S. L. Jackson

Detectors for intense, pulsed active detection

In intense, pulsed active detection, a single, intense pulse of radiation is used to induce photofission in fissionable material, increasing its detectability. The Mercury pulsed-power generator was converted to positive polarity (+3.7 MV, 325 kA, 50-ns FWHM) to drive an intense, pulsed radiation source based on the FIGARO active detection concept. The probing radiation source consisted of an ion-beam diode and a thick PTFE (Teflon) converter where 6–7 MeV γ-rays were produced via the 19F(p,αγ)16O reaction. A suite of radiation detectors was used both to detect the presence of irradiated fissionable material and to characterize the probing radiation source. Four types of detectors were used for the source characterization. Thermoluminescent dosimeters were used to measure the angular distribution of the dose associated with x-rays and γ-rays from the ion-beam diode. Plastic scintillator-photodiode detectors were used to characterize the time dependence of this dose. A plastic scintillator-photomultiplier detector was used to monitor the γ-ray intensity of the probing radiation source and to monitor changes in the production of background neutrons by the diode and PTFE converter. A set of rhodium foil activation counters was used to measure the absolute yield of these background neutrons. Two types of detectors with comparable sensitivities were used to measure delayed neutrons resulting from photofission: 3He proportional counters and a 6Li-loaded-glass-scintillator detector. The neutron detection rate from each detector following the probing radiation pulse was over 100 times higher with depleted uranium present than with lead.

Ion diode performance on a positive polarity inductive voltage adder with layered magnetically insulated transmission line flow

A pinch-reflex ion diode is fielded on the pulsed-power machine Mercury (R. J. Allen, et al., 15th IEEE Intl. Pulsed Power Conf., Monterey, CA, 2005, p. 339), which has an inductive voltage adder (IVA) architecture and a magnetically insulated transmission line (MITL). Mercury is operated in positive polarity resulting in layered MITL flow as emitted electrons are born at a different potential in each of the adder cavities. The usual method for estimating the voltage by measuring the bound current in the cathode and anode of the MITL is not accurate with layered flow, and the interaction of the MITL flow with a pinched-beam ion diode load has not been studied previously. Other methods for determining the diode voltage are applied, ion diode performance is experimentally characterized and evaluated, and circuit and particle-in-cell (PIC) simulations are performed. Results indicate that the ion diode couples efficiently to the machine operating at a diode voltage of about 3.5 MV and a total current of about...

Conversion of Mercury (a 2-TW inductive voltage adder) to positive polarity

After 616 shots in a negative polarity configuration, Mercury, a 6-MV and 300-kA inductive voltage adder (IVA), has been converted to positive polarity in order to extract ion beams. Conversion to positive polarity was achieved by rotating all six of the adder cells by 180°. In principle, we could have chosen to instead insert the center conductor from the other end of the adder to change polarity, but rotating the cells minimized the time required to make the transition. Although most of the same pieces were used, the center conductor had to be reconfigured in order to align the transition pieces with the cell feed gaps. Because the electron flow was anticipated to be very different in positive polarity, a result of emission from surfaces of different potential, a simple blade diode was fielded for the initial shots to gain a better understanding of operation in positive polarity. The blade diode consisted of the same cathode used as a dummy load in the first negative polarity shots on Mercury, but with a different carbon anode that just covered the end of the center conductor. After a few short circuit and initializing shots, a series of shots were taken where only the blade diode AK gap was varied in order to characterize self-limited and load-limited operation and to compare measurements with theory and simulation.

High-voltage, high-impedance ion beam production

A high-power, high-impedance ion beam diode has been fielded on the Mercury inductive voltage adder at typical parameters of 4 MV, 360 kA, and 70–90-kA ion current. These results are consistent with theory and with LSP calculations. This beam is focused onto a CF2 target to produce characteristic gammas via the 19F(p,αγ)16O reaction. Diagnostics characterize the ion emission from the anode, the beam distribution on the target, and the gamma yield. Shaping the anode surface improves beam focusing on the target. Our experimental arrangement allows us to resolve the gamma signal in the presence of the diode bremsstrahlung, and to minimize spurious neutron production. The calculated gamma yield exceeds 1011 gammas/sr.

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.

Room scattering effects on the measured spatial distribution of delayed photofission neutrons from depleted uranium

A series of experiments has been performed on the Mercury generator (8 MV, 200 kA, 50 ns) to investigate the use of a single, intense radiation pulse to induce photo fission.1 In these studies, a plate of depleted uranium (DU) is irradiated by the 8 MV-endpoint bremsstrahlung pulse produced by Mercury. Within this x-ray pulse, photons whose energies lie above the threshold of 5.26 MeV induce fission in the DU, producing about 2.7 neutrons per fission. The neutrons consist of two populations: prompt neutrons, which escape the DU within a few hundred ns of the bremsstrahlung pulse, and delayed neutrons, which continue to appear for minutes after the pulse. The results reported here pertain only to the delayed neutron population. These neutrons are unambiguously detected using an array of He-3 detectors deployed 1.0–4.5 m from the DU. However, the measured neutrons do not follow an inverse square law as a function of detector-to-DU distance. The farthest detector measured more than twice the counts, relative to the closest detector, that would be expected for an inverse-square distribution. Containment and scattering of the fission neutrons by the walls, floor, and ceiling of the Mercury chamber, and various nearby objects such as electrical boxes, are the likeliest explanation for the measured spatial distribution. Modeling of the experiment by the Monte Carlo transport code MCNPX 2 supports this hypothesis. Results showing the influence of various features of the Mercury chamber, as well as their composition, are presented.

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.