We present the design of the gamma-ray and neutron spectrometer (GR/NS) for Dawn, which is a NASA Discoveryclass 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 C to 60 C) must be made to ensure that the spectrometer will function optimally over the nine-year mission.

Abstract-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.

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 affected by the conductivity of the surface.

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