Toshisuke Kashiwagi

Observations of TeV Gamma-Ray Flares from Markarian 501with the Telescope Array Prototype

We will report the observations of TeV gamma ray flares from Markarian 501 using Telescope Array Prototype. The observation were carried out continuously from the end of March to the end of July in 1997. The energy spectrum, and the time variation of the gamma ray intensities are shown. The intensity has been changed by the order of magnitude in this period and the possible quasi periodic oscillation of 12.7days were discovered.We report the observations of TeV gamma-ray flares from Markarian 501 using the Telescope Array Prototype. The observations were carried out continuously from the end of March to the end of July of 1997. The energy spectrum and the time variation of the gamma-ray intensities are shown. The intensity has been changed by an order of magnitude in this period, and the possible quasi-periodic oscillation of 12.7 days were discovered.

LET distribution measurement with a new real-time radiation monitoring device-III onboard the Space Shuttle STS-84

Abstract A new type of Real-time Radiation Monitoring Device, RRMD-III, consisting of three double-sided silicon strip detectors (DSSDs) has been constructed and used onboard the Space Shuttle mission STS-84. The Space Shuttle cruised at an altitude of 300–400 km and an inclination angle of 51.6° for 221.3 h. RRMD-III succeeded in measuring the linear energy transfer (LET) distribution over the range of 0.2– 600 keV / μ m for 178 h . The obtained LET distribution of particles was investigated in detail by classifying it into galactic cosmic ray (GCR) particles and trapped protons in the South Atlantic Anomaly (SAA) region. The result shows that GCR particles contribute 60% to the total dose equivalent. The total absorbed dose rate during the mission was 0.516 mGy / day , the effective quality factor was 1.81 by ICRP-Pub.26, and the dose equivalent rate was 0.935 mSv / day . The average absorbed dose rates are 0.120 μGy / min for GCR particles and 4.80 μGy / min for trapped protons. The effective quality factors are 3.16 for GCR particles and 1.19 for trapped protons. RRMD-III data were also compared with the data of the tissue equivalent proportional counter (TEPC), proving that RRMD-III is a reliable device for deriving the true-LET distribution in real time for evaluating space radiation.

The first observation of sulfur in anomalous cosmic rays by the Geotail and the wind spacecrafts

The Geotail high-energy particle instruments have observed cosmic-ray particles in the energy range from 3 MeV n-1 to 150 MeV n-1 at 1 AU during the period 1992 September-1995 August. A remarkable enhancement of anomalous cosmic-ray (ACR) N, O, Ne, and C is observed during the period. A measurable enhancement of the sulfur flux below about 20 MeV n-1 was observed. This is the first evidence showing the existence of sulfur in the anomalous component. The flux increase of anomalous sulfur, with a first ionization potential (FIP) of 10.4 eV, is smaller than that of ACR carbon with an FIP of 11.3 eV and much smaller than those of high-FIP elements, which suggests that the fractions of neutral carbon and sulfur atoms are significantly low in the very local interstellar medium.

Gamma-ray spectrometer for Japanese lunar polar orbiter

Abstract 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.

Rapid recovery of anomalous cosmic ray flux at 1 Au in solar cycle 22

A new observation of low energy cosmic ray particles with the energies from 4 to 120 MeV/n by the HEP instrument onboard the GEOTAIL satellite orbiting at 1 AU shows a remarkable enhancement of anomalous cosmic ray (ACR) N, O and Ne in the period September 1992–December 1993 before approaching to the solar minimum in solar cycle 22. The ACR fluxes obtained by the GEOTAIL observation are in good agreement with the results earlier obtained from the SAMPEX satellite [Mewaldt et al. 1993a,b]. The ACR carbon is also evident though the C enhancement is small compared with those of N, O and Ne. We confirm the new SAMPEX finding that the ACR fluxes have recovered more rapidly in 1992–1993 than in previous solar cycles and find this tendency continues to the end of 1993.

High-purity germanium gamma-ray spectrometer with stirling cycle cryocooler

Abstract 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.

Investigation of basic characteristics of synthetic diamond radiation detectors

Diamonds have attractive properties for use as radiation detectors. Recent advances in diamond fabrication techniques have made it possible to produce almost perfect single crystal materials that have electrical properties suitable for radiation detectors. We therefore developed detectors made of high-purity synthetic IIa diamonds grown by a high-pressure, high-temperature method and investigated their basic properties. These detectors have Schottky and Ohmic electrodes, and the leakage current is less than 20 pA with reverse bias voltages up to 2.4 kV, demonstrating that the detector withstands high bias voltage. The results of testing the electrical properties show that the electrodes work well. The best detector achieved an energy resolution of 15.4 keV (FWHM) for 5.486 MeV alpha particles from /sup 241/Am.

Radiation Dosimetry Measurements with Real Time Radiation Monitoring Device (RRMD)-II in Space Shuttle STS-79

The real-time measurement of radiation environment was made with an improved real-time radiation monitoring device (RRMD)-II onboard Space Shuttle STS-79 (S/MM#4: 4th Shuttle MIR Mission, at an inclination angle of 51.6 degrees and an altitude of 250-400km) for 199 h during 17-25 September, 1996. The observation of the detector covered the linear energy transfer (LET) range of 3.5-6000 keV/micrometer. The Shuttle orbital profile in this mission was equivalent to that of the currently planned Space Station, and provided an opportunity to investigate variations in count rate and dose equivalent rate depending on altitude, longitude, and latitude in detail. Particle count rate and dose equivalent rate were mapped geographically during the mission. Based on the map of count rate, an analysis was made by dividing whole region into three regions: South Atlantic Anomaly (SAA) region, high latitude region and other regions. The averaged absorbed dose rate during the mission was 39.3 microGy/day for a LET range of 3.5-6000 keV/micrometer. The corresponding average dose equivalent rates during the mission are estimated to be 293 microSv/day with quality factors from International Commission on Radiological Protection (ICRP)-Pub. 60 and 270 microSv/day with quality factors from ICRP-Pub. 26. The effective quality factors for ICRP-Pub. 60 and 26 are 7.45 and 6.88, respectively. From the present data for particles of LET > 3.5keV/micrometer, we conclude that the average dose equivalent rate is dominated by the contribution of galactic cosmic ray (GCR) particles. The dose-detector depth dependence was also investigated.

The CALET, CALorimetric Electron Telescope, mission for the International Space Station

Abstract The CALorimetric Electron Telescope, CALET, mission is proposed for the Japanese Experiment Module Exposure Facility of the International Space Station. Major goals of the mission are precise measurements of the electrons in a few GeV - 10 TeV and the gamma-rays in 100 MeV - several TeV, keeping an energy resolution of a few % over 100 GeV. From the measurements, a systematic investigation of high-energy electromagnetic process in universe will be performed. A detection of SUSY particle which is a candidate of the dark matter would also be expected. The detector is composed of an imaging calorimeter of scintillating fibers and a total absorption calorimeter. Total thickness of the absorber is 45 r.1 for electro-magnetic particles and 2.1 m.f.p for protons. Total weight of the payload is nearly 2,200 kg, and the effective geometrical factor should be ∼ 1.0 m 2 sr. The hadron rejection power can be 10 6 for electrons.

Liquid Xenon Gamma-Ray Imaging Telescope (LXeGRIT) for medium energy astrophysics

As part of our ongoing research program to develop a liquid xenon gamma-ray imaging telescope (LXe-GRIT) for medium energy astrophysics, we have built a liquid xenon time projection chamber (LXeTPC) with a total volume of 10 liters and a sensitive are of 20 cm by 20 cm. The detector has been successfully tested with gamma-ray sources in the laboratory and is currently being prepared as balloon-borne payload for imaging MeV gamma-ray emission from the Crab Nebula, Cygnus X-1 and the Orion molecular cloud region. The LXe-TPC, sensitive to gamma-rays from 300 keV to 30 MeV, measures the energy and the 3-D location of each gamma-ray interaction with a resolution of 6% FWHM and 1 mm RMS at 1 MeV, within a 1 sr FOV. Its detection efficiency for Compton events is about 4% in the 1 - 3 MeV, an energy band of great astrophysical interest for both continuum and line emission. Its 3 sigma continuum sensitivity of 1.8 multiplied by 10-7 ph cm-2s-1keV-1 for a nominal 10 hr observation time, will allow us to study a variety of sources with an imaging accuracy as good as 1 degree. We plan to pursue a vigorous program of balloon flights with this telescope to achieve the maximum science return while continuing a strong R&D laboratory program on LXe technology. The ultimate goal is an optimized design of a satellite implementation of a liquid xenon gamma-ray imaging instrument that will lead to drastic improvements in sensitivity and angular resolution in the 0.3 - 30 MeV band and beyond.