The prospects for an improved absorption correction in electron probe microanalysis

Abstract

It is shown that the absorption correction f (χ) is dependent on two parameters: the mean depth which essentially determines the correction for f (χ)>0·8 and which accounts for variations with incident beam energy, and a shape function that is a function of overvoltage ratio and the mean atomic number of the specimen. The commonly used modified Philibert correction represents the mean depth reasonably well but has a very inadequate shape function. The general effect of the shape function can be demonstrated conveniently by the behaviour of the ratio f (χ)/ f sq (χ) where f sq (χ) is the absorption correction predicted by a square model for the depth distribution of ionization. For many practical purposes the overvoltage variation of the shape function can be neglected, but there is always a substantial atomic number dependence in the range 0·05< f (χ)<0·5. Although there are not sufficient consistent experimental data available on which to base an accurate absorption correction covering the full range of f (χ), possible forms for such a correction procedure are discussed. A simple expression for use in the case of high absorption, f (χ)<0·2, is proposed.