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GRaND science instrument moves closer to launch from Cape
By Tom Prettyman
    Schematic of Dawn GRaND Instrument referring to pulse height
       
   

 

Figure 1: Cutaway view of the GRaND instrument and example date of products.
Top graph: for neutron spectroscopy (Phoswich):
Middle graph: gamma ray spectroscopy (Cd, Zn, Te array)
Bottom graph: gamma ray spectroscopy (BGO)

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.

What will GRaND tell us?
by Nancy Ambrosiano

The gamma-ray spectrum provides a fingerprint of the elements within the surface that can be analyzed to determine their abundance. The neutrons provide information about light elements, such as hydrogen and carbon, which are the constituents of ices, as well as strong thermal neutron absorbers, such as iron, titanium, chlorine, gadolinium, and samarium. GRaND can measure many rock-forming elements on Vesta and Ceres.

GRaND's measurements of hydrogen are needed to determine the distribution of water, which may be present on Ceres as ice or hydrated minerals. From a circular, polar mapping orbit, GRaND will map the abundance of these elements over the entire surface of Vesta and Ceres.

GRaND will help answer a number of questions about how the asteroids formed and evolved by showing maps of where key elements are found and in what abundance. For example, the ratio of potassium to thorium will provide information about the material that grew to form the asteroids and may be useful in determining how the composition of the solar nebula changed with heliocentric distance. The abundance of rock-forming elements, such as iron and titanium, provides information about how igneous rock forms from magma on Vesta. The large impact basin at the south pole of Vesta provides an opportunity to determine the composition of the interior of this planet, providing additional constraints on structure and thermal evolution. The elemental abundance data will also help verify that Vesta is the source of a certain type of meteorites.

Measurements of the abundance of hydrogen and carbon will enable scientists to understand aqueous processes that probably shaped Ceres. Data from all three payload instruments will be combined to provide a complete picture of surface composition, mineralogy, and structure that can be used to answer many questions about the conditions under which planets formed.

The Dawn mission to asteroid Vesta and dwarf planet Ceres is managed by the California Institute of Technology's Jet Propulsion Laboratory in Pasadena, for NASA's Science Mission Directorate. The University of California, Los Angeles is responsible for overall Dawn mission science. Other scientific partners include the Laboratory; German Aerospace Center, Berlin; Max Planck Institute for Solar System Research, Katlenburg, Germany; and Italian National Institute of Astrophysics, Palermo. Orbital Sciences Corporation of Dulles, Virginia, designed and built the Dawn spacecraft.

   
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