Comparison of Bayesian and classical reconstructions of tomographic gamma scanning for assay of nuclear materials

Tomographic gamma scanning has been used to assay special nuclear material for the past several years. Field experience suggests that the data analysis techniques can significantly affect the assay uncertainty. For example, a positive bias has been observed for low-activity samples. Recent attempts to reduce the bias without unacceptable increase in variance have taken a non-Bayesian approach. This paper will compare some of these non-Bayesian approaches to a Bayesian approach which is a modification of an approach used in photon emission computed tomography. The Bayesian approach is both more computationally demanding and more satisfying, though the choice of the prior probability for the distribution of nuclear material can impact the analysis. Assay results for scaled-down versions of the full-dimensioned problem will be presented for several methods and cases

Deep Mapping of Small Solar System Bodies with Galactic Cosmic Ray Secondary Particle Showers

We will investigate the use of galactic cosmic ray (GCR) secondary particles to probe the deep interiors of small solar system bodies (SSBs), including comets, asteroids, and geologic structures on the surfaces of airless bodies. Applications include solar system science, planetary defense, and resource utilization. Our Phase I study demonstrated that muons, the long-range charged component of GCR showers, can penetrate SSBs up to a km in diameter, providing information on their interior structure. Muons produced in Earths atmosphere have been applied to image the interior of large objects for science and engineering. In Phase I, we found that the production of muons in the solid surfaces of airless bodies is much smaller than in Earths atmosphere. Nevertheless, the flux of transmitted muons is sufficient to detect inclusions within an asteroid or comet in a reasonable amount of time, ranging from hours to weeks, depending on the size of the SSB and the density contrast, position and size of the inclusion. For asteroids and comets, large density variations (e.g., porous soil or ice versus solid rock) are relatively easy to detect. The intrinsic spatial resolution of muon radiography (muography) is on the scale of a few meters. The spatial resolution that can be achieved in practice depends on signal intensity and integration time (counting statistics), the angular resolution of the muon tracker (hodoscope) and details of data reduction and analysis methodology. Our Phase II project will assess remaining unknowns for the application of muography to determining the interior structure of SSBs, assess risks for implementation, and provide a roadmap for development of SSB muography beyond the NIAC program. To achieve our objectives, we will focus on four interrelated tasks: Task1) Signal and background characterization: Characterize the production and transmission of muons and secondary particle backgrounds made by cosmic ray showers in SSBs; and near-surface features from radiographic and tomographic data; Task2) Imaging studies: Develop methods to determine the density structure of SSB interiors and near-surface features from radiographic and tomographic data; Task3) Instrument design: Using simulations and bench-top laboratory experiments, investigate specific concepts for the design of compact hodoscopes and components; Task4) Synthesis: Combine the results of the first three tasks to determine the range of applicability of the method, identify the steps needed for maturation of the concept, and explore concepts for a pilot muography mission

Depth, distribution, and density of CO2 deposition on Mars

Observations by the Mars Orbiter Laser Altimeter have been used to detect subtle changes of the polar surface height during the course of seasonal cycles that correlate with the expected pattern of CO2 deposition and sublimation. Using altimetric crossover residuals from the Mars Orbiter Laser Altimeter, we show that while zonally averaged data capture the global behavior of CO2 exchange, there is a dependence of the pattern on longitude. At the highest latitudes the surface height change is as high as 1.5–2 m peak to peak, and it decreases equatorward. Decomposition of the signal into harmonics in time allows inspection of the spatial pattern and shows that the annual component is strongly correlated with the residual south polar cap deposits and, to a lesser extent, with the north polar cap. In the north, the second harmonic (semiannual) component correlates with the location of the ice deposits. The phases of the annual cycles are in agreement with observations by the Thermal Emission Spectrometer of the timing of the annual disappearance of CO2 frost from the surface at the high latitudes. At lower latitudes, frost sublimation (“Crocus date”) predates the mean depositional minima, as expected. These global-scale, volumetric measurements of the distribution of condensed CO2 can be combined with measurements of the deposited column mass density derived from the Neutron Spectrometer on board Mars Odyssey to yield an estimate of the density of the seasonally exchanging material of 0.5 ± 0.1 g/cm^3. These constraints should be considered in models of the Martian climate system and volatile cycles

TGS measurements of pyrochemical salts at Rocky Flats

A new skid-mounted tomographic gamma scanner (TGS) was designed to assist in the decommissioning of Rocky Flats Building 37 1, This instrument was used to assay pyrochemical salts as a prerequisite for disposal at the Waste Isolation Pilot Plant (WIPP). The following paper discusses measurement challenges and results from the first year of operation of the instrument

Chemical Mapping of Vesta and Ceres

Following successful science operations at Vesta, the Dawn spacecraft is headed for an encounter with Ceres in 2015. What have we learned at Vesta? And, what do we expect to learn by comparing Vesta and Ceres? We will address these questions from the standpoint of geochemistry. Dawn's Gamma Ray and Neutron Detector (GRaND) is sensitive to the elemental composition of surface materials to depths of a few decimeters [1]. Gamma rays and neutrons, produced by the steady bombardment of galactic cosmic rays and by the decay of naturally ]occurring radioisotopes (K, Th, U), provide a chemical fingerprint of the regolith. Analysis of planetary radiation emissions enables mapping of specific elements (such as Fe, Mg, Si, Cl, and H) and compositional parameters (such as average atomic mass), which provide information about processes that shaped the planet1s surface and interior. Dawn has exceeded operational goals for GRaND at Vesta, accumulating an abundance of nadir-pointed data during five months in a 210 km, low altitude mapping orbit around Vesta (265-km mean radius). Chemical information from gamma ray and neutron measurements was used to test the connection between Vesta and the howardite, eucrite, and diogenite (HED) meteorites [2]. Additionally, GRaND searched for evolved, igneous lithologies [3], mantle and upper crustal materials exposed in large impact basins, mesosiderite compositions, and hydrogen in Vesta1s bulk regolith. Results of our analyses and their implications for thermal evolution and regolith-processes will be presented. The possibility of a subcrustal ocean [4, 5] and lack of cerean meteorites makes water-rich Ceres a compelling target of exploration [6]. If Ceres underwent aqueous differentiation, then crustal overturn or gas driven volcanism may have significantly modified its primitive surface; and products of aqueous alteration (e.g. [7]) would detectable by GRaND [1]. For example, the presence of Cl in salts, associated with liquid-water-processes, would have a profound effect on the thermal neutron leakage flux. GRaND is sensitive to H and H-layering, which may be in the form of endogenic water ice or hydrous minerals on Ceres. Ammonia ice (e.g., from recent cryovolcanism) would produce a distinctly different neutron signature than water ice [1]. Prospective results for GRaND at Ceres will be presented in the context of what we have learned about Vesta

CdZnTe gamma ray spectrometer for orbital planetary missions

Knowledge of surface elemental composition is needed to understand the formation and evolution of planetary bodies. Gamma rays and neutrons produced by the interaction of galactic cosmic rays with surface materials can be detected from orbit and analyzed to determine composition. Using gamma ray spectroscopy, major rock forming elements such as Fe, Ti, Al, Si, Mg, and Ca can be detected. The accuracy of elemental abundance is limited by the resolution of the spectrometer. For space missions, scintillators such as BGO and NaI(Tl) have been used for gamma ray spectroscopy. New planetary science missions are being planned to explore Mars, Mercury, the asteroid belt, and the outer planets. Significant improvements in the pulse height resolution relative to scintillation detectors can be made using CdZnTe, a new room temperature detector technology. For an orbiting instrument, a CdZnTe detector at least 16 cm{sup 3} in size is needed. A 4 x 4 array of 1-cm{sup 3} coplanar grid detectors can be manufactured that meets requirements for resolution and counting efficiency. The array will shielded from gamma rays produced in the spacecraft by a BGO detector. By improving pulse height resolution by a factor of three at low energy, the CdZnTe detector will be able to make accurate measurements of elements that are currently difficult to measure using scintillation technology. The BGO shield will provide adequate suppression of gamma rays originating in the spacecraft, enabling the gamma ray spectrometer to be mounted on the deck of a spacecraft. To test this concept, we are constructing a flight qualified, prototype CdZnTe detector array. The prototype consists of a 2 x 2 array of coplanar grid detectors. We will present the results of mechanical and electronic testing and radiation damage tests, and the performance of the array for gamma ray spectroscopy

The Geology of the Marcia Quadrangle of Asteroid Vesta: Assessing the Effects of Large, Young Craters

We used Dawn spacecraft data to identify and delineate geological units and landforms in the Marcia quadrangle of Vesta as a means to assess the role of the large, relatively young impact craters Marcia (approximately 63 kilometers diameter) and Calpurnia (approximately 53 kilometers diameter) and their surrounding ejecta field on the local geology. We also investigated a local topographic high with a dark-rayed crater named Aricia Tholus, and the impact crater Octavia that is surrounded by a distinctive diffuse mantle. Crater counts and stratigraphic relations suggest that Marcia is the youngest large crater on Vesta, in which a putative impact melt on the crater floor ranges in age between approximately 40 and 60 million years (depending upon choice of chronology system), and Marcia's ejecta blanket ranges in age between approximately 120 and 390 million years (depending upon choice of chronology system). We interpret the geologic units in and around Marcia crater to mark a major Vestan time-stratigraphic event, and that the Marcia Formation is one of the geologically youngest formations on Vesta. Marcia crater reveals pristine bright and dark material in its walls and smooth and pitted terrains on its floor. The smooth unit we interpret as evidence of flow of impact melts and (for the pitted terrain) release of volatiles during or after the impact process. The distinctive dark ejecta surrounding craters Marcia and Calpurnia is enriched in OH- or H-bearing phases and has a variable morphology, suggestive of a complex mixture of impact ejecta and impact melts including dark materials possibly derived from carbonaceous chondrite-rich material. Aricia Tholus, which was originally interpreted as a putative Vestan volcanic edifice based on lower resolution observations, appears to be a fragment of an ancient impact basin rim topped by a dark-rayed impact crater. Octavia crater has a cratering model formation age of approximately 280-990 million years based on counts of its ejecta field (depending upon choice of chronology system), and its ejecta field is the second oldest unit in this quadrangle. The relatively young craters and their related ejecta materials in this quadrangle are in stark contrast to the surrounding heavily cratered units that are related to the billion years old or older Rheasilvia and Veneneia impact basins and Vesta's ancient crust preserved on Vestalia Terra

Chemical Mapping of Vesta

Vesta s surface mineralogy and composition have been studied for decades via telescopic spectroscopy and laboratory analyses of the howardite, eucrite, and diogenite (HED) meteorites, which are thought to originate from Vesta. Visible and infrared reflectance measurements by Dawn have broadly confirmed the paradigm established by Earth-based work, strengthening the Vesta-HED connection. The Dawn mission has achieved a milestone by completing the first chemical measurements of a main-belt asteroid using nuclear spectroscopy. Dawn s Gamma Ray and Neutron Detector (GRaND) has globally mapped the composition of Vesta, including the portions of the northern hemisphere not illuminated by solar radiation. GRaND is sensitive to the composition of the bulk regolith to depths of several decimeters. Abundances and/or detection limits for specific elements and elemental ratios, such as H, Fe, Si, Fe/O, Fe/Si, and K, have been measured. Variations in the average atomic mass and neutron macroscopic absorption cross section have been characterized. The measurements constrain the relative proportions of HED whole-rock end-members, providing measurements of the pyroxene and plagioclase content of the regolith, thereby constraining the processes underlying Vesta s differentiation and crustal evolution. The spatial resolution of GRaND is sufficient to determine basin-average compositions of Veneneia and Rheasilvia, which may contain outcrops of Vesta s olivine-rich mantle. While the elemental composition of Vesta s regolith is similar to the meteorites, there are notable departures from HED whole-rock compositions. While these differences are not sufficient to topple the Vesta-HED paradigm, they provide insight into global-scale processes that have shaped Vesta s surface. Questions addressed by the analysis of GRaND data include: (i) Is Vesta the source of the Fe-rich mesosiderites? (ii) Are evolved, igneous lithologies present on Vesta s surface? (iii) What are the origins of exogenic materials found in Vesta s regolith? (iv) Is the vestan mantle exposed within the southern basins

The Main Belt Comets and ice in the Solar System

We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies