Publications

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FY2014 technical report of our project funded by DNN R&D that leverages advanced inference methods developed for medical and adaptive imaging to address arms control applications. We seek a method to acquire and analyze imaging data of declared treaty-accountable items without creating an image of those objects or otherwise storing or revealing any classified information. Such a method would avoid the use of classified-information barriers. We present our progress on FY2014 tasks defined in our life-cycle plan. We also describe some future work that is part of the continuation of this project in FY2015 and beyond as part of a venture that joins ours with a related PNNL project.

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.

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.

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 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.

Future arms control treaty verification regimes may use radiation imaging measurements to confirm and track nuclear warheads or other treaty accountable items (TAIs). This project leverages advanced inference methods developed for medical and adaptive imaging to improve task performance in arms control applications. Additionally, we seek a method to acquire and analyze imaging data of declared TAIs without creating an image of those objects or otherwise storing or revealing any classified information. Such a method would avoid the use of classified-information barriers (IB).

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An ideal ³He detector replacement for the near- to medium-term future will use materials that are easy to produce and well understood, while maintaining thermal neutron detection efficiency and gamma rejection close to the ³He standard. Toward this end, we investigated the use of standard alkali halide scintillators interfaced with ⁶Li and read out with photomultiplier tubes (PMTs). Thermal neutrons are captured on ⁶Li with high efficiency, emitting high-energy and triton (³H) reaction products. These particles deposit energy in the scintillator, providing a thermal neutron signal; discrimination against gamma interactions is possible via pulse shape discrimination (PSD), since heavy particles produce faster pulses in alkali halide crystals. We constructed and tested two classes of detectors based on this concept. In one case ⁶Li is used as a dopant in polycrystalline NaI; in the other case a thin Li foil is used as a conversion layer. In the configurations studied here, these systems are sensitive to both gamma and neutron radiation, with discrimination between the two and good energy resolution for gamma spectroscopy. We present results from our investigations, including measurements of the neutron efficiency and gamma rejection for the two detector types. We also show a comparison with Cs₂LiYCl₆:Ce (CLYC), which is emerging as the standard scintillator for simultaneous gamma and thermal neutron detection, and also allows PSD. We conclude that ⁶Li foil with CsI scintillating crystals has near-term promise as a thermal neutron detector in applications previously dominated by ³He detectors. The other approach, ⁶Li-doped alkali halides, has some potential, but require more work to understand material properties and improve fabrication processes.

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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