S. Koshio

3D imaging of human cells using PIXEμCT

We report imaging of human lung epithelial cells exposed to cobalt oxide (Co3O4) microparticles using three-dimensional (3D) particle-induced X-ray emission microcomputed tomography (PIXEμCT). The use of energy-selectable quasi-monochromatic low-energy X-rays generated via proton microbeam bombardment led to high-quality images. We also carried out two-dimensional (2D) micro-PIXE imaging. The 3D PIXEμCT imaging data are complementary with 2D micro-PIXE images and the CT value ratios of the cells show that the strong absorption stems from Co3O4. The 2D micro-PIXE analysis provides projection images of the elemental distribution, and this in combination with the 3D PIXEμCT imaging revealed details of the internal distribution of Co3O4, and hence provides insight into the mechanisms of Co3O4 toxicity via intracellular perinuclear accumulation.

Measurement of distributions of cesium and rubidium in rice grains using micro-PIXE for detailed examinations of contaminated food

We used micro-PIXE to investigate the distribution of Cs and Rb in rice grains. We succeeded, for the first time, in measuring the elemental distributions in entire rice grains (with dimensions of 6.25 mm × 6.25 mm) at micrometer spatial resolution. We found that Cs and Rb accumulated in the bran and germ of the rice. The distributions of P, K, Rb and Cs were similar within the rice grain. The concentrations of Cs and Rb in the rice were proportional to those in the soil, as well as to the exposure time. The uptake of Rb was significantly larger than that of Cs. Furthermore, the behavior of Rb was similar to that of Cs in the micrometer-scale regions in plants. It follows that the distribution of Rb can be used to investigate the behavior of radioactive Cs in plants.

Development of a high-current microbeam system

We report on the development of a high-current microbeam system for wavelength-dispersive X-ray micro particle-induced X-ray emission (WDX-μ-PIXE) for chemical state mapping. The microbeam system is composed of two slits and a quadrupole doublet lens mounted on a heavy rigid support. The microbeam system is installed immediately after a switching magnet. A beam brightness of 2.4 pA⋅μm−2⋅mrad−2⋅MeV−1 is obtained at a half-divergence of 0.1 mrad. A beam current of more than 300 pA is obtained for object sizes of 40×10 μm2 with a half-divergence of 0.2 mrad, which corresponds to a beam spot size of 1×1 μm2. The calculated spot size of the beam was 1×1 μm2 and the measured spot size was 1×1.5 μm2. The WDX-μ-PIXE system with the microbeam system is now operational.

Accumulation and localization of alkali elements in Lentinula edodes studied by PIXE analysis

The spatial distributions of potassium and rubidium in Lentinula edodes mushrooms contaminated with radioactive cesium were measured using particle-induced X-ray emission (PIXE) analysis by a beam scanning technique with a submillimeter sized proton beam to study relationships in elemental concentration in the mushrooms between radioactive cesium and other alkali elements. The results of PIXE analysis were compared with the spatial distribution of radioactive cesium in the samples measured by an autoradiographic method. Radioactive cesium was not uniformly distributed and strongly concentrated in the peripheral region of pileus of the mushroom while the elemental maps evaluated from PIXE analysis showed that potassium and rubidium are highly accumulated in the same area as radioactive cesium. The present results suggest a common mechanism of the accumulation of alkali elements including radioactive cesium in Lentinula edodes mushrooms.

Development of a method for analyzing the composition of ambient PM2.5 floating dust particles by micro-PIXE

In this paper, we developed a technique for analyzing individual PM2.5 particles using micro-PIXE. PM2.5, a designation for extremely small particulate matter (PM) in the air, has recently become the center of attention because high levels of PM2.5 were recorded in parts of western Japan, especially Fukuoka Prefecture, in January 2013. For a better understanding of their formation mechanism, analysis of individual particles is indispensable. We collected PM2.5 on a 4 μm thick Prolene foil using a multi-nozzle cascade impactor at Fukuoka Women’s University, Fukuoka, Japan. Its elemental analysis was carried out using a micro-PIXE system at Tohoku University. Although elemental concentration ratios of scanned areas were similar, those of individual particles were quite different from each other. Elemental concentration ratios for individual particles were categorized into five groups, indicating that the PM2.5 particles came from at least five different sources. Although elemental concentrations obtained by averaging over single particles formed in different processes will lose detailed information, we were able to derive comprehensive elemental compositions of individual PM2.5 particles using our novel technique. The individual particle analysis technique for PM2.5 will provide important information to identify pollution sources and particle formation mechanisms.

Evaluating radiocesium retention ability of root-mat horizon using micro-PIXE analysis

A lot of radiocesium had been deposited onto pastures and grasslands following Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. More radiocesium was accumulated in root-mat horizon than in both above ground plant bodies and mineral soils. In this study, factors causing higher radiocesium concentrations in root-mat horizon were evaluated by the addition of stable cesium solution and particle-induced X-ray emission (PIXE) analysis. Results suggest that adsorption onto root surfaces played a significant role in Cs accumulation in root-mat horizon. Furthermore, absorption of Cs was key to its long-term preservation. The adsorption of Cs by clay minerals also contributed to its retention. A slow water infiltration rate may also affect the enrichment of radiocesium in root-mat horizon. Based on these results, it is concluded that both biotic and abiotic factors contributed to the effective retention of radiocesium in root-mat horizons following the FDNPP accident.