Hisashi Kitamura

Specific activity and activity ratios of radionuclides in soil collected about 20km from the Fukushima Daiichi Nuclear Power Plant: Radionuclide release to the south and southwest

Soil samples at different depths (0-2, 5-7 and 10-12cm) were collected from J Village, about 20km south of Fukushima Daiichi Nuclear Power Plant (FNPP) to determine their radionuclide specific activities and activity ratios. The concentrations and activity ratios of (131)I, (134, 136, 137)Cs and (129m)Te were obtained, but only trace amounts of (95)Nb, (110m)Ag and (140)La were detected which were too low to provide accurate concentrations. Radionuclides such as (95)Zr, (103, 106)Ru and (140)Ba that were found in Chernobyl fallout, were not found in these soil samples. This suggests that noble gasses and volatile radionuclides predominated in the releases from FNPP to the terrestrial environment. The average activity ratios of (131)I/(137)Cs, (134)Cs/(137)Cs, (136)Cs/(137)Cs and (129m)Te/(137)Cs were 55, 0.90, 0.22 and 4.0 (corrected to March 11, 2011) in the 0-2cm soil samples of April 20 and 28, 2011.

Analysis of the calibration results obtained with Liulin-4J spectrometer-dosimeter on protons and heavy ions.

We are developing a portable dosimeter (Liulin-4J) based on a silicon semiconductor detector for use in measuring the absorbed dose from primary or secondary cosmic rays to astronauts and airplane crews. The dosimeter can measure not only the flux and dose rate, but also the deposited energy spectrum for silicon in per unit time. In order to calibrate the dosimeter, we have carried out exposures at the NIRS cyclotron and HIMAC heavy ion synchrotron facilities. We obtained a detector response function for using in measuring energy deposition and LET.

Effects of particle irradiations on vortex states in iron-based superconductors

Various kinds of energetic particles are irradiated into iron-based superconductors, and their effects on the critical current density (Jc) and vortex dynamics have been systematically studied. It is found that Jc is enhanced and vortex dynamics is strongly suppressed by energetic particles having a sufficient energy deposition rate, similar to the case of high temperature cuprate superconductors. The enhancement of Jc, in general, persists up to much higher irradiation doses than in cuprates. However, details of the effect of irradiation depend on the kind of ion species and their energies. Even with the same ions and energies, the effect is not universal for different kinds of iron-based superconductors. The correlated nature of defects created by heavy-ion irradiation is confirmed by the angular dependence of irreversible magnetization.

Single extreme low dose/low dose rate irradiation causes alteration in lifespan and genome instability in primary human cells

To investigate the long-term biological effect of extreme low dose ionising radiation, we irradiated normal human fibroblasts (HFLIII) with carbon ions (290 MeV u−1, 70 keV μm−1) and γ-rays at 1 mGy (total dose) once at a low dose rate (1 mGy 6–8 h−1), and observed the cell growth kinetics up to 5 months by continuous culturing. The growth of carbon-irradiated cells started to slow down considerably sooner than that of non-irradiated cells before reaching senescence. In contrast, cells irradiated with γ-rays under similar conditions did not show significant deviation from the non-irradiated cells. A DNA double strand break (DSB) marker, γ-H2AX foci, and a DSB repair marker, phosphorylated DNA-PKcs foci, increased in number when non-irradiated cells reached several passages before senescence. A single low dose/low dose rate carbon ion exposure further raised the numbers of these markers. Furthermore, the numbers of foci for these two markers were significantly reduced after the cells became fully senescent. Our results indicate that high linear energy transfer (LET) radiation (carbon ions) causes different effects than low LET radiation (γ-rays) even at very low doses and that a single low dose of heavy ion irradiation can affect the stability of the genome many generations after irradiation.

Radiation measurements with heat-proof polyethylene terephthalate bottles

This study demonstrates that the energy resolution of a newly developed 100 per cent pure polyvinyltoluene (PVT) plate allows its use as a base material for a plastic scintillator. The energy resolution, which is a key element for high-performance radiation detectors, was ΔE/E=8.41±0.07% (full width at half maximum (FWHM)) for 976 keV K-line conversion electrons from a 207Bi source. On the basis of results from 207Bi and 137Cs sources, the observed energy resolution of the PVT plate, ΔE/E=8.2/E1/2% (FWHM), was slightly better than that of a typical plastic scintillator (BC-408), ΔE/E=8.7/E1/2% (FWHM), with E in units of MeV. These results prompted us to search for other new base materials for plastic scintillators. In this study, we examined polyethylene terephthalate (PET) bottles, a common source of domestic plastic waste. We demonstrated that a lump of heat-proof PET bottles is fluorescent; moreover, there is excellent compatibility of the fluorescence with the quantum efficiency of typical photomultiplier tubes. This inexpensive source of plastic appears suitable for radiation measurements and as a base material for plastic scintillators. Future studies on the radiation response of plastics should lead to the development of higher performance and more eco-friendly radiation detectors.

Overview of the Liulin type instruments for space radiation measurement and their scientific results

Ionizing radiation is recognized to be one of the main health concerns for humans in the space radiation environment. Estimation of space radiation effects on health requires the accurate knowledge of the accumulated absorbed dose, which depends on the global space radiation distribution, solar cycle and local shielding generated by the 3D mass distribution of the space vehicle. This paper presents an overview of the spectrometer-dosimeters of the Liulin type, which were developed in the late 1980s and have been in use since then. Two major measurement systems have been developed by our team. The first one is based on one silicon detector and is known as a Liulin-type deposited energy spectrometer (DES) (Dachev et al., 2002, 2003), while the second one is a dosimetric telescope (DT) with two or three silicon detectors. The Liulin-type instruments were calibrated using a number of radioactive sources and particle accelerators. The main results of the calibrations are presented in the paper. In the last section of the paper some of the most significant scientific results obtained in space and on aircraft, balloon and rocket flights since 1989 are presented.

Development of polystyrene-based scintillation materials and its mechanisms

Scintillation materials based on polystyrene (PS) have been investigated. Para-terphenyl was employed as a fluorescent molecule (fluor) that functions as a wavelength shifter. A clear increase in photon yield of the scintillation materials relative to the pure PS was observed, which cannot be explained by the conventional theory of scintillation mechanism. Furthermore, the photon yield increased with flour concentration in accordance with a power-law. Here we reveal the emergence of a luminescence of PS-based scintillation materials and demonstrate that their photon yields can be controlled by the fluor concentration.

Enhancement of critical current density and vortex activation energy in proton-irradiated Co-dopedBaFe2As2

The effect of proton irradiation in Ba(Fe0.93Co0.07)2As2 single crystals is reported. We analyze temperature dependence of current density and normalized flux relaxation rate in the framework of collective creep model. Glassy exponent and barrier height for flux creep are directly determined by Maleys method. Our model functions for barrier height and critical current density in the absence of flux creep are explained by the superposition of \deltaTc- and \deltal-pinning. We also approach true critical current density by means of generalized inversion scheme, and the obtained result is in reasonable agreement with our model function. Proton irradiation effect on temperature dependence of current density and normalized relaxation rate can be summarized as doubling of barrier height at the beginning of flux creep.

Mechanism of wavelength conversion in polystyrene doped with benzoxanthene: emergence of a complex

Fluorescent guest molecules doped in polymers have been used to convert ultraviolet light into visible light for applications ranging from optical fibres to filters for the cultivation of plants. The wavelength conversion process involves the absorption of light at short wavelengths followed by fluorescence emission at a longer wavelength. However, a precise understanding of the light conversion remains unclear. Here we show light responses for a purified polystyrene base substrates doped with fluorescent benzoxanthene in concentrations varied over four orders of magnitude. The shape of the excitation spectrum for fluorescence emission changes significantly with the concentration of the benzoxanthene, indicating formation of a base substrate/fluorescent molecule complex. Furthermore, the wavelength conversion light yield increases in three stages depending on the nature of the complex. These findings identify a mechanism that will have many applications in wavelength conversion materials.

Light propagation characteristics of high-purity polystyrene

Organic scintillation materials involve short wavelength light emitted from polymers containing aromatic ring moieties. We have characterized high-purity (>99.9%) polystyrene (PS) as a potential scintillator. It emits ultraviolet light with a 310-nm emission maximum. We demonstrate that the effective refractive index (1.67) for PS is a function of the emission spectrum. Light yield distributions generated by 137Cs and 207Bi radioactive sources were also characterized. The light attenuation length is 41.6 ± 0.5 mm, which is ten times than expected. These results demonstrate that high-purity PS has important light propagation characteristics needed for organic scintillation materials.