Monte Carlo Evaluation of the Improvements in Nuclear Materials Identification System (NMIS) Resulting From a DT Neutron Generator

Nuclear safeguards active measurements that rely on the time correlation between fast neutrons and gamma rays from the same fission are a promising technique. Previous studies have shown the feasibility of this method, in conjunction with the use of artificial neural networks, to estimate the mass and enrichment of fissile samples enclosed in special, sealed containers. This paper evaluates the use of the associated particle sealed tube neutron generator (APSTNG) as the interrogation source in correlation measurements. The results show that its use is of particular importance when floor reflections are present. The Nuclear Materials Identification System (NMIS) presently uses 252Cf ionization chambers as interrogation sources for the time-dependent coincidence measurements. Because triggers from this source are associated with neutrons emitted in any direction, adjacent materials such as the floor and nearby containers could affect the measurements and should be accounted for. Conversely, the APSTNG, together with an alpha particle detector, defines a cone of neutrons that can be aimed at the item under verification, thus removing the effects of nearby materials from the time-dependent coincidence distributions. Monte Carlo calculations were performed using MCNP-POLIMI, a modified version of the standard MCNP code. The code attempts to calculate more correctly quantities that depend on the second moment of the neutron and gamma distributions. The simulations quantified the sensitivity enhancements and removal of the effects of nearby materials by substituting the traditional 252Cf source with the APSTNG