X-Ray-Emission Microanalysis of Rock-Forming Minerals I. Experimental Techniques

Abstract

In order to provide a basis for calibration of X-ray-emission analyses, existing theories and experimental measurements on the generation, absorption, and fluorescence of X-rays produced by electron bombardment of mixed targets are reviewed and compared with new measurements for a variety of targets, principally common silicates. Theoretically, the efficiency of X-ray generation should depend strongly on the mean atomic number of a mixed target, and its variation is commonly described by a so-called atomic-number factor. Using hard radiation for which the absorption is trivial, measurements have shown that the Philibert factor (1 + h) varies too little with mean atomic number. A new empirical factor, fits the data to within 2 per cent, except for close to (is incident electron accelerating voltage in kilovolts, is ionization voltage for K-electrons, {\langle}Z{\rangle} is mean atomic number of target weighted according to mass concentration, {\langle}Z/A{\rangle} is a similar average for ratio of atomic number to atomic weight, R is Green's backscatter coefficient to express proportion of potential ionization energy retained in target). For soft radiation, empirical absorption curves have been derived for simple and complex oxides on the assumption that the above efficiency-of-generation factor is valid. These curves differ markedly from those suggested by measurements on single-element targets, and it is recommended that the new curves be used for analyses in which the same experimental conditions are used. It is concluded that analyses of major elements in common minerals can be carried out to a relative accuracy of 1-5 per cent by suitable choice of standards and experimental conditions; such conditions are described.