Publications

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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.

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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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