Simulation of delayed gamma rays from neutron-induced fissions using MCNP 6.1

As part of its R&D activities in the fields of radioactive waste drum storage and homeland security, the Nuclear Measurement Laboratory of CEA Cadarache has started studies related to the detection of induced delayed fission gamma rays as a signature of U/Pu presence either in radioactive wastes or in cargo containers and luggage. The study described in the present paper explores the feasibility of detecting fission delayed gamma rays of nuclear materials interrogated by a pulsed neutron generator. For this purpose, Monte Carlo simulations have been performed with ACT, the MNCP6 Activation Control Card. Simulated results have been compared with experimental data to validate the numerical model. Samples of uranium and plutonium have been irradiated for 2 hours with a pulsed D-T neutron generator delivering 14 MeV neutrons with an average emission of 8.107 n/s, which are thermalised in a graphite cell called REGAIN. At the end of irradiation, activated nuclear materials were placed in a low-background, high-resolution gamma spectroscopy station in order to detect delayed gamma rays emitted by fission products. Anomalies have been observed in the calculated time decay curve of fission delayed gamma rays with MCNP6 ACT card, but the time behavior is correct for non-fission activated materials like aluminum or copper. On the other hand, the number of counts recorded in the main simulated gamma ray lines from activated nuclear material fission products is consistent with the experimental results, thus validating the simulation scheme in view of further studies on the characterization of radioactive waste drums or special nuclear material detection in cargo containers.