A calculation of the effective absorption coefficient μeff in multi-phase powders is presented. Characteristic in
the calculation is, that the distribution function of the grains
is similar to the correlation function for fluids. It is shown,
that for average grain diameters comparable to the inverse of the
absorption coefficients, the commonly used formulae for μeff (either a linear combination of the contituting μ's or simply μ for a single component) do not hold.The analytical expressions derived for μeff are compared with numerical simulations, which show that, within certain limits, the former give reasonable estimates of μeff. The simulations also show, however, that shape effects can have influence on μeff which are not included (yet) in the formulae.Also shown is that surface roughness can give effects comparable to those of microabsorption, but can be much more sensitive to particle statistics.
