Photofission for Active SNM Detection II: Intense Pulsed ~(19)F(p,αγ)~(16)O Characteristic y Source

摘要

An ongoing programme investigating the active detection of special nuclear material (SNM) is being undertaken by the Atomic Weapons Establishment (AWE) 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,αγ)~(16)O 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 ~3He 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, 8.5g/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 ~3He tubes fielded. The results have 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.