International Journal of PIXE

Hair is the unique biological material which, because of its growth, reflects the biomedical and environmental history of the subject. Because of convenience in handling and sampling and relatively high concentrations of metals, trace-element analysis of human hair has been applied widely for different purposes.

Although even the measurements of the bulk trace element levels contain some information, the real meaning is contained in longitudinal and radial concentration profiles. This is best achievable by X-ray emission spectroscopy measurements.

A soft X-ray crystal spectrometer designed for chemical state analysis by PIXE is described. A windowless, microchannel plate (MCP) electron multiplier combined with a photodiode array was used as a position-sensitive detector for X-rays diffracted by a plane crystal. The spectrometer was tested for Al Kα and Fe L X-ray spectra induced by 15 MeV N²⁺ ion impact. An energy resolution (FWHM) of 1.5 eV for the 1487 eV Al Kα_{1, 2} line was obtained for metallic Al, and satellite structures of Al Kα arising from multiply ionized states were well resolved. Fe L X-ray spectra were measured for Fe₂O₃ and metallic Fe. Chemical effects were clearly recognized in the Fe Lα/Lβ intensity ratio and in the intensity distributions of their multiple vacancy satellites.

This paper reports on an attempt to investigate how much the sensitivity in PIXE analysis could be improved by optimization of the experimental procedure. The composition and origin of the spectral background in proton excitation of thin plastic foils were studied. It was found that the background could be reduced by good collimation of the beam and efficient shielding of the detector. Minimum detection limits of the order of 0.05–0.2 ppm were obtained. Further improvement with detection limits in the range 0.02–0.03 ppm was achieved by using 5 MeV α-particles for excitation.

A new PIXE facility combined with a small cyclotron (alias Baby Cyclotron) established exclusively for medical use is described. Sufficient amount of 2.9-MeV protons can be extracted from the cyclotron by accelerating H₂⁺-ions with acceleration parameters for 8.3-MeV deuterons. Some technical problems for applying the cyclotron to PIXE analysis, such as elimination of deuteron contamination from proton beams and radiation shielding of the PIXE room during radioisotope production in the cyclotron vault, are reported.

An improvement has been made on a liquid target preparation for an external proton beam PIXE which is regarded as an excellent technique for multi-elemental analysis of a liquid specimen. A 2.5 MeV proton beam taken out to the atmosphere through an exit Kapton foil bombards an acidic solution target. Such bombardment enables the comparison of X-ray yields of a liquid sample with those of standard solutions so that one can determine the elemental concentration in a liquid sample more precisely and easily. The dose limit of a 7.5 µm Kapton foil was found to be 0.8~1.0 mC for a 2 MeV proton beam of 10 ~ 20 nA in intensity. An example is given for the analysis of heavy metals in stream water.

The K-shell ionization cross sections for Ar gas were measured from 100 keV down to 40 keV of H ions. The results are compared with theoretical calculations of PWBABC, CPSSR, and ECPSSR. They indicate that the discrepancy between experimental results and theoretical calculation increases as the H⁺ energy decreases in the range below ~50 keV. This is consistent with previous results on K-shell X-rays for Si.

Elemental concentrations of light and medium weight elements ranging from about 0.2% to less than 1ppm were determined by thick target PIXE in a variety of South African woods from different localities. Correspondence analysis, using the measured contents of K, Ca, Mn, Fe, Cu, Zn, and Rb, resulted in groupings which were type-dependent and locality-dependent. The possibility of substituting one variety for another in the same group, for paper production, is discussed.

Trace elements in thick samples of aluminum metal reference standards were analyzed by means of PIXE and ICP-AES. The samples were SRM1258 and SRM1259 from National Institute of Standards and Technology (U.S.A.) and 1-A, 3, 4-A and 83 from Japan light metal association. The observed PIXE spectrum was converted “into that of a hypothetical thin target and it was analyzed using a relative intensity database for thin target. A good agreement was obtained between the certified and the found amount of the trace elements. The analytical results for SRM1258 and SRM1259 obtained by ICP-AES method also agreed well with the present results confirming the analytical accuracy.