The X-Ray Microbeam Spectrometer of LNS-Catania

A x-ray microbeam spectrometer has been installed at the LNS-INFN (Catania). It basically consists of a x-ray tube, a straight shaped capillary (10 cm length) and a Si(Li) detector. The data are acquired using a portable MCA and they are stored on a standard PC. The spot of the x-ray beam is 400 um wide. It is well known that the capillary deforms the spectral distribution emitted by the x-ray tube. However the output spectrum is necessary to perform the quantitative analysis of the samples. In order to determine this spectrum, scattering measurements of x-rays on a polyethylene sample have been performed. Using the known relations for the elastic and Compton cross sections, and taking into account the air absorption, the capillary spectrum has been calculated. The spectrum determined with this procedure was used to perform a quantitative analysis of six metallic standards. The samples were composed by Cu, Sn and Pb in different concentrations. The standard concentrations have been reproduced with an error of 3%. The device has been used in a quantitative study of petrologic samples. It is well known that light trace elements in minerals are of particular interest in petrologic and mineralogical research. The x-ray microbeam is particularly suitable for the determination of light trace elements since the energy of the beam lies in the range 4-12 keV. As an example we have performed a quantitative analysis on a single olivine mineral whose major and trace elements have been previously determined through the use of an electron microprobe for major elements and proton microprobe for trace elements. Another application of the device was the study of a Russian icon. Pointing the x-ray beam in different points of the icon surface the substratum and the painting colours have been studied. In the substratum the x-ray spectra show the presence of S,Ca and Ag; probably it was composed of a first layer of CaSO4 and a second one of Silver. Above the silver layer the author painted a scene. In the colour case each x-ray spectrum shows the presence of the characteristics elements present in the pigments.