Active Interrogation of Depleted Uranium Using a Single Pulse, High-Intensity Photon and Mixed Photon-Neutron Source

Active interrogation is a method used to enhance the likelihood of detection of shielded special nuclear material (SNM); an external source of radiation is used to interrogate a target and to stimulate fission within any SNM present. Radiation produced by the fission process can be detected and used to infer the presence of the SNM. The Atomic Weapons Establishment (AWE) and the Naval Research Laboratory (NRL) have carried out a joint experimental study into the use of single pulse, high-intensity sources of bremsstrahlung x-rays and D(gammab, n)H photoneutrons in an active interrogation system. The source was operated in both x-ray-only and mixed x-ray/photoneutron modes, and was used to irradiate a depleted uranium (DU) target which was enclosed by up to 150 g·cm ^{- 2} of steel shielding. Resulting radiation signatures were measured by a suite of over 80 detectors and the data used to characterise detectable fission signatures as a function of the areal mass of the shielding. This paper describes the work carried out and discusses data collected with ³He proportional counters, NaI(Tl) scintillators and Eljen EJ-309 liquid scintillators. Results with the x-ray-only source demonstrate detection ( > 3sigmab) of the DU target through a minimum of 113 g·cm ^{- 2} of steel, dropping to 85 g·cm^- 2 when using a mixed x-ray/photoneutron source. The ³He proportional counters demonstrate detection ( > 3sigmab) of the DU target through the maximum 149. 7 g·cm ^{- 2} steel shielding deployed for both photon and mixed x-ray/photoneutron sources.