Thomas Humphrey Prettyman

Composition from fast neutrons: Application to the Moon

Planetary neutron leakage fluxes provide a measure of surface composition. However to be used in geological studies, a quantitative relationship between measured fluxes and surface composition is needed. The present work shows that neutron production is expected to be a function of the atomic mass, and that the fast leakage flux in the energy range between 0.6 and 8 MeV is linearly related to the average soil atomic mass. This result is consistent with laboratory measurements, and with Lunar Prospector observations of the Moon. When calibrated with returned lunar samples, this relationship is used to construct a map of the average atomic mass of lunar soils.

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.

Gamma-ray and neutron spectrometer for the Dawn mission to 1 Ceres and 4 Vesta

We present the design of the gamma-ray and neutron spectrometer (GR/NS) for Dawn, which is a NASA Discovery-class mission to explore two of the largest main-belt asteroids, 1 Ceres and 4 Vesta, whose accretion is believed to have been interrupted by the early formation of Jupiter. Dawn will determine the composition and structure of these protoplanetary bodies, providing context for a large number of primitive meteorites in our sample collection and a better understanding of processes occurring shortly after the onset of condensation of the solar nebula. The Dawn GR/NS design draws on experience from the successful Lunar Prospector and Mars Odyssey missions to enable accurate mapping of the surface composition and stratigraphy of major elements, radioactive elements, and hydrogen at both asteroids. Here, we describe the overall design of the GR/NS and compare the expected performance of the neutron spectrometer subsystem to the neutron spectrometer on Mars Odyssey. We also describe radiation damage studies carried out on CdZnTe detectors, which will be components of the primary gamma-ray spectrometer on Dawn. We conclude that provisions for annealing at moderate temperatures (40/spl deg/C to 60/spl deg/C) must be made to ensure that the spectrometer will function optimally over the nine-year mission.

CdZnTe gamma ray spectrometer for orbital planetary missions

We present the design and analysis of a new gamma ray spectrometer for planetary science that uses an array of CdZnTe detectors to achieve the detection efficiency needed for orbital measurements. The use of CdZnTe will provide significantly improved pulse height resolution relative to scintillation-based detectors, with commensurate improvement in the accuracy of elemental abundances determined by gamma ray and neutron spectroscopy. The spectrometer can be flown either on the instrument deck of the spacecraft or on a boom. For deck-mounted systems, a BGO anticoincidence shield is included in the design to suppress the response of the CdZnTe detector to gamma rays that originate in the spacecraft. The BGO shield also serves as a backup spectrometer, providing heritage from earlier planetary science missions and reducing the risk associated with the implementation of new technology.

EFFECT OF SURFACES ON THE PERFORMANCE OF CdZnTe DETECTORS

Surface processing plays a major role in manufacturing CdZnTe semiconductor devices used for radiation detection. We are conducting a thorough, systematic study of surfaces and contacts and their effect on charge transport and signal formation in CdZnTe devices. We are investigating wet chemical processing techniques as well as treatment of surfaces with energetic neutral atoms. Our goal is to develop and implement improved surface treatment methods and device manufacturing techniques for large-volume CdZnTe detectors. In addition, we will determine how surfaces and electrical contacts affect the performance of CdZnTe devices used for radiation detection. In this paper, we will show how surface electronic properties influence carrier transport and signal formation in devices designed to simulate coplanar grid detectors. By altering the surface using a wet chemical process, we will show that charge collection is significantly effected by the conductivity of the surface.

Design and characterization of cylindrical CdZnTe detectors with coplanar grids

This paper describes the development of cylindrical coplanar grid CdZnTe detectors for gamma ray spectroscopy. Cylindrical detector offer a number of advantages over established designs. For example, grid structures for cylindrical detectors are simpler than those for rectangular designs. The goal of our work is to design a cylindrical coplanar grid detector with excellent resolution at low- and high-energy. Information on detector design and manufacturing is presented. Six detectors are characterized. The pulse height resolution of the best detector is 13.5 keV full width at half maximum at 662 keV and 5.5 keV FWHM at 122 keV.

Characterization of a large-volume multi-element CdZnTe detector

In this paper, we present results of experiments to characterize a large-volume (4 cm3), multi-element CdZnTe detector for high-efficiency, gamma-ray spectroscopy. The module includes an array of eight 0.5 cm3 coplanar grid detectors manufactured by eV Products. An eight-channel data acquisition system with list mode output is used to record gamma-ray events for each detector in the array. The list mode data are analyzed to determine the efficiency for coincidence events and to demonstrate different modes of operation (e.g., Compton suppression). The total efficiency of the array is found to match Monte Carlo calculations to within a few percent; however, the full-energy (photopeak) efficiency is significantly lower than predicted by Monte Carlo. The observed difference is probably caused by a combination of electrode design and charge transport properties. Approaches to improve full-energy efficiency are proposed.

Mapping the elemental composition of Ceres and Vesta: Dawn’s gamma ray and neutron detector

Dawn is a NASA discovery mission that will explore the main belt asteroids (1) Ceres and (4) Vesta. Ceres and Vesta are among the oldest bodies in the solar system and represent very different evolutionary paths. By studying these ancient, complementary asteroids, we will answer fundamental questions about the early solar system and planetary formation processes. The Dawn payload consists of a Framing Camera (FC), a visual and infrared mapping spectrometer (VIR), and a Gamma Ray and Neutron Detector (GRaND). The instruments provide data needed to investigate the structure, geology, mineralogy, and geochemistry of the asteroids. GRaND provides the data for the geochemistry investigation, including maps of most major elements and selected radioactive and trace elements. An updated description of the GRaND instrument is given along with the expected performance of GRaND at Vesta and Ceres. Approaches to combine data from FC, VIR and GRaND are discussed.

Charge collection in multielectrode CdZnTe detectors

Preliminary results of experiments to investigate charge collection in CdZnTe detectors are presented. The experiments support the development of semiconductor- modeling tool for device engineering that will be used to design large volume CdZnTe detectors for gamma ray spectroscopy. Improved diagnostic methods are described, including an automated alpha particle scanner for charge pulse mapping. Semiconductor modeling techniques are presented along with methods to visualize charge transport. Experimental results are compared to a physical model that has been used routinely in research on room temperature devices for gamma ray detection.

CHAPTER 41 – Remote Chemical Sensing Using Nuclear Spectroscopy

Nuclear spectroscopy techniques are used to determine the elemental composition of planetary surfaces and atmospheres. Radiation, including gamma rays and neutrons, is produced steadily by cosmic ray bombardment of the surfaces and atmospheres of planetary bodies and by the decay of radionuclides within the solid surface. The leakage flux of gamma rays and neutrons contains information about the abundance of major elements, selected trace elements, and light elements such as H and C. Gamma rays and neutrons can be measured from high altitudes (less than a planetary radius), enabling global mapping of elemental composition by an orbiting spacecraft. Radiation that escapes into space originates from shallow depths (<1 m within the solid surface). Consequently, nuclear spectroscopy is complementary to other surface mapping techniques, such as reflectance spectroscopy, which is used to determine the mineralogy of planetary surfaces.