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Calibration of Dawn’s Gamma Ray and Neutron Detector (GraND) Instrument

Thomas H. Prettyman
GRaND Team Lead and Dawn Co-I, Los Alamos National Laboratory

The gamma ray and neutron detector (GRaND) is designed to measure the chemical composition of the surfaces of Vesta and Ceres. GRaND will map the near-surface abundance of major rock forming elements, long-lived radioactive elements, and volatiles such as H, C, N and O which are the major constituents of ices. A cutaway view of the instrument is shown in Fig. 1. GRaND uses heritage technology from Lunar Prospector and 2001 Mars Odyssey, including a bismuth germanate (BGO) scintillator for high efficiency gamma ray spectroscopy and boron loaded plastic scintillators for fast and epithermal neutron detection. GRaND also includes new sensor technologies to improve the accuracy of elemental abundance measurements. These include a 16-element, CdZnTe (CZT) semiconductor detector array for high resolution gamma ray spectroscopy, and boron-loaded plastic/Li-loaded-glass phoswiches (“phosphor sandwiches”) to separately measure thermal, epithermal, and fast neutrons originating from the asteroids.

GRaND has undergone extensive calibration and characterization at LANL facilities and following integration with the spacecraft. Examples of data products for gamma ray and neutron spectroscopy are shown in Fig 1. The pulse height spectrum shown in Fig. 1a for a phoswich sensor, was acquired using a laboratory neutron source with an energy distribution similar to that of a planetary leakage spectrum. Thermal and epithermal neutrons interacting in the Li-loaded glass produce a distinct peak associated with the recoil energy of the reaction products for the 6Li(n,t)4He reaction. Epithermal neutrons that interact with the B-loaded plastic produce a separate peak at 93 keVeq. The two peaks are well separated in energy and can be used together to measure the thermal and epithermal components of the neutron spectrum.

The BGO sensor has high efficiency for gamma ray detection and can measure gamma rays over a wide energy range (see Fig. 1c for a spectrum acquired for neutrons incident on an Fe slab). The CZT array has a more restrictive energy range (0- to 3-MeV), but has somewhat higher energy resolution than the BGO sensor (better than 3% at 662 keV). An ore sample spectrum, showing prominent gamma rays from the decay of 214Bi (from the 238U decay chain) is shown, for example, in Fig. 1b. The CZT array enables improved accuracy for the analysis of the low energy region of the spectrum, which is densely populated by gamma rays from radioactive decay and nuclear reactions.

Dawn’s operational plan provides ample integration time and coverage at each asteroid, sufficient to globally map surface elemental composition. The geochemical data provided by GRaND will, for example, provide strong constraints on thermal evolution, including the role of water and other volatiles in planetary development, context for the HED meteorites, and the degree of volatile depletion in the source material from which the asteroids accreted.

Figure 1. Cutaway view of the GRaND instrument and example data products for: a) neutron spectroscopy (phoswich); b) gamma ray spectroscopy (CdZnTe array); and c) gamma ray spectroscopy (BGO). Click for larger view.