International Journal of PIXE

A microbeam analysis system has been developed at Tohoku University for biological applications. Spatial resolution of less than 1 μm has been achieved with a beam current of ~40 pA. In microbeam analysis of biological specimens, simultaneous measurement of structural and elemental properties is very important. Our system is applicable to simultaneous in-air/vacuum PIXE, RBS and STIM analyses. Typical results of biological application are shown.

We recently developed and reported a three-detector measuring system making use of a pure-Ge detector combined with two Si(Li) detectors. The efficiency curve of the pure-Ge detector was determined as relative efficiencies to those of the existing Si(Li) detectors and accuracy of it was confirmed by analyzing a few samples whose elemental concentrations were known. It was found that detection of fluorine becomes possible by analyzing prompt γ-rays and the detection limit was found to be less than 0.1 ppm for water samples. In this work, a method of quantitative analysis of fluorine has been established in order to investigate environmental contamination by fluorine. This method is based on the fact that both characteristic x-rays from many elements and 110 keV prompt γ-rays from fluorine can be detected in the same spectrum. The present method is applied to analyses of a few environmental samples such as tealeaves, feed for domestic animals and human bone. The results are consistent with those obtained by other methods and it is found that the present method is quite useful and convenient for investigation studies on regional pollution by fluorine.

Since biological or biomedical studies by nuclear microscopy are limited by irradiation damage, we developed an in-air system in order to cool down the biological samples by convection. The behavior of the beam extraction window is an important parameter of our system. Then, we investigated the behavior of a polyethylene teraphtalate (PET) thin-film used as beam extraction window during in-air irradiation by STIM (Scanning Transmission Ion Microscopy) and RBS (Rutherford Backscattering Spectrometry) methods. We found that it is better to flow helium than air atmosphere because in-air irradiation enhances the degradation of the PET film.

Rat basophilic leukaemia cells (RBL-2H3) were analyzed by in air micro-PIXE analysis to investigate the relationship between the distribution of elements and histamine release in immune system. RBL-2H3 cells were sensitized with monoclonal immunoglobuline E(IgE) and stimulated with antigen. In the stimulated cells, it is understood that the level of Ca²⁺ increases in cytoplasm and then the degranulation or the release of chemical mediators occur. In this study, stimulated RBL-2H3 cells with antigen and control cells were analyzed. After the stimulation, the Na in the cells was increased. Moreover some stimulated cells showed the characteristic distribution of sulfur that is not accompany with the distribution of phosphorus. In the same measurement, the aggregations of Na, Ca, Fe, and Zn were observed.

Phytoremediation is a technology for remediation of contaminated soils. In this study, we used a submilli-PIXE camera to analyze plants and soils from a shooting range. Some heavy metals were rapidly and easily detected in these samples. Element dot-maps of the plant show Cu and Pb accumulated in the epidermis of subterranean stems and the venation of leaves. From these findings, it is possible to map the distribution of heavy metals and to detail their location in the plant, using the submilli-PIXE camera. PIXE analysis is an effective tool for undertaking phytoremediation research.

We attempted in-air-PIXE analysis of SPM using spot samples on a filter-tape mounted in an automated beta-ray absorption mass monitor. Al, Si, S, Fe and Zn, etc., which are of interest for identifying the behavior and characteristics of SPM, were detected on the SPM spot samples on a glass-fiber filter-tape, but the peaks of these elements were nearly identical to those of blank glass-fiber filter-tape. As such, it was difficult to detect elements present in SPM from the X-ray spectra of the spot samples. On the other hand, in the case of a PTFE membrane filter-tape, the S peak was distinct and the Fe peak was also clear, and peaks for elements Al, Mn and Zn, etc., were also confirmed. Consequently, if a method for determining quantity is established, direct multi-elemental analysis by in-air-PIXE of high time-resolution SPM spot samples collected on a PTFE membrane filter-tape mounted in a SPM monitor will be possible.

Thick-target PIXE analysis was applied to the study of electrokinetic soil remediation technique. In order to simulate radioactive soil contamination by ¹³⁷Cs, salt of stable cesium was mixed with soil samples. These soil samples were subjected to electrolysis for up to 36 hours with a field gradient of ≈ 1-3 V/cm. After the electrolysis, we measured the distribution of Cs concentration in the soil along the electrolysis cell by PIXE analysis. LX-rays of cesium were used for the analysis. The Cs concentrations in the drain water sampled from the cathode well were also evaluated. After the electrolysis the migration of cesium from the anode to the cathode was clearly observed. Water supply into the anode well enhanced the removal rate. We found that the main driving force of the migration of Cs⁺ observed in this work was not electrophoretic migration, but electroosmotic flow in the soil samples. Owing to the spectral interference by major metallic elements in the soil, the minimum detectable concentration of cesium by the present method was limited to ≈ 800 ppm.

The aim of this study is to develop a method for monitoring water pollution caused by dissolved sulfides and other harmful elements by means of PIXE offering the advantage of multielemental nature, high sensitivity and speed of analysis for a wide variety of samples. Sulfide ions dissolved in an aqueous sample were converted to insoluble CuS compounds and then collected on a thin organic filter suitable as a backing foil for PIXE measurements. The standard method for scavenging CuS precipitate is founded on an investigation of the pH-dependence of the recovery of dissolved sulfides and the calibration curve covering the concentration range from 10 to 1000 ppb. The recovery-factor of CuS showed a maximum value at around pH 8 although it changed with the intrinsic pH of samples. The recovery factor for pH adjusted to 7.50 was kept constant in a wide concentration range of sulfide ions from 10 to 1000 ppb. The concentration of tens of ppm sulfur compound was detected in both drain water from the industrial waste disposal and in peripheral agricultural water, but the fraction of dissolved sulfide was only about 1%. The low concentration of dissolved sulfides was discussed from the viewpoint of the oxidation condition originated from the soluble and insoluble components detected in samples by PIXE analysis.

In anoxic marine sediments sulfur is considered to be essential to the formation of humic substances. It is also estimated that sulfur may play a crucial role in the degradation of wood buried in marine sediments. In order to confirm this prediction, we observed the tissue of fossil wood from the marine clay aged 1.1 million years by polarizing microscopy and SEM before the determination of its sulfur content by PIXE and EDXA, and its distribution by μ-PIXE. Results show that the sample contained about 6000 μg/g of total sulfur and tracheid cells with birefringence, which have probably preserved the original wood components contain less sulfur compared to those without birefringence, which are heavily degraded. This might suggest the participation of sulfur in the degradation or the formation of humic substances occurred in wood tissue during diagenesis.