Photofission for active SNM detection II: Intense pulsed 19F(p,αγ)¹⁶O characteristic γ source

Abstract

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.

Active detection of special nuclear material - Recommendations for interrogation source approach for UK prototype active detection system

“…The second set concerned quantification of the output of interrogating neutrons from 7 Li(p,nfBe reactions with pulsed power generated proton sources. Results from these experimental campaigns are included in these conference proceedings [2,3,4]. objects Three important results were derived from these experiments in relation to the source optioneering question for a prototype active detection system.…”

“…Practical experience from the range of detectors used in these experimental campaigns suggested that typically radiation detector systems might be unresponsive for a 'dead time' of a few ms post flash of interrogating radiation. During this dead time any prompt neutron radiation from fission, or delayed gamma or neutron signals could not be recorded and these data were useful in interpretation of predicted levels of neutron and gamma signal from flash sources [3]. Thirdly, it was clear in the characteristic gamma experiments that the pulsed ion beam diodes used with the pulsed accelerators resulted in a small population of accelerated deuterons.…”

“…This resulted in a background of neutrons being generated from the source (via. interactions with carbon or fluorine components in the diode) and these, in turn, confused signature fission neutrons from material under detection [3]. This deuteron induced neutron background represents a disadvantageous feature of beam-target generated sources (such as 19 F(p,uy) 16 0) in comparison with bremsstrahlung sources where relatively few ions are accelerated in the diode.…”