Lee J. Mitchell

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.

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.

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.

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.

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.

Development of the strontium iodide coded aperture (SICA) instrument

The work reports on the development of a Strontium Iodide Coded Aperture (SICA) instrument for use in space-based astrophysics, solar physics, and high-energy atmospheric physics. The Naval Research Laboratory is developing a prototype coded aperture imager that will consist of an 8 x 8 array of SrI2:Eu detectors, each read out by a silicon photomultiplier. The array would be used to demonstrate SrI2:Eu detector performance for space-based missions. Europium-doped strontium iodide (SrI2:Eu) detectors have recently become available, and the material is a strong candidate to replace existing detector technology currently used for space-based gamma-ray astrophysics research. The detectors have a typical energy resolution of 3.2% at 662 keV, a significant improvement over the 6.5% energy resolution of thallium-doped sodium iodide. With a density of 4.59 g/cm and a Zeff of 49, SrI2:Eu has a high efficiency for MeV gamma-ray detection. Coupling this with recent improvements in silicon photomultiplier technology (i.e., no bulky photomultiplier tubes) creates high-density, large-area, low-power detector arrays with good energy resolution. Also, the energy resolution of SrI2:Eu makes it ideal for use as the back plane of a Compton telescope.

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.

Activation of sodium iodide detectors in an Active Interrogation environment

Applications such as the Active Interrogation of shielded nuclear material that require the detection of specific gamma and neutron signatures in a high mixed radiation field background, require detector materials and device fabrications that are not highly susceptible to neutron or photon activation. This paper presents an activation analysis from Active Interrogation experiments where a pulsed-power accelerator was used to deliver 2.5 MeV protons onto a lithium target to produce a spectrum of neutrons up to 1 MeV. The experimental objectives were to characterize the neutron energy spectrum and spatial distribution produced from the ion beam diode utilized with the pulsed accelerator with a view to its utility for the purposes of active detection. A variety of neutron and gamma detectors were used to record the resulting signatures including sodium iodide (NaI), which is the main focus of this paper, and which is also a candidate detector technology to detect fission signatures from active detection systems. A High Purity Germanium (HPGe) detector was used to measure the amount of activation created in the NaI detector. The results showed significant activation of the core detection material, with activation product decay time constants of the order of seconds and minutes. Activation of the detector construction materials was also observed. From this type of analysis, the minimum time between active interrogations can be determined based on the recovery time of the detectors following activation, or alternatively, materials can be sought that are less susceptible to activation.