Richard S. Woolf

Pulsed power active interrogation of shielded fissionable material

We report on a collaborative test campaign conducted at the Naval Research Laboratorys Mercury pulsed power facility in December of 2012. The experiment sought to use Mercury in the Intense Pulsed Active Detection (IPAD) [1] mode to interrogate a fissionable material target (depleted uranium, DU) and benchmark the effects of shielding the target with either a low-Z (2% borated high-density polyethylene, BPE) or high-Z (steel) material. A large suite of instrumentation, including 3He, BF3, NaI(Tl), and liquid scintillation detectors were used to measure the delayed γ and neutron signatures from the DU. The test campaign consisted of a series of single IPAD pulses, i.e., “shots,” employing incremental shielding configurations of BPE (up to 50 g/cm2) and steel (up to 150 g/cm2) encapsulating the DU target. We show the results from each detector array, for varying amounts of shielding, in terms of the signal-to-noise vs. time.

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.

Neutrons for active detection of special nuclear material: An intense pulsed 7 Li(p,n) 7 Be source

An ongoing program me looking at 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, pulsed-power driven neutron experiments were conducted at the NRL Mercury accelerator. Mercury was used in a positive polarity mode to produce and accelerate protons into lithium metal foils, generating neutrons via the 7Li(p,n)7Be reaction. 13 shots were carried out at varying machine voltages and over 30 separate neutron and gamma-ray diagnostics were fielded to characterise the angular distribution and energy spectrum of the neutrons generated. Machine performance, neutron, and gamma-ray data are presented and discussed. Neutron yields of up to 1011 neutrons/steradian were recorded, with yields at 60° off axis being approximately 50% of the on axis yield. Previously published analysis [1] of data has been used to validate GEANT4 modelling of the experiments (2). Machine performance data has been used in conjunction with modelled neutron spectra to predict the performance of the Mercury 7Li(p,n)7Be source as a system for detecting SNM.

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.

Standoff detection of thermal and fast neutrons

Improved detection of weapons of mass destruction is one of the Science and Technology priorities of the Secretary of Defense for Fiscal Years 2013-2017. Unfortunately, the remote detection of special nuclear materials is difficult because the materials are not very radioactive, the radiation signatures decrease rapidly with distance, and faint sources of radiation can be obscured by naturally occurring and man-made radioactive sources. The Radiation Detection Section of the High Energy Space Environment Branch of the U.S. Naval Research Laboratory has developed a containerized fast and thermal neutron standoff detection system. The instrument was characterized with neutron sources at different standoff distances and at different drive-by speeds to determine the on-water performance of the system. Results of this measurement campaign will be discussed.

Comparative study of the pulse shape discrimination (PSD) performance of fast neutron detectors

Certain scintillating materials are sensitive to both gamma and neutron radiation and can give information about the type of interacting radiation due to differences in the light output response. By collecting the light pulses and converting them to electrical signals the nature of the radiation can be determined by measuring the amount of electrical charge in the pulse tail - for neutrons, the pulses are longer, with more charge in the tail than for the shorter gamma pulses. This determination called Pulse Shape Discrimination (PSD) can nowadays be performed in real-time onboard digitisers during data collection. In this work several detectors (EJ301, EJ309 liquids; EJ299-33 plastic and p-terphenyl scintillators) of various shapes and sizes were connected to several digital Data Acquisition (DAQ) systems as well as the established digital / analogue hybrid Mesytec MPD8 / MADC-32 set up in a comparative study. The aim of the campaign was to produce a Figures of Merit (FOM) for the PSD performance of the various detector / DAQ combinations to give relative performance estimates of the CAEN V1751 10-bit 1 GSample/s digitiser in comparison with other DAQ solutions within a near-standardised experimental environment. It is likely that the DAQ set ups were not equivalent as significant differences in the matching of the detector outputs to the dynamic range of the digitisers were observed - however, with the configurations used in this campaign the CAEN V1751 digitiser showed superior FOM values to the Struck SiS3320, Bridgeport usbBase and Mesytec MPD-8 DAQ systems tested. Furthermore, there seemed little difference between the FOM from the faster but lower voltage resolution (1 GSample/s with 10 bits) CAEN V1751 compared to the slower but higher resolution (250 MSample/s with 12 bits) CAEN N6720 digitiser for this application.

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

64-element fast-neutron, coded-aperture imager

We report on the characterization efforts on a largescale, coded-aperture imager for fast neutrons at the U.S. Naval Research Laboratory. The fast neutron imager discussed in this study is designed for a mobile unit (20 ft. ISO container) and consists of a 64-element array (arranged in an 8 by 8 grid pattern) of 15 cm × 15 cm × 15 cm liquid scintillation detectors (EJ-309) mounted behind a 12 × 12 pseudorandom coded aperture. The coded aperture is composed of 15 cm × 15 cm × 10 cm blocks of high-density polyethylene (HDPE). The 12 × 12 pseudorandom coded aperture produces a shadow pattern on the detector array behind the mask. With knowledge of the mask pattern and the number of neutrons detected in a masked and unmasked detector, a source image can be deconvolved to obtain a 2-d image. The number of neutrons detected in each detector was found by processing the fast signal from each photomultiplier tube (PMT) in flash digitizing electronics. The γ-ray background was filtered out using digital pulse shape discrimination (PSD). The 64-element instrument was tested at an indoor facility using four γ/neutron sources: 1.3-µCi 252Cf (5.6 × 103 neutrons/s), 9.3-µCi 252Cf (4.0 × 104 neutrons/s), 99.8-µCi 252Cf (4.3 × 105 neutrons/s), and a 46-µCi 241AmBe (2.0 × 105 neutrons/s) at three standoff distances of 9, 15, and 26 m (maximum allowed in the facility) over various integration times. The source was moved at one pixel increments to test the imaging capabilities of the instrument. We found the detection significance at each of these distances and pixel locations with the three sources after 5 min to 25 min of integration depending on the strength and location of the source. We show the detection significance as a function of time at some locations and compare the detected significance to our groups previous 32-element array detected significances [1]. We also shielded 252Cf sources with 2.5 cm, 5.1 cm, and 10.2 cm thick HDPE and show the detection significance as a function of time at 9 m, 15 m, and 26 m.

Mobile outdoor SuperMISTI Compton imager

The SuperMISTI coded aperture imager trailer has been modified as a Compton imager with a 5 × 5 array of either NaI or plastic detectors and a 6 × 13 array of NaI detectors separated by 1.66 m. Gamma-ray sources were located by both the NaI-NaI and the Plastic-NaI imagers at 10s of meters standoff distances. The results from the new Compton cameras are compared to the coded aperture imager that also utilizes the 6 × 13 array of NaI detectors.

Modelling of the 7Li(p,n)7Be neutron yield from mercury using GEANT-4 and LSP

Summary form only given. An ongoing programme looking at the active detection of special nuclear material is being undertaken by the Atomic Weapons Establishment in collaboration with the Naval Research Laboratory (NRL). As part of this programme, proton beam generation experiments were conducted at the NRL Mercury pulsed power accelerator using a lithium metal cathode to generate neutrons through the 7Li(p,n)7Be reaction. A modification to GEANT4 to include the 7Li(p,n)7Be reaction for the purposes of modeling the doubly differential neutron yield from these experiments has been implemented. The Large Scale Plasma (LSP) Particle-in-Cell (PiC) code was used to obtain the proton kinematics and flux at the lithium conversion target; an algorithm was subsequently written to transform this data to a complete GEANT4 source description. The modification to the GEANT4 simulation toolkit is presented and the output compared to experimental data with good agreement. The work presented shows that the new GEANT4 process alongside the existing packages can be used to model the 7Li(p,n)7Be reaction with reasonable accuracy given an input proton phase space from a suitable simulation code such as LSP.