theory and performance of AIMS

A hybrid Monte Carlo and deterministic methodology has been developed for application to active interrogation systems for detection of HEU (Highly Enriched Uranium) in a cargo container. The methodology consists of four steps: 1) determination of neutron flux distribution due to neutron source transport and subcritical multiplication; 2) generation of gamma source distribution from (n, γ) interactions; 3) determination of gamma current at a detector window; and 4) detection of gammas by the detector. This paper discusses the theory and results of the first three steps for the case of a cargo container with a sphere of HEU in third-density water. In the first step, a response-function formulation has been developed to calculate the subcritical multiplication and neutron flux distribution. Response coefficients are pre-calculated using the MCNP5 Monte Carlo code. The second step uses the calculated neutron flux distribution and Bugle-96 (n, γ) cross sections to find the resulting gamma source distribution. Finally, in the third step the gamma source distribution is coupled with a pre-calculated adjoint function to determine the gamma flux at a detector window. A code, AIMS (Active Interrogation for Monitoring Special-Nuclear-materials), has been written to output the gamma current for an source-detector assembly scanning across the cargo using the pre-calculated values and takes significantly less time than a reference MCNP5 calculation. It is concluded that AIMS predictions are accurate for cases where the HEU sphere is not directly between the source and detector or near the container wall. For these cases, it is necessary to extend the database by determining response coefficients and adjoint functions corresponding to the true model. (authors)