This PDF file contains the front matter associated with SPIE Proceedings Volume 8854 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

Some applications, particularly in homeland security, require detection of both neutron and gamma radiation. Typically, this is accomplished with a combination of two detectors registering neutrons and gammas separately. We have investigated a new type of neutron/gamma (n/γ) directional detection capability. We explored a new class of scintillator, cerium (Ce)-doped Elpasolites such as Cs₂LiYCl₆:Ce (CLYC), Cs₂LiLaCl₆ (CLLC), Cs₂LiLaBr_6:Ce (CLLB), or Cs₂LiYBr₆:Ce (CLYB). These materials are capable of providing energy resolution as good as 2.9% at 662 keV (FWHM), which is better than that of NaI:Tl. Because they contain ⁶Li, Elpasolites can also detect thermal neutrons. In the energy spectra, the full energy thermal neutron peak appears near or above 3 GEE_n MeV. Thus, very effective pulse height discrimination is possible. In addition, the core-to-valence luminescence (CVL) provides Elpasolites with different temporal responses under gamma and neutron excitation, and, therefore, may be exploited for effective pulse shape discrimination. For instance, the CLLC emission consists of two main components: (1) CVL spanning from 220 nm to 320 nm and (2) Ce emission found in the range of 350 to 500 nm. The former emission is of particular interest because it appears only under gamma excitation. It is also very fast, decaying with a 2 ns time constant. The n/γ discrimination capability of Elpasolite detectors may be optimized by tuning the cerium doping content for maximum effect on n/γ pulse shape differences. The resulting Elpasolite detectors have the ability to collect neutron and gamma data simultaneously, with excellent discrimination. Further, an array of four of these Elpasolites detectors will perform directional detection in both the neutron and gamma channels simultaneously.

The Fano factor for an integer-valued random variable is defined as the ratio of its variance to its mean. Light from various scintillation crystals has been reported to have Fano factors from sub-Poisson (Fano factor < 1), Poisson (Fano factor = 1) to super-Poisson (Fano factor > 1). For a given mean, a smaller Fano factor implies a smaller variance and thus less noise. We investigate if lower noise in the scintillation light results in better spatial and energy resolution in a scintillation imaging detector. The impact of Fano factor on estimation of position of interaction and energy deposited in simple gamma-camera geometries is estimated by calculating the Cramer-Rao bound. The calculated Cramer-Rao bound is quantitatively validated by estimating the variance of the maximum likelihood estimator.

Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr₃) and cerium-activated lanthanum tribromide (LaBr₃:Ce), their commercial availability and application is limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. The objective of this investigation was to employ aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca²⁺ ) was investigated as a dopant to strengthen CeBr₃ without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca²⁺ dopant were grown. Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca²⁺-doped CeBr₃ exhibited little or no change in the peak fluorescence emission for 371 nm optical excitation for CeBr₃. The structural, electronic, and optical properties of CeBr₃ crystals were investigated using the density functional theory within generalized gradient approximation. The calculated lattice parameters are in good agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr₃, including the dielectric function, were calculated.

Neutron counting using large arrays of pressurized ³He proportional counters from an aerial system or in a maritime environment suffers from the background counts from the primary cosmic neutrons and secondary neutrons caused by cosmic ray‒induced mechanisms like spallation and charge-exchange reaction. This paper reports the work performed at the Remote Sensing Laboratory–Andrews (RSL-A) and results obtained when using two different methods to reduce the cosmic neutron background in real time. Both methods used shielding materials with a high concentration (up to 30% by weight) of neutron-absorbing materials, such as natural boron, to remove the lowenergy neutron flux from the cosmic background as the first step of the background reduction process. Our first method was to design, prototype, and test an up-looking plastic scintillator (BC-400, manufactured by Saint Gobain Corporation) to tag the cosmic neutrons and then create a logic pulse of a fixed time duration (~120 μs) to block the data taken by the neutron counter (pressurized 3He tubes running in a proportional counter mode). The second method examined the time correlation between the arrival of two successive neutron signals to the counting array and calculated the excess of variance (Feynman variance Y2F)¹ in the neutron count distribution from Poisson distribution. The dilution of this variance from cosmic background values ideally would signal the presence of manmade neutrons.² The first method has been technically successful in tagging the neutrons in the cosmic-ray flux and preventing them from being counted in the ³He tube array by electronic veto—field measurement work shows the efficiency of the electronic veto counter to be about 87%. The second method has successfully derived an empirical relationship between the percentile non-cosmic component in a neutron flux and the Y2F of the measured neutron count distribution. By using shielding materials alone, approximately 55% of the neutron flux from man-made sources like ²⁵²Cf or Am-Be was removed.

Although there has been progress in applying GPU-technology to Computed-Tomography reconstruction algorithms, much of the work has concentrated on optimizing reconstruction performance for smaller, medical-scale datasets. Industrial CT datasets can vary widely in size and number of projections. With the new advancements in high resolution cameras, it is entirely possible that the Industrial CT community may soon need to pursue a 100-megapixel detector for CT applications. To reconstruct such a massive dataset, simply adding extra GPUs would not be an option as memory and storage bottlenecks would result in prolonged periods of GPU downtime, thus negating performance gains. Additionally, current reconstruction algorithms would not be sufficient due to the various bottlenecks in the processor hardware. Past work has shown that CT reconstruction is an irregular problem for large-scale datasets on a GPU due to the massively parallel environment. This work proposes a high-performance, multi-GPU, modularized approach to reconstruction where computation, memory transfers, and disk I/O are optimized to occur in parallel while accommodating the irregular nature of the computation kernel. Our approach utilizes a dynamic MIMD-type of architecture in a hybrid environment of CUDA and OpenMP. The modularized approach showed an improvement in load-balancing and performance such that a 1 trillion voxel volume was reconstructed from 10,000 100 megapixel projections in less than a day.

This work will present the utilization of the massively multi-threaded environment of graphics processors (GPUs) to improve the computation time needed to reconstruct large computed tomography (CT) datasets and the aris- ing challenges for system implementation. Intelligent algorithm design for massively multi-threaded graphics processors differs greatly from traditional CPU algorithm design. Although a brute force port of a CPU algo- rithm to a GPU kernel may yield non-trivial performance gains, further measurable gains could be achieved by designing the algorithm with consideration given to the computing architecture. Previous work has shown that CT reconstruction on GPUs becomes an irregular problem for large datasets (10GB-4TB),¹ thus memory band- width at the host and device levels becomes a significant bottleneck for industrial CT applications. We present a set of GPU reconstruction kernels that utilize various GPU-specific optimizations and measure performance impact.

Successful images of hard x-rays were taken at the OMEGA Laser at the Laboratory for Laser energetics ant he University of Rochester. This facility served as a surrogate for the National Ignition Facility for which this system was designed. Eleven plastic shells filled with ³He pellets were imploded producing soft and hard x-rays. As the system was designed to image 4.44MeV gammas the hard x-rays were of particular interest. These bremsstrahlung x-rays were emitted for the outer plastic shell and imaged using the gamma ray imaging system 13 meters away. A number of filtering arrangements were used to do transmission radiography of the source providing spectrum information. A 200-micron pinhole aperture was used to image the source. These shots provide information critical in characterizing the performance of the system

One of the scientific goals of the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory, Livermore CA, is to obtain thermonuclear ignition by compressing 2.2 mm diameter capsules filed with deuterium and tritium to densities approaching 1000 g/cm³ and temperatures in excess of 4 keV. Thefusion reaction d + t → n + a results in a 14.03 MeV neutron providing a source of diagnostic particles to characterize the implosion. The spectrum of neutrons emanating from the assembly may be used to infer the fusion yield, plasma ion temperature, and fuel areal density, all key diagnostic quantities of implosion quality. The neutron time-of-flight (nToF) system co-located along the Neutron Imaging System line-of-site, (NIToF), is a set of 4 scintillation detectors located approximately 27.3 m from the implosion source. Neutron spectral information is inferred using arrival time at the detector. The NIToF system is described below, including the hardware elements, calibration data, analysis methods, and an example of its basic performance characteristics.

Los Alamos has used penetrating radiography extensively throughout its history dating back to the Manhattan Project where imaging dense, imploding objects was the subject of intense interest. This interest continues today as major facilities like DARHT¹ have become the mainstay of the US Stockpile Stewardship Program² and the cornerstone of nuclear weapons certification. Meanwhile, emerging threats to national security from cargo containers and improvised explosive devices (IEDs) have invigorated inspection efforts using muon tomography, and compact x-ray radiography. Additionally, unusual environmental threats, like those from underwater oil spills and nuclear power plant accidents, have caused renewed interest in fielding radiography in severe operating conditions. We review the history of penetrating radiography at Los Alamos and survey technologies as presently applied to these important problems.

Image intensifiers combined with columnar scintillators have found application in x-ray and gamma-ray, biomedical imaging and other fields. In scintillator imaging, hundreds or thousands of optical photons can illuminate the faceplate of the image intensifier in a small area, essentially simultaneously. This is a situation not found in the typical design application for an image intensifier, night vision or low-light-level imaging. Microchannel plates (MCPs) are known to exhibit gain saturation that could result in non-linear signal response in scintillator imaging, limiting quantitative measurement capabilities. A calibrated LED photon source was developed that can provide a known average number of photons per unit area in a small spot size, similar to that seen due to a gamma-ray interaction in a BazookaSPECT imager. A BazookaSPECT imager is composed of a columnar scintillator and an image intensifier, with output light optically imaged onto a CCD camera. The calibrated source was used to investigate gain-saturation effects for two Proxivision, GmbH image intensifiers, a single-stage BV 2583 EZ and a two stage BV 2583 QZ-V 100N in a BazookaSPECT imaging configuration. No gain saturation was found for the single-stage image intensifier up to more than 100 optical photons per microchannel, but significant gain-saturation non-linearities were measured in the two-stage image intensifier at high gains for >12 optical photons per microchannel. Implications for scintillator imaging using such systems are discussed.