J. O'Malley

Detector requirements for a cosmic ray muon scattering tomography system

Cosmic ray muon scattering tomography is one of four techniques currently being investigated at AWE for the detection of special nuclear material (SNM). In order to develop a prototype muon detection system, it is necessary to consider the requirements of the radiation detectors with respect to; coincidence timing for system triggering; tracking of the muon trajectory; and determination of muon energy. The detector requirements for a prototype muon scattering tomography system are presented and a variety of detector types considered and assessed against these requirements. The advantages, disadvantages, potential compromises and compatibility with other complementary detection techniques are discussed. Future plans are outlined for an initial prototype and future, long-term development of a muon scattering tomography system for detection of SNM.

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

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,αy)16O characteristic gamma sources and the use of DD, DT, and lower energy beam-target neutron sources such as those produced via. 7Li(p,n)7Be. 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,αy)16O 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.

Photofission for active SNM detection I: Intense pulsed 8MeV bremsstrahlung source

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). As part of this programme, the NRL Mercury IVA was operated in negative polarity mode to produce an 8MeV endpoint bremsstrahlung spectrum, which in turn was used to induce photofission in a depleted uranium (DU) sample. Twenty-six experiments were fielded in March 2011 in which twenty-seven 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) shielding configurations was employed and positive detection was made up to the maximum shielding tested, 75g/cm2. A detailed source has been modelled using MCNPX and MCNP6 to predict the number of (n,p) detector events within four of the 3He tubes fielded. The modelling is consistent with the experiment to within a factor of two, when integrating between 0.1 and 20s.

AWE development of active interrogation techniques for the detection of SNM

Active interrogation techniques using photons (<10 MeV) and low energy neutrons are currently being investigated at AWE for the remote detection of special nuclear material (SNM). To identify the presence of SNM the induced fission signatures are measured. AWE is investigating the combination of the photon and neutron techniques to provide robust detection for shielded scenarios involving either hydrogenous or high Z materials. A brief description of the interrogation sources is given with consideration to the extraction of high fidelity fission signatures in the presence of typical naturally occurring radioactive material and other background signals generated by the interrogation process. Initial results are presented from MCNPX simulations of prompt and delayed neutrons and γ-rays produced from the induced fission of SNM. Photon and low energy neutron interrogation simulations are compared to identify requirements for an initial common detection system.

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 ( > 3\sigmab) 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 ( > 3\sigmab) of the DU target through the maximum 149. 7 g·cm - 2 steel shielding deployed for both photon and mixed x-ray/photoneutron sources.

Prompt sub-MeV neutron production from the 7Li(p,n)7Be reaction on mercury

Summary form only given. Proton-beam-generation experiments have been conducted on the NRL Mercury pulsed-power generator operating in positive polarity with a lithium metal target embedded in the cathode. The accelerating voltage was limited to below 2.7 MV in order to limit the energy of neutrons produced in the 7Li(p,n)7Be reaction (Q = 1.88 MeV) to below 1 MeV. Analyses based on published results1 and calculations presented here are used to predict the angular distribution of neutron yield and spectrum for each shot. Predicted neutron yields are compared to Rh-counter and Al-activation measurements. The results of these comparisons are quite encouraging, showing better than factor-of-2 agreement between the two sets of measurements and the analysis over the voltage range of the shot series. In order to achieve this level of agreement, a series of MCNPX computations has been carried out to determine the spectral contribution of neutrons reflected from the Mercury test-cell environment2 and the associated changes in detector calibrations. The agreement between measurements and modeling provides a check on the voltage calculated using a positive-polarity ion-diode model. For operation at 2.5–2.6 MV, on-axis neutron yields from the p-Li reaction are in the 1011 neutrons/steradian range.

Current status of AWE programmes for remote detection of SNM

AWE has recently commenced a research programme to develop techniques for the remote detection of illicitly trafficked, special nuclear material. The result of a review of potential techniques to remotely detect special nuclear material (SNM) conducted by AWE has stimulated a programme at AWE to further investigate four remote detection techniques. These are a photonuclear based and a neutron based active interrogation technique, the use of nuclear resonance fluorescence and cosmic ray muon scattering tomography. The current status of work in the development of these techniques is presented with particular emphasis on the photonuclear and neutron techniques. Developments in physics modeling capabilities, active source technologies, detector technologies and data fusion and analysis approaches are summarised for each technique where applicable. Particular attention is given to the development of high current, pulsed power based, active interrogation sources for remote detection of SNM. Potential technologies for active source technology are discussed and requirements for further technology developments in this area are presented

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

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 19P(p,αγ)160 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.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

Advanced active interrogation sources for remote detection of special nuclear material

AWE has recently commenced a research programme to develop techniques for the remote detection of illicitly trafficked, special nuclear material. Three techniques that are being investigated as part of this programme utilise an active interrogation source to stimulate a radiation signature within SNM. A photonuclear technique and nuclear resonance fluorescence require an active gamma source while a technique stimulating fission with low energy neutrons requires a low energy neutron source. The potential for consolidation of the photonuclear and neutron technique is discussed. Conceptual designs of intense, pulsed, ion beam diodes to generate suitable sources of 6-7MeV gamma photons for the photonuclear technique and ≪100keV neutron sources for the active neutron technique are presented. The sources required for remote detection via the 60keV neutron or photofission techniques are contrasted with the source requirements for nuclear resonance fluorescence.

Current status of AWE programmes into stand-off detection of special nuclear material (SNM)

A brief overview of an AWE survey of existing techniques to detect SNM will be presented. This survey has identified two techniques that incorporate active gamma sources for further study. These are a photonuclear based technique and the use of nuclear resonance fluorescence. The current status of work in the development of these techniques is presented. Developments in physics modelling capabilities, active source technologies, detector technologies and data fusion and analysis approaches are summarised. Particular attention will be given to the potential for high current, pulsed power based, active interrogation sources for remote detection of SNM.