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 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 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 proportional counters, NaI(Tl) scintillators and Eljen EJ-309 liquid scintillators. Results with the x-ray-only source demonstrate detection ( ) of the DU target through a minimum of of steel, dropping to when using a mixed x-ray/photoneutron source. The proportional counters demonstrate detection ( ) of the DU target through the maximum steel shielding deployed for both photon and mixed x-ray/photoneutron sources.Index Terms-Active interrogation, bremsstrahlung x-ray, electron accelerators, nuclear security, photoneutron, special nuclear material (SNM).
We irradiated a depleted uranium ( ) target with intense, single 50 ns pulses of bremsstrahlung to study the behavior of , , NaI(Tl), and liquid scintillation detectors in a harsh radiological environment. The target was exposed unshielded, and shielded with borated high-density polyethylene, or steel, and delayed -ray and neutron signatures were measured. We found that a high confidence measurement of the delayed emission could be obtained in this environment and show the results from each detector array, for varying amounts of shielding, in terms of the signal-to-noise ratio vs. time and the relationship between the mean of the signal-to-noise ratio vs. areal mass density.
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