Fast Neutron Detection in Nuclear Material Photofission Assay Using a 15 MeV Linear Electron Accelerato

The purpose of this research was to use a 15 MeV (K15 model by Varian) linear electron accelerator (linac) for the photon assay of special nuclear materials (SNM). First, the properties of the photon radiation probe were determined. The stochastic radiation transport code, MCNP5, was used to develop computational models for the linac. The spectral distribution of photons as well as dose rate contour maps of the UNLV accelerator facility were computed for several linac operating configurations. These computational models were validated through comparison with experimental measurements of dose rates.;The linac model was used to simulate the photon interrogation of SNM targets of various compositions and shielding materials. The spectra of neutrons produced by the irradiation of shielded SNM was characterized. The effects of shielding material and the SNM enrichment on the neutron yields following photon assay were determined. It was determined that the radiation signatures following the photon assay of SNM consisted of photons and neutrons produced from the fissions, in addition to neutrons produced from photonuclear reactions.;The EJ-299-33A plastic scintillator was evaluated for this study due to its ability to discriminate between fast neutrons and gamma rays. The neutron coincidence measurement option was also evaluated. The detector response functions were determined for different incident neutron energies. Further, it was computationally shown that an array of EJ-299-33A detectors allows to measure neutron multiplicity, enabling discrimination between fission neutrons and the photoneutrons.