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The low rate at which some fissile isotopes, such as  235 U,  233 U, and  239 Pu, undergo spontaneous fission leads to a weak signal, resulting in a high-uncertainty in applying passive neutron counting techniques. Stimulating fission through active neutron interrogation can overcome this issue. At Canadian Nuclear Laboratories, a  252 Cf and a deuterium-deuterium neutron source are available. In this study, a neutron counting system was designed to perform passive measurements and active neutron interrogation for a search of special nuclear material. The detection system consists of a cylindrical cavity surrounded by a polyethylene moderator with  3 He detectors interspersed throughout. When used for passive measurements, the sample is placed in the cylindrical cavity, whereas in active interrogation mode, the  252 Cf neutron source and the sample are placed in close proximity to each other in the cylindrical cavity. Measurements that actively interrogated samples, notably containing (among other isotopes) either  235 U or  239 Pu whose mass was on the order of fractions of a gram, carried out using the  252 Cf neutron source found that the average delayed neutron count rate was on the same order of magnitude as those obtained from passive measurements using several kilograms of natural uranium. The Monte Carlo N-Particle 6 version 2.0 radiation transport code was used to simulate the aforementioned active interrogations and to inform the experimental results. Results showed that, due to the close proximity of the polyethylene moderator to the  252 Cf source, the neutron energy spectrum traversing the fissile sample has a significant thermal component that maximizes the fission reaction rate in the interrogated fissile samples, thereby allowing for successful measurements.

nuclear technology

Benchmarking Neutron Counting System for Passive Measurements and Active Interrogation of Unknown Objects for Fissile and Fissionable Materials Determination