Anthony L. Hutcheson

Maritime detection of radiological/nuclear threats with hybrid imaging system

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 the SuperMISTI stand-off detection system for maritime environments. The instrument was deployed at Norfolk Naval Station in July 2012 as part of the Manta technology demonstration to determine the on-water performance of the system. Detailed descriptions of the SuperMISTI system and its operation are given.

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.

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.

Stand-off detection with an active interrogation photon beam

Active interrogation measurements with a bremsstrahlung photon beam were taken using a hybrid coded imaging and spectroscopic identification system developed at the Naval Research Laboratory (NRL). Measurements were taken using a bremsstrahlung photon beam produced by the Photonuclear Inspection and Threat Assessment System (PITAS) bremsstrahlung photon source at the Idaho Accelerator Centers (IAC) Pocatello Airport facility. The 3 to 7 MeV delayed gamma-ray signature for depleted uranium (DU) was observed and was not seen with a lead target. The delayed gamma-rays were localized to the position of the DU using the coded imager.

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.

An active interrogation detection system (ACTINIDES) based on a dual fast neutron/gamma-ray coded aperture imager

We report on the initial characterization efforts for an active interrogation detection system (ACTINIDES) for applications in maritime security. The ACTINIDES concept is based on neutron/gamma-ray detection, measurement and imaging using a coded aperture mask and an array of liquid scintillator detectors. The coded mask is based on a modified uniformly redundant array (MURA) with hybrid mask elements comprised of high-density polyethylene and lead. The detector array is composed of thirty liquid scintillators. Liquid scintillator detectors are sensitive to both neutrons and gamma rays, with discrimination between the two accomplished by measurement of differences in the de-excitation light pulses. The proof-of-concept study of the instrument has been conducted passively in the laboratory; ultimately, the fully scaled-up instrument is designed to be used in tandem with an active interrogator. Results from the full laboratory test campaign will be presented, along with future prospects for work with an active interrogator.

Outdoor stand-off interrogation of fissionable material with a hybrid coded imaging system

A hybrid coded imaging and detection system developed at the U.S. Naval Research Laboratory (NRL) was used for active interrogation measurements with pulsed bremsstrahlung at the Hermes-III facility at Sandia National Laboratories, Albuquerque. This work follows previous experiments performed in 2011 [1] and explores different targets and system conditions to the previous work. The techniques used and challenges encountered during this work are described.

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.

Neutron/gamma pulse shape discrimination (PSD) in plastic scintillators with digital PSD electronics

Pulse shape discrimination (PSD) is a common method to distinguish between pulses produced by gamma rays and neutrons in scintillator detectors. This technique takes advantage of the property of many scintillators that excitations by recoil protons and electrons produce pulses with different characteristic shapes. Unfortunately, many scintillating materials with good PSD properties have other, undesirable properties such as flammability, toxicity, low availability, high cost, and/or limited size. In contrast, plastic scintillator detectors are relatively low-cost, and easily handled and mass-produced. Recent studies have demonstrated efficient PSD in plastic scintillators using a high concentration of fluorescent dyes. To further investigate the PSD properties of such systems, mixed plastic scintillator samples were produced and tested. The addition of up to 30 wt. % diphenyloxazole (DPO) and other chromophores in polyvinyltoluene (PVT) results in efficient detection with commercial detectors. These plastic scintillators are produced in large diameters up to 4 inches by melt blending directly in a container suitable for in-line detector use. This allows recycling and reuse of materials while varying the compositions. This strategy also avoids additional sample handling and polishing steps required when using removable molds. In this presentation, results will be presented for different mixed-plastic compositions and compared with known scintillating materials