Detection of special nuclear material in hydrogenous cargo using differential die-away analysis

Differential Die Away Analysis is a sensitive technique to detect presence of fissile materials like 235U and 239Pu. In DDAA, a neutron generator produces repetitive pulses of neutrons which are directed into a cargo being inspected. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. The thermalization process is very rapid and the population of all neutrons from source to epithermal neutrons decays away within microseconds. The population of thermal neutrons however decays much slower with the diffusion decay time of the inspected medium (thermal neutron die-away time), on the order of hundreds of microseconds. If special nuclear material (SNM) is present, the thermalized neutrons from the source will cause fissions that produce a new source of neutrons. These fast fission neutrons decay with a time very similar to that of the thermal neutron die away of the surrounding cargo.;Improvement of DDAA sensitivity can be obtained by advanced knowledge of the thermal-neutron kinetic properties of the inspected medium. The standard way to obtain such information is by measuring thermal neutron die-away by a detector inside of the medium. Since this is not practical in a real system, a method of determining information about thermal die-away properties of a medium from external measurements is examined. This method allows inspected media to be grossly characterized by their neutron moderating and attenuating characteristics.;The DDAA method provides a binary decision regarding the presence or absence of special nuclear material in an inspection medium. A detection algorithm was developed that utilizes advance knowledge of detector response to improve the decision quality.;The sensitivity of DDAA, for a given source of neutrons, critically depends on optimizing the fast and epithermal neutron detection system. The optimization involves both time response and detection efficiency. The optimized detectors were able to detect fissile material throughout pallets of hydrogenous cargo.