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A GRaND New Instrument

Bill Feldman and Tom Prettyman
Los Alamos National Laboratory (LANL)

Figure 1. Cut-away view of the GRaND instrument showing gamma ray and neutron sensors along with structural and electronic components. Click image to enlarge.

The Los Alamos National Laboratory (LANL) is responsible for the Gamma Ray and Neutron Detector (GRaND) for the Dawn mission. The GRaND was designed to provide an accurate and robust measurement system that fits within the cost and resource constraints of the mission, with ample margins and sufficient heritage to maximize the science return during the nine-year mission to the asteroid belt. Features of the GRaND instrument include provisions to suppress and subtract spacecraft background and the ability to fully resolve gamma ray peaks for most elements. The latter is provided by new technology, CdZnTe, which will make its debut in planetary science on the Dawn mission.

Using a combination of gamma ray and neutron spectros-copy, the GRaND will measure the abundance of major elements, including O, Si, Ti, Al, Fe, Ca, and Mg. Knowl-edge of the composition of all major rock-forming elements is needed to provide context for meteoritic data and to constrain models of planetary structure and evolution. The gamma ray spectrometer will also determine the abundance of radioactive elements, including K, U, and Th. The ratio of the volatile element K to the refractory element U provides a measure of the depletion of volatile elements in the source material from which the asteroid was accreted. The neutron spectrometer along with the gamma ray spectrometer will measure the abundance of H and its stratigraphy. H can be in the form of water or hydrated minerals. Ceres, for example, in addition to having a wet, clay veneer at mid-latitudes, may have polar caps that consist of water ice.

A diagram of the GRaND is shown in Fig. 1. The instrument contains a 4x4 array of CdZnTe semiconductor radiation detectors which will serve as the primary gamma ray spec-trometer. The CdZnTe array is mounted on top of a BGO crystal that acts as an anticoincidence shield to suppress gamma rays that come from the spacecraft. The BGO sen-sor is also used to acquire gamma ray spectra and will serve to augment the detection efficiency of the CdZnTe array (in coincidence mode) and as a backup, stand-alone, spectrometer if needed. The CdZnTe array has sufficient pulse height resolution and counting efficiency to map all major elements and radioactive elements with improved accuracy compared to the BGO-based spectrometer flown on Lunar Prospector.

Figure 2. Distribution of the abundance of near surface hydrogen on Mars expressed as the percentage by mass of hydrogen if it were in the form of water. The map was determined using epithermal neutron counting data aquired by the 2001 Mars Odyssey neutron spectrometer. Click image to enlarge.

Surrounding the BGO crystal and CdZnTe array are four boron-loaded plastic scintillators (labeled BC454 in the figure). Note that structural materials in Fig. 1 hide the zenith facing scintillator. The nadir (asteroid) facing and zenith facing elements are primarily sensitive to fast and epithermal neutrons because they are shielded by ⁶Li-loaded materials that absorb thermal neutrons. The two side facing elements will respond to neutrons of all energies and are primarily intended for cosmic ray suppression. The plastic scintillators are arranged so that the flux of neutrons originating from the spacecraft can be suppressed, separately measured, and subtracted from the neutrons that originate in the asteroid. Light produced in the scintillating components is measured by five photomultiplier tubes (shown in Fig. 1), one for the BGO crystal and four for the BC454 scintillators. Thermal neutrons from the asteroids will be detected by the ⁶Li-loaded glass (GS20), which is optically coupled to the nadir facing plastic scintillator, forming a phoswhich. Signals from the plastic and glass will be separated and analyzed electronically using a time-domain filter.

Figure 3. Distribution of Th on the portion of the lunar surface visible from Earth. The scale gives Th mass-abundance in units of parts per million. The map was determined from gamma ray spectra acquired by the Lunar Prospector gamma ray spectrometer. Click image to enlarge.

The GRaND instrument on Dawn derives its heritage from the 2001 Mars Odyssey neutron spectrometer and the Lu-nar Prospector gamma ray and neutron spectrometers. Data acquired by the GRaND will be comparable in quality to these previous missions. The neutron spectrometer will map the near surface abundance of water-equivalent hydrogen on Ceres and Vesta (as illustrated in Fig. 2 for Mars). The gamma ray spectrometer, in some cases com-bined with the neutron spectrometer, will map the abun-dance of major- and radioactive elements (as illustrated in Fig. 3 for the Moon). The GRaND instrument on Dawn ex-ploits the rich heritage of past successful investigations to answer key questions concerning the divergent evolution of Vesta and Ceres. Its new technology CdZnTe detectors also pave the way for a new generation of more capable and compact instruments.