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The responses of a piezoelectric lead zirconate titanate (PZT) element to hypervelocity collisions were experimentally studied. In this study, the particles of masses ranging from 0.3 to 10 fg were made to collide with PZT at velocities between 20 and 96 km/s. The amplitude and the corresponding rise time of the single-pulse output signals that were produced in the piezoelectric PZT element were measured to determine the possible collision states. The results revealed an apparently multimodal output; three classes were assumed to be involved in the pulse formation mechanism. The amplitude and rise time were sensitive to the collision velocity. The multimodal behavior implied that the PZT-based cosmic dust detectors should be calibrated according to the class they belong to.

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.

The SELENE-B, a lunar landing mission, has been studied in Japan, where a scientific investigation plan is proposed using a robotic rover and a static lander. The main theme to be investigated is to clarify the lunar origin and evolution, especially for early crustal formation process probably from the ancient magma ocean. The highest priority is placed on a direct in situ geology at a crater central peak, “a window to the interior”, where subcrustal materials are exposed and directly accessed without drilling. As a preliminary study was introduced by Sasaki et al. [Sasaki, S., Kubota, T., Okada, T. et al. Scientific exploration of lunar surface using a rover in Japanse future lunar mission. Adv. Space Res. 30, 1921–1926, 2002.], the rover and lander are jointly used, where detailed analyses of the samples collected by the rover are conducted at the lander. Primary scientific instruments are a multi-band stereo imager, a gamma-ray spectrometer, and a sampling tool on the rover, and a multi-spectral telescopic imager, a sampling system, and a sample analysis package with an X-ray spectrometer/diffractometer, a multi-band microscope as well as a sample cleaning and grinding device on the lander.

A comparison between the abundances of major elements on the Moon determined by Lunar Prospector gamma ray spectrometer and those in returned lunar samples is performed. Lunar Prospector shows higher Mg and Al content and lower Si content in western maria in comparison with the lunar sample collection. Lunar Prospector overestimated the Mg content by about 20%. There are no elemental anomalies at the lunar poles: this is additional evidence for the presence of polar lunar hydrogen. Using Mg, Fe, and Al abundances, petrologic maps containing information about the abundances of ferroan anorthosites, mare basalts, and Mg-rich rocks are derived. This approach is useful for searching for cryptomaria and Mg-rich rocks deposits on the lunar surface. A search is implemented for rare rock types (dunites and pyroclastic deposits). Ca-rich, Al-low small-area anomalies are detected in the far side highlands.

著者人数: 14名, 資料番号: SA1003696000

The Japanese lunar polar orbiter SELENE carries a gamma-ray spectrometer which uses a high-purity Ge detector cooled to 80-90 K by a Stirling mechanical cooler. The Gamma-Ray Spectrometer (GRS) consists of a large volume n-type Ge detector (252 cc) as the main detector and bismuth-germanate (BGO) and plastic scintillators as an active shielding. The engineering model still maintains excellent energy resolution even after severe vibration testing. The Gamma-Ray Spectrometer will globally map of the Moon for the major elements of O, Mg, Al, Si, Ti, Fe, etc. and natural radioisotopes of K, Th and U with a high precision. The energy resolution of the GRS is such that it would identify prompt gamma-ray line from hydrogen and the location and the amount of ice, if it exists at the polar regions.

We have investigated the relationship between solar wind speeds and the energetic proton intensity associated with corotating interaction regions (CIR) observed at 1 AU during 1973 – 1995 using NSSDC Data. The speed-difference between the high-speed solar wind and the slow-speed solar wind, which forms CIR at outer heliosphere, shows a good correlation with the peak intensity of 1 – 2 MeV protons during the solar quiet periods. The speed-difference is probably related to the shock strength of the reverse shock of the CIR formed beyond ∼ 1 AU. We have also found the solar activity dependence of the correlation between these quantities in CIR events during the period between 1973 and 1995. The intensity of CIR particles during the solar active periods is higher than that during the quiet periods, suggesting that the density of seed particles in CIR events during the solar active periods is larger than that during the solar quiet period.

We review the current status of the development of Gamma-Ray Spectrometer (GRS) for the Lunar mission SELENE. The GRS instrument will measure gamma-rays in the energy range from 100 keV to 9 MeV. The instrument is a high-purity Ge detector surrounded by BGO and plastic scintillators which are operated as an anticoincidence shield, and is cooled by a Stirling cycle cryocooler. The primary objective is to provide global maps of the lunar composition. Measurements are anticipated for Fe, Ti, U, Th, K, Si, Mg, Al, O, Ca and Na over the entire lunar surface. The abundance of water ice in the permanently shaded craters at both the lunar poles will be measured with this instrument.

Long-term decrease events in the intensity of galactic cosmic rays have been investigated using solar wind parameters. We investigated distinct long-term decreases in 1993/94 near the solar minimum phase. In this period, the corotating interaction regions were observed over half a year. Recurrent increases of the solar wind speed were observed during this time period, forming corotating interaction regions. Simultaneously recurrent energetic proton events and recurrent decrease of galactic cosmic rays are measured. The baseline counts of neutron monitors was well anti-correlated with the peak speed of solar wind. It is suggested that long-lasting enhancements of solar wind velocity or corotating interaction regions formed by them can produce the long-term decrease in the galactic cosmic ray intensity.

The energy spectra (40 to 300 MeV/n) of galactic cosmic ray particles (C, N, O, Ne, Mg, Si, and Fe) observed by the HEP-HI silicon detector telescope onboard the GEOTAIL satellite were investigated over the period from January 1993 to May 1996, from the viewpoint of solar modulation. From the GEOTAIL data and those of relativistic heavy particles observed by HEAO-3, the modulation potential φ was obtained by using a solution of the Fokker-Plank equation with the “force-field” approximation. In the equation the diffusion coefficient was used as adjustable parameter, under the assumptions of a fixed velocity of solar wind (400km/sec) and the radius of the heliosphere of 50AU. The time variation of the modulation potential thus obtained is compared with that obtained from the CLIMAX neutron monitor. A reasonable correlation between both time variations is found when five months delay is applied to the results obtained by the HEP-HI telescope.

The composition and energy spectra of anomalous cosmic-rays (ACRs) in the energies 6 – 200 MeV/n have been measured during the period from September 1992 to August 1995 using the High Energy Particle instrument onboard the GEOTAIL satellite orbiting at 1 AU. A remarkable enhancement of ACR N, O, Ne and Ar is observed during the period. A flux enhancement of anomalous argon at 1 AU is confirmed by the GEOTAIL satellite. The first possible evidence for the emergence of a measurable sulfur component in ACR below about 20 MeV/n is found. The flux increase of anomalous sulfur with 10.4 eV FIP is smaller than that of ACR carbon with 11.3 eV FIP, and much smaller than those of high FIP elements, which suggests that the fraction of neutral carbon and sulfur atoms is significantly lower in the very local interstellar medium.