Publications

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In this work we investigate a method that confirms the operability of neutron detectors requiring neither radiological sources nor radiation-generating devices. This is desirable when radiological sources are not available, but confidence in the functionality of the instrument is required. The “source”, based on the production of neutrons in high-Z materials by muons, provides a tagged, low-background and consistent rate of neutrons that can be used to check the functionality of or calibrate a detector. Using a Monte Carlo guided optimization, an experimental apparatus was designed and built to evaluate the feasibility of this technique. Through a series of trial measurements in a variety of locations we show that gated muon-induced neutrons appear to provide a consistent source of neutrons (35.9 ± 2.3 measured neutrons/10,000 muons in the instrument) under normal environmental variability (less than one statistical standard deviation for 10,000 muons) with a combined environmental + statistical uncertainty of ~18% for 10,000 muons. This is achieved in a single 21-22 minute measurement at sea level.

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.

The time-correlated pulse-height (TCPH) distribution can be used to differentiate between multiplying (e.g 235U, 239Pu) and non-multiplying (e.g Am-Li, 252Cf) sources. In the past, this approach proved effective at characterizing the multiplication of alpha phase plutonium metal through a passive measurement. Recently, Sandia National Laboratories has completed a measurement campaign with its new Correlated Radiation Signature (CoRS) system involving active interrogation of highly enriched uranium (HEU) with an Am-Li source. An additional obstacle was introduced to the measurement configuration by shielding the HEU with depleted uranium (DU). Simulation results have proven Am-Li source to be a suitable interrogating source because of its relatively low-energy neutron spectrum. The TCPH distribution was successfully used to determine the presence of a multiplying medium inside DU shells. The correlation between multiplication and an empirical parameters broke down for externally driven configurations, but in all cases the presence of a multiplying source was detected.

Our previous conference report on this instrument emphasized its use for fast-neutron imaging spectroscopy. We describe here its additional measurement capabilities, namely active interrogation, time-correlated pulse-height multiplication measurements, and gamma imaging.

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A transportable fast neutron detection system has been designed and constructed for measuring neutron energy spectra and flux ranging from tens to hundreds of MeV. The transportability of the spectrometer reduces the detector-related systematic bias between different neutron spectra and flux measurements, which allows for the comparison of measurements above or below ground. The spectrometer will measure neutron fluxes that are of prohibitively low intensity compared to the site-specific background rates targeted by other transportable fast neutron detection systems. To measure low intensity high-energy neutron fluxes, a conventional capture-gating technique is used for measuring neutron energies above 20 MeV and a novel multiplicity technique is used for measuring neutron energies above 100 MeV. The spectrometer is composed of two Gd containing plastic scintillator detectors arranged around a lead spallation target. To calibrate and characterize the position dependent response of the spectrometer, a Monte Carlo model was developed and used in conjunction with experimental data from gamma ray sources. Multiplicity event identification algorithms were developed and used with a Cf-252 neutron multiplicity source to validate the Monte Carlo model Gd concentration and secondary neutron capture efficiency. The validated Monte Carlo model was used to predict an effective area for the multiplicity and capture gating analyses. For incident neutron energies between 100 MeV and 1000 MeV with an isotropic angular distribution, the multiplicity analysis predicted an effective area of 500 cm2 rising to 5000 cm2. For neutron energies above 20 MeV, the capture-gating analysis predicted an effective area between 1800 cm2 and 2500 cm2. The multiplicity mode was found to be sensitive to the incident neutron angular distribution.

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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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The intent of the proposed work, in collaboration with University of Michigan, is to develop the algorithms that will bring the analysis from qualitative images to quantitative attributes of objects containing SNM. The first step to achieving this is to develop an indepth understanding of the intrinsic errors associated with the deconvolution and MLEM algorithms. A significant new effort will be undertaken to relate the image data to a posited three-dimensional model of geometric primitives that can be adjusted to get the best fit. In this way, parameters of the model such as sizes, shapes, and masses can be extracted for both radioactive and non-radioactive materials. This model-based algorithm will need the integrated response of a hypothesized configuration of material to be calculated many times. As such, both the MLEM and the model-based algorithm require significant increases in calculation speed in order to converge to solutions in practical amounts of time.

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A Bayesian approach is proposed for pulse shape discrimination of photons and neutrons in liquid organic scinitillators. Instead of drawing a decision boundary, each pulse is assigned a photon or neutron confidence probability. In addition, this allows for photon and neutron classification on an event-by-event basis. The sum of those confidence probabilities is used to estimate the number of photon and neutron instances in the data. An iterative scheme, similar to an expectation-maximization algorithm for Gaussian mixtures, is used to infer the ratio of photons-to-neutrons in each measurement. Therefore, the probability space adapts to data with varying photon-to-neutron ratios. A time-correlated measurement of Am–Be and separate measurements of ¹³⁷Cs, ⁶⁰Co and ²³²Th photon sources were used to construct libraries of neutrons and photons. These libraries were then used to produce synthetic data sets with varying ratios of photons-to-neutrons. Probability weighted method that we implemented was found to maintain neutron acceptance rate of up to 90% up to photon-to-neutron ratio of 2000, and performed 9% better than the decision boundary approach. Furthermore, the iterative approach appropriately changed the probability space with an increasing number of photons which kept the neutron population estimate from unrealistically increasing.

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This report summarizes the discussion and conclusions reached during a table top exercise held at Sandia National Laboratories, Albuquerque on September 3, 2014 regarding a recently described approach for nuclear warhead verification based on the cryptographic concept of a zero-knowledge protocol (ZKP) presented in a recent paper authored by Glaser, Barak, and Goldston. A panel of Sandia National Laboratories researchers, whose expertise includes radiation instrumentation design and development, cryptography, and arms control verification implementation, jointly reviewed the paper and identified specific challenges to implementing the approach as well as some opportunities. It was noted that ZKP as used in cryptography is a useful model for the arms control verification problem, but the direct analogy to arms control breaks down quickly. The ZKP methodology for warhead verification fits within the general class of template-based verification techniques, where a reference measurement is used to confirm that a given object is like another object that has already been accepted as a warhead by some other means. This can be a powerful verification approach, but requires independent means to trust the authenticity of the reference warhead - a standard that may be difficult to achieve, which the ZKP authors do not directly address. Despite some technical challenges, the concept of last-minute selection of the pre-loads and equipment could be a valuable component of a verification regime.

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In coded aperture imaging, one of the most important factors determining the quality of reconstructed images is the choice of mask/aperture pattern. In many applications, uniformly redundant arrays (URAs) are widely accepted as the optimal mask pattern. Under ideal conditions, thin and highly opaque masks, URA patterns are mathematically constructed to provide artifact-free reconstruction however, the number of URAs for a chosen number of mask elements is limited and when highly penetrating particles such as fast neutrons and high-energy gamma-rays are being imaged, the optimum is seldom achieved. In this case more robust mask patterns that provide better reconstructed image quality may exist. Through the use of heuristic optimization methods and maximum likelihood expectation maximization (MLEM) image reconstruction, we show that for both point and extended neutron sources a random mask pattern can be optimized to provide better image quality than that of a URA.

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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.

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This report provides a short overview of the DNN R&D funded project, Time-Encoded Imagers. The project began in FY11 and concluded in FY14. The Project Description below provides the overall motivation and objectives for the project as well as a summary of programmatic direction. It is followed by a short description of each task and the resulting deliverables.

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A wide range of NSC (Neutron Scatter Camera) activities were conducted under this lifecycle plan. This document outlines the highlights of those activities, broadly characterized as system improvements, laboratory measurements, and deployments, and presents sample results in these areas. Additional information can be found in the documents that reside in WebPMIS.

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A series of field experiments were undertaken to evaluate the performance of the one dimensional time encoded imaging system. The significant detection of a Cf252 fission radiation source was demonstrated at a stand-off of 100 meters. Extrapolations to different quantities of plutonium equivalent at different distances are made. Hardware modifications to the system for follow on work are suggested.

We have developed two neutron detector systems based on time-encoded imaging and demonstrated their applicability toward non-proliferation missions. The 1D-TEI system was designed for and evaluated against the ability to detect Special Nuclear Material (SNM) in very low signal to noise environments; in particular, very large stand-off and/or weak sources that may be shielded. We have demonstrated significant detection (>5 sigma) of a 2.8e5 n/s neutron fission source at 100 meters stand-off in 30 min. If scaled to an IAEA significant quantity of Pu, we estimate that this could be reduced to as few as ~5 minutes. In contrast to simple counting detectors, this was accomplished without the need of previous background measurements. The 2D-TEI system was designed for high resolution spatial mapping of distributions of SNM and proved feasibility of twodimensional fast neutron imaging using the time encoded modulation of rates on a single pixel detector. Because of the simplicity of the TEI design, there is much lower systematic uncertainty in the detector response typical coded apertures. Other imaging methods require either multiple interactions (e.g. neutron scatter camera or Compton imagers), leading to intrinsically low efficiencies, or spatial modulation of the signal (e.g., Neutron Coded Aperture Imager (Hausladen, 2012)), which requires a complicated, high channel count, and expensive position sensitive detector. In contrast, a single detector using a time-modulated collimator can encode directional information in the time distribution of detected events. This is the first investigation of time-encoded imaging for nuclear nonproliferation applications.

The overall goal of the WATCHMAN project is to experimentally demonstrate the potential of water Cerenkov antineutrino detectors as a tool for remote monitoring of nuclear reactors. In particular, the project seeks to field a large prototype gadolinium-doped, water-based antineutrino detector to demonstrate sensitivity to a power reactor at ~10 kilometer standoff using a kiloton scale detector. The technology under development, when fully realized at large scale, could provide remote near-real-time information about reactor existence and operational status for small operating nuclear reactors.

A series of laboratory experiments were undertaken to demonstrate the feasibility of two dimensional time-encoded imaging. A prototype two-dimensional time encoded imaging system was designed and constructed. Results from imaging measurements of single and multiple point sources as well as extended source distributions are presented. Time encoded imaging has proven to be a simple method for achieving high resolution two-dimensional imaging with potential to be used in future arms control and treaty verification applications.

A spallation based multiplicity detector has been constructed and deployed to the Kimballton Underground Research Facility to measure the cosmogenic fast neutron flux anti-coincident from the initiating muon. Two of the three planned measurements have been completed (~380 and ~600 m.w.e) with sufficient statistics. The third measurement at level 14 (~4450 m.w.e.) is currently being performed. Current results at ~600 m.w.e. compare favourably to the one previous measurement at 550 m.w.e. For neutron energies between 100 and 200 MeV measurements at ~380 m.w.e. produce fluxes between 1e^-8 and 7e^-9 n/cm²/s/MeV and at ~600 m.w.e. measurements produce fluxes between 7e^-9 and 1e^-11 n/cm²/s/MeV.

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The time-correlated pulse-height technique can distinguish multiplying (special nuclear material) from non-multiplying sources. The technique relies upon the measurement of correlated photon-neutron pairs using organic liquid scintillation detectors. For such interactions, the distribution of measured neutron recoil energy versus the time-of-flight difference between correlated photons and neutrons are imprinted with the fission chain dynamics of the source. The theoretical time-of-arrival assuming the photons and neutrons are created in the same fission is calculated. Correlated pairs with longer time-of-arrival indicate delays caused by self-induced fission chains in a multiplying source. For the specific circumstances of simulated measurements of 25.4 kg of highly enriched uranium at 50 cm source to detector distance, correlated pairs from fission chains can arrive upwards of 40 ns later than correlated pairs with the same neutron energies from non-multiplying sources like 252Cf at the same source detector distance. The use of detectors with ns scale time resolution and the use of pulse digitization allows for the distinction of these events. This method has been used successfully in the past to measure a variety of plutonium-bearing samples. The particle transport code MCNPX-PoliMi has been used to simulate and validate these measurements as well. Due to the much lower signature emission rate of 235U, this technique has not yet been used to measure the presence of highly enriched uranium. In this work we therefore explore the use of the time-correlated pulse-height technique with the introduction of an interrogating neutron source to stimulate fission. The applicability of 252Cf, AmLi and a DD generator neutron sources is explored in a series of simulations. All three sources are viable options with their own pros and cons with the choice of appropriate source depending upon the intended application. The TCPH technique is envisioned as a viable measurement solution of special nuclear material in situations in which the presence of shielding material disqualifies the use of passive gamma spectroscopy or gamma spectroscopy reveals classified information on the special nuclear material's isotopic composition. © 2014 Elsevier Ltd. All rights reserved.

Time-encoded imaging (TEI) is a new approach to directional detection of energetic radiation that produces images by inducing a time-dependent modulation of detected particles. TEI-based detectors use single-scatter events and have a low channel count, reducing complexity and cost while maintaining high efficiency with respect to other radiation imaging techniques such as double-scatter or coded aperture imaging. The scalability of TEI systems makes them a very promising detector class for weak source detection. Extension of the technique to high-resolution imaging is also under study. With a prototype time-encoding detector, we demonstrated detection of a neutron source at 60 m with neutron output equivalent to an IAEA significant quantity of WGPu. We have since designed and built a full-scale detector based on the time-encoding concept. We will present results from characterization of very large liquid scintillator cells, including pulse shape discrimination, as well as from studies of the detector system performance in weak source detection scenarios. © 2013 SPIE.

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Time-encoded imaging is an approach to directional radiation detection that is being developed at SNL with a focus on fast neutron directional detection. In this technique, a time modulation of a detected neutron signal is inducedtypically, a moving mask that attenuates neutrons with a time structure that depends on the source position. An important challenge in time-encoded imaging is to develop high-resolution two-dimensional imaging capabilities; building a mechanically moving high-resolution mask presents challenges both theoretical and technical. We have investigated an alternative to mechanical masks that replaces the solid mask with a liquid such as mineral oil. Instead of fixed blocks of solid material that move in pre-defined patterns, the oil is contained in tubing structures, and carefully introduced air gapsbubblespropagate through the tubing, generating moving patterns of oil mask elements and air apertures. Compared to current moving-mask techniques, the bubble mask is simple, since mechanical motion is replaced by gravity-driven bubble propagation; it is flexible, since arbitrary bubble patterns can be generated by a software-controlled valve actuator; and it is potentially high performance, since the tubing and bubble size can be tuned for high-resolution imaging requirements. We have built and tested various single-tube mask elements, and will present results on bubble introduction and propagation as a function of tubing size and cross-sectional shape; real-time bubble position tracking; neutron source imaging tests; and reconstruction techniques demonstrated on simple test data as well as a simulated full detector system.

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