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Imaging of γ rays with 1–5 MeV with a xenon time projection chamber (Xe-TPC) has been simulated with the GEANT4 code. The clear image of a γ-ray source was obtained using the Xe-TPC. The dependence of angular resolution and imaging efficiency on xenon density were also investigated over the density range from 0.06 to 0.18 g/cm 3 . It was found that a Xe-TPC is promising for imaging of high-energy γ-rays (∼5 MeV) on planetary surface.

The high precision gamma-ray spectrometer (GRS) is scheduled to be launched on the lunar polar orbiter of the SELENE mission in 2007. The GRS consists of a large Ge crystal as a main detector and massive bismuth germanate crystals as an anticoincidence detector. A Stirling cryocooler was adopted in cooling the Ge detector. The flight model of SELENE GRS has been completed and an energy resolution of 3.0 keV (FWHM) at 1.332 MeV has been achieved. The spectrometer aims to observe nuclear line gamma rays emitted from the lunar surface in a wide energy range from 100 keV to 12 MeV for one year and more to obtain chemical composition on the entire lunar surface. The gamma-ray data enable us to study lunar geoscience problems including crust and mantle composition, and volatile reservoirs at polar regions.

Assuming different abundances of the Apollo lunar soil samples and the average spectrum of galactic cosmic ray protons, energy spectra of neutrons and gamma rays and emission rates of gamma-ray lines from major elements have been estimated by using the reviewed Monte Carlo simulation library Geant4 and nuclear data. Previously, such libraries were not able to reproduce gamma-ray lines properly for the planetary application. Results clearly show that the emission rate of gamma rays heavily depends not only on the chemical abundance but also neutron flux within the lunar subsurface. While the intensities of gamma-ray lines are mostly proportional to elemental abundances, the intensity per unit elemental abundance can vary. Such a complex correlation is attributed to the change in neutron flux within the lunar subsurface and petrological restriction of elemental variation.

High-pressure xenon is an attractive medium for radiation detection in that the time projection chambers can be constructed by combined measurements of charge and light signals. The electron transport properties are essential information for developing and operating high-pressure xenon detectors. In this paper, our recent experimental results of electron diffusion coefficients in high-pressure xenon are presented. We measured the longitudinal diffusion coefficient of electrons under external applied electric fields in high-pressure xenon, ranging from 0.17 to 5.0 MPa in pressure at room temperature. A significant pressure dependence was found in the density-normalized longitudinal diffusion coefficient for low electric field region. We compared the longitudinal diffusion coefficient with the transverse one at a pressure of 1.0 MPa, and obtained the difference between both the diffusion coefficients. The longitudinal diffusion was found to become smaller than the transverse one when increasing the external electric field.

We summarize the present R&D status of high-pressure xenon time projection chamber (HPXe-TPC) which is a new gamma-ray camera for planetary science mission.

Research Institute for Science and Engineering., Waseda Univ., Japan College of Health Science, Saitama Prefectural University,, Japan Inst. of Space and Astronautical Science, JAXA, Japan Nippon Medical School, Japan Dep. of Chemistry, Tokyo Metropolitan Univ., Japan National Institute of Polar Research, Japan Japan Agency for Marine-Earth Science and Technology, Japan Research Institute, Chiba Institute of Technology, Japan Centre d’Etude Spatiale des Rayonnements,Toulouse, France Dep. of Meteorites, Univ. of New Mexico, USA *nhasebe@waseda.jp

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.