Mixed Source Interrogation of Steel Shielded Special Nuclear Material Using an Intense Pulsed Source

This paper explores the benefits of using a mixed photon and neutron radiation source for active detection of special nuclear material. More than fifty irradiations were performed using an 8 MV electron accelerator employing and induction voltage adder (IVA). The experiments used a high atomic number converter to produce a Bremsstrahlung photon spectrum which was then used to create a neutron source via a nuclear interaction with heavy water (deuterium oxide, D2O). This mixed particle source was used to irradiate a depleted uranium (DU) sample, inducing fission in the sample. Several thicknesses of steel shielding were tested in order to compare the performance of the mixed photon and neutron source to a Bremsstrahlung-only source. An array of detectors were fielded to record both photons and neutrons emitted by the fission reactions. A correlation between steel shielding and a detection figure-of-merit can be seen in all cases where the Bremsstrahlung-only source was used. The same relationship for the mixed photon-neutron source is less consistent. The data collected from the fielded detectors is compared to MCNP6 calculations and good agreement is found.