Kiyoaki Okudaira

Gamma-ray observations from Hinotori

Some interesting results on gamma-ray line emission and its time profiles observed by Hinotori are presented. Possible explanations of gamma-ray line and hard X-ray emissions for the impulsive and gradual flares are discussed. Relationship between the gamma-ray line emission and acceleration and escape of the solar particles is also studied.

On the Analysis of the Slow Particles Emitted from Cosmic-Ray Stars (II)

An analysis of the nature of tracks of the stars ending in the G5 emulsion was made by measuring their width (diameter). Six groups of tracks having different width were observed. We identified two of these groups as that of α particles and Li nuclei by comparing their width with those of α particles emitted from radiothorium stars and with those of hammer tracks. As for the more thicker tracks, the charge determination on the tracks of each group was made by measuring their thin-down lengths. These measurements seem to indicate that the width of track was approximately proportional to \(\sqrt{Z}\) ( Z : atomic number). The energy and the angular distribution of each group were also examined. The curves of the energy distribution of protons and α particles approximately agreed with those deduced from the evaporation model, assuming the appropriate nuclear temperature and Coulomb barrier height. The angular distributions of these were nearly isotropic. It was noted That the number of slower a particles mea...

Gamma-ray spectrometer aboard solar flare observation satellite HINOTORI

Abstract A gamma-ray spectrometer aboard the solar flare obsevation satellite HINOTORI launched on 21 Feb. 1981 is described. The gamma-ray spectrometer consists of a phoswich type scintillator of CsI(Tl) crystal surrounded by a plastic scintillator, which can discriminate between gamma-rays and charged particles. The CsI(Tl) scintillator, whose size is 62 cm2 in area and 5.1 cm in height. covers the energy range 0.21–6.67 MeV. The energy resolution is 0.1E 1 2 (MeV) fwhm and the full energy peak efficiency is 0.40 at 0.662 MeV. The primary data output is a full resolution 128 channel pulse height spectrum every 2 s. Since the launch HINOTORI recorded about forty solar gamma-ray flares and four cosmic gamma-ray bursts, and several interesting results were reported. The typical results of the large solar flare on 27 April 1981 and the cosmic gamma-ray burst on 21 July 1981 are presented in this paper. These observed data could provide important clues for advancing our understanding of high enegry phenomena in solar flares and cosmic gamma-ray bursts.

Particle acceleration as viewed from the observations of flare-associated gamma-ray emissions

Since 1972, many solar gamma-ray events are observed. Much advances have been made in the understandings of the solar flare phenomena. A brief description of the gamma-ray observations is given. Several particle acceleration mechanisms that are consistent with these observations are presented. All these mechanisms can work well. But it is difficult at this stage to decide which one is really working at the Sun. There remain many problems for future studies, especially the observations during the next solar maximum.

The wide band spectrometer for the solar flare satellite SOLAR-A

Abstract The Wide Band Spectrometer (WBS) for the solar flare satellite SOLAR-A scheduled for launch in 1991 consists of three kinds of detectors to observe the wide band spectrum from soft X-rays to gamma-rays. The soft X-ray spectrometer (gas proportional counter), hard X-ray spectrometer (NaI scintillation counter and gamma-ray spectrometer (two BGO scintillation counters) cover the 2–30 keV, 20–400 keV and 0.2–100 MeV bands, respectively. Further, the WBS contains a radiation belt monitor consisting of a NaI scintillation counter which is capable of detecting cosmic gamma-ray bursts. The details of these detectors, electronics systems and data collection are described.

Development of an auroral X-ray imager using a two-dimensional array of Si(Li) semiconductor detectors

Abstract In order to observe aurotal X-ray images, a horizontal two-dimensional array of lithium-drifted silicon (Si(Li)) semiconductor detectors has been developed. The auroral X-ray imager has a 5 × 5 pixel array. The sensor element of one pixel is a Si(Li) detector, the area of which is 1.8 cm × 1.8 cm and the thickness of which is 0.5 cm. Noises of the Si(Li) detectors have been decreased for X-ray measurements. Although the noise signals are too large for X-ray measurements at room temperature, they become much smaller at 5° C. Thus, accurate X-ray measurements by the Si(Li) detectors are fully possible at such a low temperature. In observations by a stratospheric balloon, cooling of the Si(Li) detectors can be made by dissipation of heat to a low-temperature environment. A heat-pipe has been used to cool the Si(Li) detectors. The energy range of the auroral X-rays to be observed is from 37 to 200 keV. The horizontal two-dimensional array of the Si(Li) detectors has been placed in a pinhole collimator. The angular resolution corresponding to one pixel of the auroral X-ray imager is 15° at the central pixel. The total field of view of the imager is 62°.

Gamma-ray bursts observed from the Hinotori satellite

During February, 1981 and June, 1982 the gamma-ray and the hard X-ray spectrometers on the Hinotori satellite observed four gamma-ray bursts on 28 February, 21 July, 1981, 26 February and 13 March, 1982. These gamma-ray bursts were simultaneously observed by other satellites. The time histories and energy spectra are shown for these gamma-ray bursts, and the burst sizes (erg cm−2) are estimated. Two possible source locations for the burst of 21 July, 1981 are roughly determined from arrival time delays between two pairs of satellites, PVO-Hinotori and ISEE-3-Hinotori. The weak gamma-ray line peak structure around 1.8 MeV was observed for the burst of 13 March, 1982. The line could be interpreted in terms of gravitationally redshifted neutron capture line at 2.22 MeV.

Unusual Time History of Hard X-Ray and Gamma-Ray Emissions from the 1981 April 4 Flare

The Hinotori satellite observed an unusual time history of hard X-ray and gamma-ray emissions from a limb flare on 4 April, 1981. The detailed comparison of the time history reveals that the gamma-ray emission precedes the hard X-ray emission by about 10 s. Following three possibilities are suggested to explain the unusual time history: (1) The ion acceleration precedes the electron acceleration, (2) the ions and electrons are simultaneously accelerated, but the nuclear reactions responsible for gamma-ray emission precede the electron bremsstrahlung responsible for hard X-ray emission, and (3) the gamma-rays and hard X-rays are simultaneously emitted, but the hard X-rays are strongly attenuated from the limb effect.

Large-area silicon semiconductor detector telescope for identification of primary cosmic-ray isotopes

Abstract Large-area silicon semiconductor detectors with an effective area of 20 cm2 have been fabricated to observe primary cosmic ray isotopes from hydrogen to oxygen with a high resolution. As a preliminary experiment the energy resolution of detectors was investigated for α particles and internal conversion electrons. A detector telescope consisting of ΔE and E detectors was constructed and the mass resolution of the detector telescope was investigated by measuring long-range 11H, 21H and 31H nuclei emitted from the radioactive 25298Cf source. The expected result was obtained from this preliminary experiment and it seems possible to separate masses of primary cosmic ray isotopes up to oxygen with a high resolution.

In-flight energy calibration for the X-ray and gamma-ray spectrometers on the solar-A satellite

Abstract The electronically gated in-flight energy calibration is applied to the X-ray and gamma-ray spectrometers on a SOLAR-A satellite. The NaI X-ray spectrometer covering the 20–400 keV energy band is calibrated by an 241 Am radioactive source which decays by the simultaneous emission of 60 keV X-rays and 5.48 MeV alpha-particles. The X-ray calibration spectrum is accumulated in coincidence with an event tag pulse generated by the simultaneous detection of an X-ray and an alpha-particle. The BGO gamma-ray spectrometer covering the 0.2–10 MeV energy band is calibrated by a 60 Co radioactive source which decays by the simultaneous emission of 1.17 and 1.33 MeV gamma-rays and a beta-ray (maximum energy is 313 keV). The gamma-ray calibration spectrum is accumulated in a manner similar to the X-ray spectrometer. Since the present method enables to select the calibration pulse without the disadvantage of introducing extra pulses, it is suitable for a space experiment where external conditions and background counting rates can significantly change.