S. Tsuboi

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.

In-Vivo Elemental Analysis by PIXE-μ-CT

We have developed “micron-CT”, using micro-PIXE for in-vivo imaging. This system comprises an X-ray CCD camera (Hamamatsu photonics C8800X9) with high resolution (pixel size: 8×8μm2, number of pixels: 1000×1000) and an X-ray-point-source with a spot size of 1.5×1.5μm2 which is generated by irradiation of a microbeam on a pure metal target. Thus we can acquire projection data with high resolution. The sample is placed in a small diameter tube and is rotated by a stepping motor. The 3D images were reconstructed from the obtained projection data by using cone-beam CT reconstruction algorithm. X-ray spectra produced by heavy charged particle bombardment, exhibit a much smaller continuous background compared to electron bombardment. Therefore, X-rays produced by ion beam can be used as a monochromatic and low energy X-ray source. The feature is very effective to investigate small insects. Moreover we can get elemental distribution image of object by choosing appropriate characteristic X-rays corresponding to the absorption edge. On the other hand, the conventional X-ray CT, in which continuous X-rays are used, provides images of the electron density in the object. Using this system, we were able to get 3D images of a living ants head with 6 μm spatial resolution. By using Fe-K-X-rays (6.40 keV) and Co-K-X-rays (6.93 keV), we can investigate the 3D distribution of Mn (K-absorption edge = 6.54 keV) in an ants head.

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.

CHARACTERIZATION OF AEROSOL PARTICLES IN A MECHANICAL WORKSHOP ENVIRONMENT

Indoor aerosols are directly affecting human lives. Especially aerosols in workshops, factories, and laboratories, where many chemical substances are used in treatment and production processes, might contain toxic elements: special care must be taken to alleviate air pollution and assure a clean breathing environment for the workers. For this study, size segregated aerosol particle sampling with a cascade impactor was performed in the machine workshop of Jožef Stefan Institute. The samples, collected during weekdays and weekend were analyzed with a microbeam facility at Tohoku University. Bulk PIXE analysis with scanning over the whole sample area was conducted along with multimodal microanalysis with microscopic scanning. Using bulk analysis, high concentrations of Pb and Ba were detected on weekend days, which was related to the removal of an old white paint from the furniture. On weekdays, concentrations of W and of soil origin elements increased, probably because of the machine operations and worker movements. At the same time high concentration of sulfur was detected. A microscopic multimodal analysis shows that it stems from a lubricant oil vapor. The combination of bulk and microanalysis of the size selected samples is an effective approach to aerosol characterization in the working environment.

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.

µ-CT IMAGES OF THE EGG OF DROSOPHILA

We are developing a µ-CT (micro computed tomography) which enables to observe the interior of living small insects. In this study, we applied the µ-CT to get CT images of the egg of drosophila, since the drosophila is applied to various basic studies, such as gene research. The interior of the living egg was observed and it was uniform. In the case of the dried egg, the shell and the interior structure were confirmed. This result comes from the growth of the living egg.

PIXE Analysis of Atmospheric Aerosols from Asian Continent

A large amount of yellow sand dust from Asian continent transported to Japan by air, affecting traffic, climate and human health. The yellow sand is known as a natural phenomenon, but it is mixed with anthropogenic aerosols over the industrial area in East Asia and causes environmental pollution. In addition, atmospheric turbid phenomena are occasionally observed in spring of recent years over western part of Japan. This phenomenon is caused by the increase in the amounts of fine particles. For a better comprehension of such turbid phenomena, we collected aerosols at Nagasaki. At the same time, particle number concentrations were observed by OPC. The samples were analyzed by an in-air PIXE system at Tohoku University. Time series of elemental concentrations and particle number concentration were obtained. It found that the yellow sand dust phenomenon occurred when coarse particles reached high concentrations, and elemental concentrations of Si, Ca, Fe, Pb and Hg showed similar variations along with that of coarse particles. On the other hand, when fumes or mists were observed, concentrations of fine particles were also correlated with concentrations of S and Zn.