Benjamin S. McDonald

Non-invasive material discrimination using spectral x-ray radiography

Current radiographic methods are limited in their ability to determine the presence of nuclear materials in containers or composite objects. A central problem is the inability to distinguish the attenuation pattern of high-density metals from those with a greater thickness of a less dense material. Here, we show that spectrally sensitive detectors can be used to discriminate plutonium from multiple layers of other materials using a single-view radiograph. An inverse algorithm with adaptive regularization is used. The algorithm can determine the presence of plutonium in simulated radiographs with a mass resolution per unit area of at least 0.07 g cm−2.

System Modeling and Design Optimization for a Next-Generation Unattended Sensor

We are developing a next-generation unattended sensor that can detect and identify radiation sources while operating on battery power for several weeks. The system achieves smaller size and weight over systems that use NaI:Tl and 3He detectors by using a relatively new scintillator, Cs2LiYCl6:Ce (CLYC). This material can detect both gamma rays and thermal neutrons, has best-case energy resolution under 4% full width at half maximum at 662 keV, and allows for particle discrimination by pulse amplitude as well as pulse shape. The overall design features an array of sixteen CLYC detectors, each read out by a photomultiplier tube and custom pulse processing electronics. A field-programmable gate array analyzes the energy spectra using computationally efficient algorithms for anomaly detection and basic isotope identification. In this paper, we report the results of a modeling study to optimize various parameters of the unattended sensor for best performance. Key parameters include the number and placement of detectors, dimensions and weight of the moderator, and location of the batteries. These results have guided the design of the proof-of-concept prototype.

Characterization of CLYC Detectors for a Next-Generation Unattended Sensor

We are developing a next-generation unattended sensor for detecting anomalous radiation sources. The system uses a scintillator material with dual sensitivity to gamma rays and neutrons, Cs2LiYCl6:Ce (CLYC), to reduce the size and complexity of the design. CLYC also offers a best-case energy resolution under 4% full width at half maximum at 662 keV, and allows for particle discrimination by pulse amplitude as well as pulse shape. The unattended sensor features sixteen one-inch CLYC detectors, each read out by a photomultiplier tube and custom readout electronics. A field-programmable gate array implements a suite of efficient processing algorithms for anomaly detection and isotope identification, and transmits alarm information to a base station via a wireless link. The system is designed to operate on battery power for several weeks. In this paper, we report the energy resolution, linearity, and temperature stability of the first CLYC detectors acquired for the project. Rather than characterizing the scintillator material under ideal conditions, we evaluate the detectors with the components selected for the unattended sensor, acknowledging the tradeoffs imposed by small size, limited power budget, and uncontrolled environmental conditions.

Transport Test Problems for Hybrid Methods Development

This report presents 9 test problems to guide testing and development of hybrid calculations for the ADVANTG code at ORNL. These test cases can be used for comparing different types of radiation transport calculations, as well as for guiding the development of variance reduction methods. Cases are drawn primarily from existing or previous calculations with a preference for cases which include experimental data, or otherwise have results with a high level of confidence, are non-sensitive, and represent problem sets of interest to NA-22.

Improvements in the method of radiation anomaly detection by spectral comparison ratios.

We present a new procedure for configuring the Nuisance-rejection Spectral Comparison Ratio Anomaly Detection (N-SCRAD) method. The procedure minimizes detectable count rates of source spectra at a specified false positive rate using simulated annealing. We also present a new method for correcting the estimates of background variability used in N-SCRAD to current conditions of the total count rate. The correction lowers detection thresholds for a specified false positive rate, enabling greater sensitivity to targets.

Determining HPGe Total Detection Efficiency Using γ–γ Coincidence

Both the peak and total detection efficiencies are generally needed in order to calculate sample activity from a gamma spectroscopic measurement, except in the case of isotope specific calibration. This is particularly true when the sample is in close proximity to the detector and corrections for cascade summing effects are required to avoid significant inaccuracy in the result. These corrections use the total detection efficiency to correct for summing-in and summing-out events, and the extent of the correction depends on both the geometry and the gamma cascade for the isotope in question. Experimentally determining the total efficiency is a labor intensive endeavor requiring multiple measurements with a set of single-gamma-emitting standards. Modeling the total efficiency vs. energy may be less time consuming, but is also likely to produce less confidence in the final result. Pacific Northwest National Laboratory’s Radiation Detection and Nuclear Sciences group has constructed a low background 14-crystal HPGe array for sample measurement; in all measurements, samples will be in close proximity to the germanium crystals. This close geometry and the sheer number of efficiency calibrations required for the system have led us to investigate methods to simplify the efficiency calibration procedure. One method we are developing uses the γ–γ coincidence plane to isolate Compton scattering event populations, allowing experimental determination of total detection efficiency values from the measurement of a single mixed isotope standard. A description of the analysis and experimental results of this method are presented.

Phase contrast x-ray imaging signatures for homeland security applications

Gratings-based phase contrast imaging is a promising new radiographic technique providing three distinct contrast mechanisms, absorption, phase, and scatter, using a conventional x-ray tube source. We investigate the signatures available in these three contrast mechanisms with particular attention towards potential homeland security applications. We find that the scatter mode in particular is sensitive to textured materials, enabling lowered detection limits than absorption for materials such as powders. We investigate the length scales to which our imaging system is sensitive.

Unattended Sensor System With CLYC Detectors

We have developed an unattended sensor for detecting anomalous radiation sources. The system combines several technologies to reduce size and weight, increase battery lifetime, and improve decision-making capabilities. Sixteen Cs2LiYCl6:Ce (CLYC) scintillators allow for gamma-ray spectroscopy and neutron detection in the same volume. Low-power electronics for readout, high voltage bias, and digital processing reduce the total operating power to 1.7 W. Computationally efficient analysis algorithms perform spectral anomaly detection and isotope identification. When an alarm occurs, the system transmits alarm information over a cellular modem. In this paper, we describe the overall design of the unattended sensor, present characterization results, and compare the performance to stock NaI:Tl and 3He detectors.

Phase Contrast X-Ray Imaging Signatures for Security Applications

Differential phase contrast imaging with a grating interferometer is a promising new radiographic technique providing three distinct contrast mechanisms-absorption, phase, and scatter (or dark field)-using a conventional X-ray tube source. We examine the signatures available in these three contrast mechanisms with attention towards potential security applications. We find that the scatter mode is uniquely sensitive to textured materials, potentially leading to enhanced material discrimination through the use of multiple contrast modes. We find that scatter signal in our imaging system increases as texture size is reduced from 800 μm to 7 μm. This range spans the transition from features that are resolved in the image to those residing below the system resolution, and corresponds to length scales of known texture or density variations in several common explosives.

Computational assessment of the impact of gamma-ray detector material properties on spectroscopic performance

Pacific Northwest National Laboratory (PNNL) is performing a computational assessment of the impact of several important gamma-ray detector material properties (e.g. energy resolution and intrinsic detection efficiency) on the scenario-specific spectroscopic performance of these materials. The research approach combines 3D radiation transport calculations, detector response modeling, and spectroscopic analysis of simulated energy deposition spectra to map the functional dependence of detection performance on the underlying material properties. This assessment is intended to help guide formulation of performance goals for new detector materials within the context of materials discovery programs, with an emphasis on applications in the threat reduction, nonproliferation, and safeguards/ verification user communities. The research results will also provide guidance to the gamma-ray sensor design community in estimating relative spectroscopic performance merits of candidate materials for novel or notional detectors.