Publications

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We present the relative timing and pulse-shape discrimination performance of a H1949-50 photomultiplier tube to SensL ArrayX-B0B6_64S coupled to a SensL ArrayC-60035-64P- PCB Silicon Photomultiplier array. The goal of this work is to enable the replacement of photomultiplier readout of scintillators with Silicon Photomultiplier devices, which are more robust and have higher particle detection efficiency. The report quantifies the degradation of these performance parameters using commercial off the shelf summing circuits, and motivates the development of an improved summing circuit: the pulse-shape descrimination figure-of- merit drops from 1.7 at 500 keVee to 1.4, and the timing resolution (a) is 288 ps for the photomultiplier readout and approximately 1 ns for the Silicon Photomultiplier readout. A degradation of this size will have a large negative impact on any device that relies on timing coincidence or pulse-shape discrimination to detect neutron interactions, such as neutron kinematic imaging or multiplicity measurements.

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We present single-sided 3D image reconstruction and neutron spectrum of non-nuclear material interrogated with a deuterium-tritium neutron generator. The results presented here are a proof-of-principle of an existing technique previously used for nuclear material, applied to non-nuclear material. While we do see excess signatures over background, they do not have the expected form and are currently un-identified.

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We present the design, characterization, and testing of a laboratory prototype radiological search and localization system. The system, based on time-encoded imaging, uses the attenuation signature of neutrons in time, induced by the geometrical layout and motion of the system. We have demonstrated the ability to detect a ∼1mCi252Cf radiological source at 100m standoff with 90% detection efficiency and 10% false positives against background in 12min. This same detection efficiency is met at 15s for a 40m standoff, and 1.2s for a 20m standoff.

This is a high-risk effort to leverage knowledge gained from previous work, which focused on detector development leading to better energy resolution and reconstruction errors. This work seeks to enable applications that require precise elemental characterization of materials, such as chemical munitions remediation, offering the potential to close current detection gaps.

We present the design and performance of a proof-of-concept 32 channel material identification system. Our system is based on the energy-dependent attenuation of fast neutrons for four elements: hydrogen, carbon, nitrogen and oxygen. We describe a new approach to obtaining a broad range of neutron energies to probe a sample, as well as our technique for reconstructing the molar densities within a sample. The system's performance as a function of time-of-flight energy resolution is explored using a Geant4-based Monte Carlo. Our results indicate that, with the expected detector response of our system, we will be able to determine the molar density of all four elements to within a 20-30% accuracy in a two hour scan time. In many cases this error is systematically low, thus the ratio between elements is more accurate. This degree of accuracy is enough to distinguish, for example, a sample of water from a sample of pure hydrogen peroxide: the ratio of oxygen to hydrogen is reconstructed to within 8 0.5% of the true value. Finally, with future algorithm development that accounts for backgrounds caused by scattering within the sample itself, the accuracy of molar densities, not ratios, may improve to the 5-10% level for a two hour scan time. Experimental performance was evaluated with various thicknesses of polyethylene. The detector response in terms of energy, particle identification, and timing are presented as well.

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We present the design and expected performance of a proof-of-concept 32 channel material identification system. Our system is based on the energy-dependent attenuation of fast neutrons for four elements: hydrogen, carbon, nitrogen and oxygen. We describe a new approach to obtaining a broad range of neutron energies to probe a sample, as well as our technique for reconstructing the molar densities within a sample. The system's performance as a function of time-of-flight energy resolution is explored using a Geant4-based Monte Carlo. Our results indicate that, with the expected detector response of our system, we will be able to determine the molar density of all four elements to within a 20–30% accuracy in a two hour scan time. In many cases this error is systematically low, thus the ratio between elements is more accurate. This degree of accuracy is enough to distinguish, for example, a sample of water from a sample of pure hydrogen peroxide: the ratio of oxygen to hydrogen is reconstructed to within 8±0.5% of the true value. Finally, with future algorithm development that accounts for backgrounds caused by scattering within the sample itself, the accuracy of molar densities, not ratios, may improve to the 5–10% level for a two hour scan time.

Dual plane neutron scatter cameras have shown promise for localizing fast neutron sources. The condition that a neutron must scatter in both planes of the camera produces low counting efficiencies. Counting efficiency can be improved using an alternative design that uses a single, optically segmented volume of scintillation material. Using Geant4, we simulated pulses from neutron elastic scatter events at different locations throughout an EJ-204 scintillator bar. We used nonlinear regression on low light pulses to determine the position along the bar where the scatter event occurred.

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We present a neutron detector system based on time-encoded imaging, and demonstrate its applicability toward the spatial mapping of special nuclear material. We demonstrate that two-dimensional fast-neutron imaging with 2° resolution at 2 m stand-off is feasible with only two instrumented detectors.

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Material Identification by Resonant Attenuation is a technique that measures the energy-dependent attenuation of 1-10 MeV neutrons as they pass through a sample. Elemental information is determined from the neutron absorption resonances unique to each element. With sufficient energy resolution, these resonances can be used to categorize a wide range of materials, serving as a powerful discrimination technique between explosives, contraband, and other materials. Our proposed system is unique in that it simultaneously down-scatters and time tags neutrons in scintillator detectors oriented between a d-T generator and sample. This allows not only for energy measurements without pulsed neutron beams, but for sample interrogation over a large range of relevant energies, vastly improving scan times. Our system’s core advantage is a potential breakthrough ability to provide detection discrimination of threat materials by their elemental composition (e.g. water vs. hydrogen peroxide) without opening the container. However, several technical and computational challenges associated with this technique have yet to be addressed. There are several open questions: what is the sensitivity to different materials, what scan times are necessary, what are the sources of background, how do each of these scale as the detector system is made larger, and how can the system be integrated into existing scanning technology to close current detection gaps? In order to prove the applicability of this technology, we will develop a validated model to optimize the design and characterize the uncertainties in the measurement, and then test the system in a real-world scenario. This project seeks to perform R&D and laboratory tests that demonstrate proof of concept (TRL 3) to establishing an integrated system and evaluating its performance (TRL 4) through both laboratory tests and a validated detector model. The validated model will allow us to explore our technology’s benefits to explosive detection in various applications.

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This note summarizes an effort to characterize the effects of adding water-based liquid scintillator to the WATCHMAN detector. A detector model was built in the Geant4 Monte Carlo toolkit, and the position reconstruction of positrons within the detector was compared with and without scintillator. This study highlights the need for further modeling studies and small-scale experimental studies before inclusion into a large-scale detector, as the benefits compared to the associated costs are unclear.

Antineutrino monitoring of nuclear reactors has been demonstrated many times (Klimov et al., 1994 [1]; Bowden et al., 2009 [2]; Oguri et al., 2014 [3]), however the technique has not as of yet been developed into a useful capability for treaty verification purposes. The most notable drawback is the current requirement that detectors be deployed underground, with at least several meters-water-equivalent of shielding from cosmic radiation. In addition, the deployment of liquid-based detection media presents a challenge in reactor facilities. We are currently developing a detector system that has the potential to operate above ground and circumvent deployment problems associated with a liquid detection media: the system is composed of segments of plastic scintillator surrounded by 6LiF/ZnS:Ag. ZnS:Ag is a radio-luminescent phosphor used to detect the neutron capture products of 6Li. Because of its long decay time compared to standard plastic scintillators, pulse-shape discrimination can be used to distinguish positron and neutron interactions resulting from the inverse beta decay (IBD) of antineutrinos within the detector volume, reducing both accidental and correlated backgrounds. Segmentation further reduces backgrounds by identifying the positron's annihilation gammas, a signature that is absent for most correlated and uncorrelated backgrounds. This work explores different configurations in order to maximize the size of the detector segments without reducing the intrinsic neutron detection efficiency. We believe that this technology will ultimately be applicable to potential safeguards scenarios such as those recently described by Huber et al. (2014) [4,5].

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