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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).

An ongoing programme investigating the active detection of special nuclear material (SNM) is being undertaken by the Atomic Weapons Establishment (A WE) in collaboration with the Naval Research Laboratory (NRL). The programme is funded through the UK Home Office, Ministry of Defence and Cabinet Office and the Naval Research Laboratory supported primarily through the US Defence Threat Reduction Agency with support also from the Office of Naval Research and the Defence Nuclear Detection Office. The process by which the UK are applying active detection techniques to border protection and a review of the current challenges and opportunities for this technology as assessed by the authors is provided. As part of this programme, the NRL Mercury IVA was operated in positive polarity mode to produce photons characteristic of the 19 F(p,ay) 16 0 reaction, at energies of 6.13, 6.92 and 7.12 MeV. Protons produced by Mercury interact with a thick Teflon (PTFE) target to produce characteristic gamma radiation. These in turn were used to induce photofission in a depleted uranium (DU) sample. Eighteen experiments were fielded in September 2011, in which thirty-five detectors were fielded, including 3 He tubes, NaI detectors, liquid scintillators and high purity germanium detectors, capable of detecting both gamma radiation and neutrons. The results from a selection of those detectors are discussed here. A variety of high-Z (lead) and hydrogenous (borated polyethylene) and hydrogenous shielding configurations was employed and positive detection was made up to the maximum shielding tested, 8.Sg/cm2. Effects of secondary reactions in the photon production are visible in the results and some employed reduction techniques are discussed. Monte Carlo modelling has been employed for a subset of the 3 He tubes fielded. The resultshave been found to agree within an order of magnitude, but have also been found to die away more quickly than observed in the experimental data.

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.

A WE is developing a prototype active interrogation system to enable robust detection of shielded nuclear material within the context of border choke point detection. This paper describes a study which took place in order to determine the optimum type of radiation and pulse structure to be used with the prototype system. A wide variety of neutron and gamma interrogation sources were considered including the use of sub 10 MeV end-point energy bremsstrahlung or 19F(p,uyi60 characteristic gamma sources and the use of DD, DT, and lower energy beam-target neutron sources such as those produced via. 7 Li(p,n) 7 Be. In each case, where appropriate, both flash systems capable of delivering intense, sub 100 ns pulses of radiation and non-flash repetitively pulsed or continuous wave (CW) sources were considered. Experimental measurements of photo-fission signatures on bare and shielded depleted uranium produced by 100 ns flash sources of 8 MeV bremsstrahlung and 19F(p,uyi60 characteristic gamma sources together with a systematic series of simulations of photofission signatures and associated backgrounds and a review of technological limitations of relevant accelerator technologies were all assessed.An estimate derived from experimental measurements of a minimum number of fissions necessary to detect target quantities of special nuclear material through shielding thickness of interest in the context of border detection is used to determine a charge which must be delivered by the interrogation source and this in turn is used to rank potential interrogation source options.Wider arguments concerned with the ease with which fission signatures may be discriminated above active backgrounds and the utility of different options given operational constraints are then presented and it is concluded that the UK will recommend a flash «100 ns), 10 MeV end-point energy bremsstrahlung source for the UK active interrogation prototype system.