Improvement of detection limit in 14 MeV neutron direct interrogation method by decreasing background

Haruyama, Mitsuo; Takase, Misao; Tobita, Hiroshi

For improvement of detection limit in the 14 MeV-neutrons direct interrogation method, the simulation calculation by the continuous energy Monte Carlo transportation code (MVP) was carried out. From the results, it was made clear that the background neutrons appearing in the nuclear-fission neutrons counting-area can be greatly decreased by adopting a stainless steel SUS-304 (an iron-base alloy) reflector in place of a graphite moderator in the conventional detection system. The background level of this modified detecting system becomes 1/10,000 of that with the conventional system and method. Namely, the background counting of 5,000 occurred in the graphite detecting system with the conventional method can be decreased down to 0.5 by adoption of the stainless steel detecting system. The detection limit for the cementation waste that is contaminated with the natural uranium is 0.000122 Bq/g with the stainless steel detecting system in a time of 10 minutes.