author)