Y. Kawamura

MICROBEAM ANALYSIS OF SINGLE AEROSOL PARTICLES AT TOHOKU UNIVERSITY

A microbeam system has been developed for the analysis of single aerosol particles. Combination of PIXE, RBS and off-axis STIM methods enabled simultaneous analysis for hydrogen to metal elements. Aerosol particles were collected on thin polycarbonate film (~0.3 μm) resulting in good signal-to-noise ratio. Quantitative elemental correlation was measured for single aerosol particles. A total of 270 particles were analyzed and clustered into 4 groups. The analysis system reveals the chemical composition of aerosol particles and is a powerful tool for source identification.

DEVELOPMENT OF AN IMAGE RECONSTRUCTION METHOD FOR MICRON-CT USING PIXE

A prototype of micron-CT for biological research is being developed at Tohoku University. This micron-CT uses a point X-ray source with a spot size of 1μm and an X-ray CCD with 1000×1000 pixels of 8μm×8μm, achieving a spatial resolutions of the order of micro-meter. The event data obtained by the X-ray CCD is statistically poor and the 3 dimensional filtered back projection (3D FBP) algorithm, generally used in image reconstruction of X-ray CT, is not suitable because it is highly sensitive to statistical noise. Hence, we applied the expectation maximization (EM) algorithm for image reconstruction and developed an image reconstruction method using 3D EM algorithm. To confirm the validity of the reconstruction method, we irradiated two hairs inside a micro tube and reconstructed the CT image applying both EM and FBP algorithm on projection data. With 200×200×200 voxels of 4μm×4μm×4μm in the field of view, the computation time was less than 2 mins per iteration on a DELL Precision 650 Workstation 3.2GHz. The resulting EM image showed a better contrast than FBP image, and in the EM reconstructed CT image, we were able to reconstruct the micro tube of 270μm diameter and 45μm wall thickness and to visualize the two hairs inside.

IMPROVEMENT OF THE PERFORMANCE OF TOHOKU MICROBEAM SYSTEM

A microbeam system was installed at Dynamitron laboratory at Tohoku University and is applicable to simultaneous in-air/in-vacuum PIXE, RBS, SE, and STIM analyses, and 3D µ-CT. Insufficient beam brightness of the source and field contamination of the microbeam line restricted spatial resolution. In order to improve the performance of Tohoku microbeam system, optimization and modification of the ion source and microbeam system were performed. By the modification of the system, the beam brightness of the system was increased to 1.0 pA·µm-2·mrad-2·MeV-1 at the half divergence of 0.2 mrad. Considering the brightness and the magnification, obtainable target current will be 200 and 900 pA for beam spot sizes of 1.0 × 1.0 and 2.0 × 2.0 µm2, respectively. The modification of the source meets both the lifetime and the performance. The parasitic field contamination of the system was reduced down to less than 0.5 % by replacing the beam scanner chamber and a part of beam duct. Both resolution and beam currents are sufficient for our applications of in-air/in-vacuum PIXE, RBS, SE, and STIM analyses and 3D PIXE-µ-CT.

PIXE STUDY ON ARSENIC ACCUMULATION BY A FERN (PTERIS VITTATA)

Pteris vittata is a fern reported to be an arsenic hyper-accumulator. To develop the practical application of the fern to a phytoremediation technique, it is necessary to explicate the effective accumulation mechanism. In this study, the arsenic distribution and the elemental correlation in the cellular level were examined in the fronds supplied with arsenate and arsenite separately via xylem vessel using an in-air micro-PIXE system at Tohoku University. The difference in transportation rate between arsenate and arsenite as well as the translocation of elements necessary for plant metabolism was revealed in different tissues of the fronds accumulating arsenic in high concentration. Hence, the in-air micro-PIXE analysis is an effective measure for undertaking phytoremediation research of hyper-accumulator plants.

PIXE STUDY ON ABSORPTION OF ARSENATE AND ARSENITE BY ARSENIC HYPERACCUMULATING FERN (PTERIS VITTATA)

Pytoremediation using an arsenic hyperaccumulator, Petris vittata L., has generated an increasing interest worldwide due to both environmentally sound and cost effectiveness. However the mechanism of arsenic accumulation by this fern is not clear at this time. This study examined the uptake of arsenate (As(V)) and arsenite (As(III)) by a hydroponic culture of Pteris vittata using both in-air submilli-PIXE for different parts of the fern and in-air micro-PIXE for the tissue cells. These PIXE analysis systems used 3 MeV proton beams from a 4.5-MV single-ended Dynamitron accelerator at Tohoku University, Japan. The fern took up both arsenate and arsenite from hydroponic solutions which were spiked with 50 mg of arsenic per litter. Final amount of arsenic accumulation in the fern is 1,500 mg per kg (wet weight) of the plant biomass in arsenite treatment and 1,100 mg per kg in arsenate treatment. Arsenic accumulation was not observed at the root parts of the ferns. The in-vivo mapping of elements by submilli-PIXE analyses on the fern laminas showed the arsenic accumulation in the edges of a pinna. The micro-PIXE analyses revealed arsenic maps homogeneously distributed in cells of the lamina, stem and rhizome of the fern. These results indicate that arsenic, both arsenate and arsenite in a contaminated medium are translocated quickly from roots to fronds of Pteris vittata, and distributes homogeneously into tissue cells of the fern laminas.

PIXE Analysis of Individual Particles in Coal Fly Ash

Consumption of coal is increasing as an alternative for petroleum. During the process, coal fly ash particles are produced and are disposed as an industrial waste. Coal ash contains toxic heavy metals, which leads to a concern about the possibility of leakage into environment. The spatial distribution and chemical form as well as elemental concentration of the toxic elements in the particles are important factors in assessing the leakage into the environment. In this study, we analyzed individual coal fly ash particles with 1 µm spatial resolution by using the simultaneous micro-PIXE/RBS/off-axis STIM system at Tohoku University. Eighty fly ash particles were analyzed. The particles are mainly composed of O, Si and Al and estimated as dioxide. Hydrogen and carbon are not observed in these particles. V, Zn, Sr, Cu, Ni, Mn, Cr and As are contained in the particles. The content of each element is quite different in each particle. These elements are distributed homogeneously. As an exception, Al, Ca, Fe, Zn, As and Zr are distributed on the surface of the particle which might be related to the combustion process.

CHARACTERIZATION OF CORROSION LAYER OF CARBON STEEL BY MICRO-PIXE/RBS ANALYSIS

Wall thinning of piping made of carbon steel in nuclear and thermal power plants affects their life time and safety. Wall thinning results from dissolution of the oxide layer formed on the surface. To get a better understanding of this mechanism, characterization of oxide layer in localized area should be examined. We applied the simultaneous micro-PIXE/RBS analysis to characterization of the oxide layer in localized area. The oxide layer formed under the dynamic condition is not uniform over the 20 × 20 µm2 area. In this case, it is difficult to characterize layer by conventional RBS analysis. The simultaneous micro-PIXE/RBS system could characterize the layer by region analysis. Micro-PIXE analysis was applicable for segmentation of the specific area Then, RBS spectra was extracted from the area and the layer for the corresponding area was characterized. By combining micro-PIXE and micro-RBS, characterization of oxide layer could be done so as to get a better understanding of the corrosion mechanism effectively.

ELEMENTAL PIXE ANALYSIS OF OOLONG TEA

The contamination of heavy metals in food becomes a serious problem. We analyzed oolong tea from different production areas by PIXE using very simple sample preparation and examined trace elements contained in these samples. From the results of this experiment, we could know oolong tea which analyzed in this experiment contains various minerals such as K, Ca, P, S, Cl, Fe, Mn, Cu and Cr but not toxic element of As which detection limit of PIXE was lower than the standard values given by the food hygiene law in Japan.

PIXE ANALYSIS OF UMEBOSHI (DRIED PLUM)

In recent years, the self-sufficiency ratio of food in Japan has gradually decreased and 60% of foods are now imported from abroad. Japanese people rely strongly upon imported food and should take care of their safety, but what is happening now is that soil pollution in country of origin grows into a serious problem and can affect imported food. Umeboshi is one of the traditional pickled food in Japan and very popular. In the case of umeboshi, more than 65% of it is imported. We analyzed imported umeboshi and domestic ones by using in-air PIXE system and submili-PIXE camera at Tohoku University. In this experiment, the concentrations of toxic heavy metals such as As, Cd, Hg and Pb were lower than standard values given by a food hygiene law. Concentrations of elements heavier than Ca show similar trend among each sample. On the other hand, lighter elements than Ca show large differences, which is related to food coloring or processing technique.

APPLICATION OF MICRO-PIXE ANALYSIS TO INVESTIGATE TRACE ELEMENTS IN DECIDUOUS TEETH ENAMEL

The early life environment has widespread consequences for later health and disease. To prevent the disease in later life, the assessment of fetal environment is very important. In Japan, birthweight has fallen rapidly during recent two decades. The reduction of birthweight represents reduced fetal nutrition. Deciduous tooth enamel contains pre- and postnatal enamel and its chemical composition reflects the status of metabolism of trace elements during formation period. Deciduous tooth enamel is considered to be a suitable indicator of trace elements exposure in utero. We applied micro-PIXE analysis to investigate the trace elemental content in deciduous tooth enamel. Two deciduous canines from one healthy Japanese boy were used for this study. The enamel section including pre-and postnatal enamel was prepared for micro-PIXE analysis. Five trace elements (Na, Mg, Cl, Zn, and Sr) were detected in the scanning area of tooth. The distribution profiles of 5 elements were obtained as X-ray maps. The distribution profiles of zinc and chlorine were specific, and showed higher concentration in surface enamel. No elements showed different profiles of X-ray maps between pre-and postnatal enamel in this sample. The results of this study suggested that micro-PIXE analysis would be able to estimate the trace elements in prenatal and postnatal enamel, respectively.