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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.

On the lunar surface, every human being would be exposed to galactic cosmic rays (GCRs) and their secondary products such as gamma rays and neutrons. For the human activity on the lunar surface in the future, it is important to estimate the effect of these particles on radiation doses. The annual ambient dose equivalent on the lunar surface was estimated on the basis of the latest observational data of GCRs. It is found that the annual ambient dose equivalent amount to about 570 mSv/yr during the intermediate period between the maximum and the minimum phases of the solar activity. This amount of dose is mainly produced from primary components of GCRs heavier than proton and helium nuclei. The annual ambient dose equivalent due to iron nuclei during this period is about 130 mSv/yr, more than 20% of the total dose on the lunar surface. Moreover, the dose due to these neutrons among the secondary particles reaches 50 mSv/yr, suggesting that the dose due to neutrons must be considered from the viewpoint of the human activity on the lunar surface.

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