Abstract
Abstract The high-precision gamma-ray spectrometer (GRS) on the lunar polar orbiter SELENE is designed to measure 200 keV—12 MeV gamma rays in order to determine elemental compositions of the lunar surface. The GRS consists of a large germanium (Ge) crystal as a main detector and a massive bismuth germanate crystal and a plastic scintillator as anticoincidence detectors. The Ge detector is cooled by a Stirling cryocooler with its compressor attached to a passive radiator facing the cold space. The cooling system maintains the Ge detector below 90 K during the observation. The flight model of the GRS has achieved an energy resolution of 3.0 keV (FWHM) at 1333 keV. Energy spectra obtained by the GRS will show sharp gamma-ray lines whose energies identify the elements and whose intensities determine the concentrations of the elements, permitting global mapping of the elemental abundances in the sub-surface of the Moon. The elemental maps obtained by the GRS with such high-energy resolution enable us to study lunar geoscience problems.
Highlights
To determine the spatial distribution of major and important trace elements in the lunar surface is an essential objective of lunar science
We review a high-resolution gamma-ray spectrometer (GRS) for the Japan’s lunar explorer SELENE, the largest lunar orbiter since the Apollo missions
Seven of major background components in energy spectra to be measured in the lunar orbit are summarized as follows: (1) Compton continuum caused by gamma rays scattered within the lunar subsurface; (2) Compton continuum caused by a partial energy deposition of line gamma rays from the lunar surface in the detector; (3) Lines and continuum by gamma rays promptly emitted from the spacecraft as a result of nuclear interactions; (4) Activation of the Ge and BGO detectors and surrounding materials by cosmic rays; (5) Natural radioactivity in the materials of GRS system, its surroundings, and the spacecraft; (6) Continuum produced by energy deposition of charged particles in the detector; (7) Gamma rays from astrophysical sources and other interactions of high energy particles
Summary
To determine the spatial distribution of major and important trace elements in the lunar surface is an essential objective of lunar science. The SELENE-GRS measures gamma rays in the energy range of 200 keV to 12 MeV with high precision to determine the composition of the lunar surface. It will provide data on the abundance of many major elements over the entire lunar surface and possibly on water ice at the polar regions. It is based on a spectrum of gamma rays calculated to be emitted from the Apollo 16 site (Yamashita et al, 2008), as taking the energy resolution of the GRS into account. The anti-coincidence processing of the Ge detector and the BGO detector is enabled or disabled by command
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