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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.

The Monte Carlo model for electron scattering described in an earlier paper has been used to calculate the absorption and back-scattering corrections met in electron-probe x-ray microanalysis. Although agreement with experimental data is on the whole good, the calculated values for the correction factors are not sufficiently accurate for general use. However, in the case of light element analysis where very high absorption corrections are needed, the corrections calculated from Monte Carlo data are the best available at present. Values for the back-scattering correction factor R calculated for incident beam angles 22·5° and 45° to the normal surface are given.

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