Kazuo Yamakoshi

Mars Dust Counter

In order to unveil the presence and characteristics of Martian dust ring/torus, Mars Dust Counter (MDC) is aboard ISAS’s spacecraft PLANET-B, which will be launched in 1998 summer and investigate the upper atmosphere and surrounding environment of Mars between 1999 and 2001. MDC PLANET-B is an improved version of impact-ionization dust detectors aboard HITEN and BREMSAT. It weighs only 730 g with the sensor aperture area of 140 cm2. To improve signal to noise ratios and to precisely determine the risetime of signals, a neutral target channel is added independent of ion and electron target channels. Detectable velocity (v) range is between 1 km/s and more than 70 km/s, which will cover all possible dust clans: circummartian (low v), interplanetary (mid v), and interstellar (high v) particles. Measurable mass range is 5 × 10−15 and 10−10 g at v = 10 km/s. Since PLANET-B takes an elliptic retrograde orbit around Mars, MDC can investigate particles from Phobos and Deimos with relative velocity higher than 1 km/s. Therefore, MDC can clarify the presence of a confined dust ring along Phobos’ orbit and an extended dust torus along Deimos’ orbit, and it may answer whether these ring and torus are self-sustained or not. Since the nominal operation of PLANET-B is longer than one Martian year, MDC may detect predicted seasonal variation of the ring/torus structure.

The Munich Dust Counter --- A Cosmic Dust Experiment on Board of the MUSES--A Mission of Japan

The Munich Dust Counter (MDC) is a scientific experiment on board of the MUSES-A mission of Japan. It is the result of a cooperation between the Institute of Space and Astronautical Science (ISAS) of Japan and the Chair of Astronautics of the Technische Universitat Munchen (TUM) of Germany. The MDC is an impact ionization detector designed to determine mass and velocity of cosmic dust. Here a short overview over the MUSES-A mission is given to show the measurement situation of the MDC experiment. The measurement principle of the instrument together with a discussion of the scientific objectives and the design of the experiment is summarized.

An extremely low-background beta-gamma coincidence spectrometer

Abstract An extremely low-background beta-gamma coincidence spectrometer has been constructed with heavy mass and anticoincidence shieldings. The main detector consists of a large well-type NaI(Tl) scintillator and two semi-cylindrical GM counters fitted into the well. The geometry of the NaI(Tl) scintillator is 6.5″ ∅ × 4.0″ (well; 3.5″ ∅ × 2.5″). The crystal has a 5 mm thick quartz window and is mounted in a 0.8 mm thick vacuum-melted copper. The crystal is matched to a quartz made photomultiplier EMI 9530 QR (photocathode 5″ in diameter). The main counter system is surrounded with a large scale kerosine scintillation counter, which acts as a guard counter. The detector system is set inside a 25 cm thick iron room, which was lined with 0.2 cm thick OFHC and 0.5 cm thick lucite plates. The iron bricks and the skeleton were made before 1945. The whole assembly was built in an underground cell heavily shielded against major cosmic ray component with a rock cover of at least 100 mwe. The background counting rate was 0.050 cpm at the 511 keV and the counting efficiency was 11.9% in beta-gamma mode for the annihilation peak.

Trace element concentrations in silicate spherules from oceanic sediments

Abstract The possible existence of meteoritic spherules was investigated among several silicate spherules separated from oceanic sediments and analyzed by means of INAA (instrumental neutron activation analysis). A 0.72 mg glassy spherule was found to have uniform enrichment of 4 ~ 5 for the refractory REE (rare earth elements) and Sc with substantial depletion of Ce relative to chondritic abundances. This implies that this spherule is meteoritic in origin and that the enrichment of refractory elements was established by high temperature heating in a high O/H environment, possibly at the time of entering the Earths atmosphere. The other three analyzed spherules showed major and trace element abundances that are consistent with an origin in the oceanic environment.

An extremely low background beta-gamma-gamma coincidence spectrometer with GMGe(Li)NaI(Tl) counter combination

Abstract An extremely low background betagammagamma coincidence spectrometer has been constructed for the measurement of cosmic ray produced radionuclides carried by extraterrestrial matter accreted on the Earth. The detectors used in this work are a large volume Ge(Li) detector, a pair of GM counters of thin layer type for beta counting, a large volume NaI(Tl) well scintillator and liquid scintillator for guard counter. Six counting modes are applicable; gamma only, gammagamma coincidence, Compton suppression, betagamma, (beta+gamma)gamma and betagammagamma counting modes. The absolute counting efficiency of the betagammagamma mode is slightly lower than that of the betagamma mode, however the figure of merit in the betagammagamma mode becomes better than that in the betagamma mode at higher energies.

Cosmic-ray-produced 59Ni in marine sediments

Abstract 59Ni in 0.4 g of nickel from deep-sea sediments was measured with an extremely low-level X-ray spectrometer. The obtained specific activity was (5.9 ± 1.8) × 10−2 dpm/kg sediments. The59Ni activity induced by proton and alpha particle irradiation in outer space can be estimated as 300 dpm/kg dust. Hence, the content of extraterrestrial dust in deep-sea sediments was not more than 200 ppm.

Isotopic compositions of potassium and calcium in magnetic spherules from marine sediments

Abstract Isotopic compositions of potassium and calcium in individual magnetic spherules were determined. No significant anomaly was observed for potassium within twice the statistical error (2σ), although for calcium isotopes enrichments of 46 Ca, 44 Ca and 42 Ca were observed in one spherule. The relative excess of 46 Ca, 44 Ca and 42 Ca in the spherule agrees with the relative yield of spallogenic calcium isotopes observed in iron meteorites. This fact indicates that the enrichment in the calcium isotopes was caused by cosmic ray irradiation of the spherule in outer space.

Studies on Isotopic Ratios of Osmium and Iridium in Cosmic Spherules Using Instrumental Neutron Activation Analysis

Studies on isotopic anomalies in cosmic meteoroids are expected to reveal the features of nuclear synthesis in various phases of star-evolution. The respective isotopes of noble metals had been produced through various reaction processes as well as in various regions of star-eruptions. However, isotopic anomalies of Os in extremely refractory inclusions of primordial carbonaceous chondrites have not been found up to now. In this work, isotopic ratios of the elements having high condensation temperatures, such as Os and Ir, in the cosmic spherules are examined using instrumental neutron activation analysis(INAA).

A Cosmic Matter Accretion Event Around 660,000 Years Before Present Found in Two Dated, Central Pacific Cores

In order to study large-scaled cosmic matter accretion events in the past, Ir enriched layers at C-T and other geological boundaries and dated sedimental cores have been searched by many scientists. In this work, Iridium contents and the ratios of (Co/Fe) in two dated, respective layers of the cores are determined. These samples were dated fortunately with the paleo-magnetic and also with the cosmogenic Be-10 methods. Ir enrichments are found at (0.660 ± 0.030) My before present.

Cosmic Ray Exposure Age Determinations of Cosmic Spherules from Marine Sediments

The cosmic ray exposure ages of deep sea metallic spherules were determined by various methods; low level countings (Ni-59), neutron activation analysis (Mn–5 3), high energy accelerator mass spectrometry (Be-10,Al-26) and mass spectrometry (K isotopes). The exposure ages of 0.3 – 50 Ma were obtained. According to Poynting-Robertson effect, the starting points (supplying sources) are located at inner region of the orbit of Saturn.