International Journal of PIXE

Many countries have promoted the multilateral commerce in the global market according to the WTO/TBT (Agreement of Technical Barrier to Trade). One of the most important factors to implement this agreement is that the trade goods should be evaluated once in the export or import country. Therefore the test laboratories for product assessment should construct the new management systems to get the same results of tests anywhere in the world. Even the research laboratories are in the same situation as well as the test laboratories because they must compare their data with others for joint study. They should be encouraged to apply the requirements of ISO/IEC 17025 to their systems.

Nishina Memorial Cyclotron Center (NMCC) has been opened for nationwide-common utilization of positron nuclear medicine (PET) and PIXE since April 1993. At the present time, nearly 40 subjects of PIXE in various research fields are pursued here, and more than 50,000 samples have been analyzed up to the present. In order to perform quantitative analyses of diverse samples, technical developments in sample preparation, measurement and data analysis have been continuously carried out. Especially, a "standard-free method for quantitative analysis" made it possible to perform analysis of infinitesimal samples, powdered samples and untreated bio samples, which could not be well analyzed quantitatively in the past. The "standard-free method" and a "powdered internal standard method" made the process for target preparation quite easier. It has been confirmed that results obtained by these methods show satisfactory accuracy and reproducibility preventing any ambiguity coming from complicated target preparation processes.

A three-detector measuring system making use of a pure-Ge detector combined with two Si(Li) detectors has been developed. The efficiency curve of the pure-Ge detector has been determined easily as relative efficiencies to those of the existing Si(Li) detectors, since this system allows us to analyze a sample with the pure-Ge and the Si(Li) detectors simultaneously under the same irradiating conditions. It is found that detection efficiencies of the pure-Ge detector decrease just above the absorption edge of Ge owing to absorption of X-rays in the dead layer of the detector. Accuracy of the efficiency curve thus obtained was confirmed by analyzing a few samples whose elemental concentrations are known. It is confirmed that a pure-Ge detector can be used in place of a Si(Li) detector for the purpose of analysis of elements Z ≧ 19, since its energy resolution is almost equal to that of a high-performance Si(Li) detector and efficiencies at high energies are far better. Moreover, it becomes possible to detect prompt γ-rays and to analyze light elements such as fluorine, which arouses much interest from the point of view of environmental contamination. Detection limit of fluorine is found to be less than 0.1 ppm for water samples.

In PIXE analysis system and Tandem Accelerator facility (PASTA) of NIRS, we are using Scanning Transmission Ion Microscopy (STIM) method and solid track detector to diagnose the spatial resolution of scanning microbeam PIXE analysis system. These methods are widely used by many microbeam facilities.

The facilities of the NMCC have been opened to other scientists and researchers of universities and institutes in Japan for their academic contribution since 1993. A lot of study subjects of PIXE are accepted every year. We tried to confirm that elemental concentrations in a sample were determined exactly at the quantitative analysis using PIXE facilities at NMCC. The known quantity of a standard liquid sample was dropped and dried on a polycarbonate membrane filter that was used ordinarily as a backing film for an aerosol sample collection. The determined values of elements in targets agree with the values evaluated from the added quantities of the standard liquid samples.

Total suspended particulate (TSP) samples were collected from three areas (commercial, residential and agricultural) in and near Akita City in northern Japan, from May – June 1996 (non-snow-clad period) and January – February 1997 (snow-clad period), over three days with a one-hour sampling interval for each area. The elemental composition and particle shape of TSP samples were determined and/or observed by Particle Induced X-ray Emission (PIXE) and a Scanning Electron Microscope (SEM) combined with Energy Dispersive X-ray (EDX) analysis. In the hourly TSP samples collected during the non-snow-clad period, 23 elements were determined for each area, and Na, Mg, Al, Si, S, Cl, K, Ca and Fe were the major components. On the other hand, 25 elements, excluding As and Rb, were found in the snow-clad period samples, and P, Ga, Nb and Mo were only found in the snow-clad-period samples. Dominant elements, i.e., major components, of the snow-clad period were the same as the non-snow-clad period. Comparing the arithmetic means of elemental concentrations in TSP for the non-snow-clad and snow-clad periods, in the commercial area the elemental concentrations in the snow-clad period were the same or lower than those in the non-snow-clad period, with the concentrations of Na, Mg, Al, Si, S, K, Ca, Ti and Fe in particular being markedly lower. In the residential area the concentrations of Na, Mg and Cl were markedly higher in the snow-clad period than in the non-snow-clad period, while the concentrations of the other elements were the same or only slightly lower. In the agricultural area, the concentrations of Na and Cl were markedly higher in the snow-clad period relative to the non-snow-clad period, and the concentrations of the other elements, except for P, were low. The concentrations of the major elements and Pb were particularly low. Thus, in the residential and agricultural areas the effect of northwesterly winter wind on TSP can be observed by Cl, and it is conjectured that soil particles rising up into the air is inhibited by snow accumulation. Soil particles rising up into the air seems to be inhibited by snow accumulation in the commercial area as well. With the aid of SEM and EDX analysis, aggregates of diesel exhaust particles (DEP), soil particles and small silicon-rich spherical particles were observed in the non-snow-clad and snow-clad period samples for each area. In addition, small aluminum-rich and iron-rich spheres were observed during the non-snow-clad period in the commercial area. Zinc-rich fiber was observed during the non-snow-clad period in the residential area.

We developed mini step samplers with low manufacturing and running costs for application in multi-site air-pollution monitoring. The miniaturization of the sampler was achieved by reducing the suction nozzle size. We tested the samplers with suction nozzle diameters of 2 and 4 mm through simultaneous exposure in the same site. Elemental concentrations of aerosol collected by these samplers were consistent within ±20% during comparison and the sample uniformity did not differ significantly. Sampling with small suction nozzle did not adversely affect aerosol collection. Aerosol samples were collected simultaneously at two sites in our laboratory and in the hall outside for 3 days and analyzed subsequently by PIXE. The time variation of elemental concentrations was high during daytime and low at night time and also during the weekend. Elemental concentrations in the hall were always higher than those in the laboratory. In our laboratory, we change shoes at the entrance and therefore, elemental concentrations inside the lab are lower than in the hall. In a second field experiment, we carried out simultaneous multi-site aerosol sampling during two periods in correlation with meteorological data (wind direction and velocity). It was observed that elemental concentrations of some soil origin elements changed periodically. On the other hand, the concentration of Cu and Zn showed irregular concentration spikes whose pattern showed a variation with the sites. Analysis using the data of wind directions showed that Cu had been transported to the sites from northeasterly direction and that the concentration of Zn was influenced by two big factories nearby. In conclusion, it has been demonstrated that the multi-site sampling system combined with meteorological data is well suited to identify sources of pollution.

Fog water samples were collected at the Akita Hachimantai mountain range in northern Japan from August to September 1997 and from June to September 1988 to 2000 to investigate the pollution mechanism of fog. Ionic components and insoluble substances in fog water were analyzed using ion chromatography and particle induced X-ray emission (PIXE) analysis, respectively. Combining a 72 h back trajectory and principal component analysis (PCA) to chemical analysis data elucidated the effect of the transport course of an air mass on samples insoluble substances and ionic components. When the air mass was transported from northern China to the Hachimantai range, the nss-SO₄^2- was greater than in other cases. That fog contained "Kosa" as an insoluble substance from northern China. Our PCA analysis revealed that the score of PC1 (which was about –0.5) was a measure of containing "Kosa".

A workshop on practical problems in biological applications of micro-PIXE analysis was held in one of the evening sessions at the 8th International Conference on Nuclear Microprobe Technology and Application which was held in Takasaki, Japan, throughout September 8-13, 2002. We summarize the participants' exchange of observations and suggestions on technical aspects of biological sample target preparation and specimen damage due to beam irradiation, which occur in micro-PIXE analysis under vacuum.