ABSFAC - A PROGRAM FOR THE CALCULATION OF THE ABSORPTION DURING SCATTERING IN MULTIFACETED CRYSTALS AND SIMILAR SAMPLES

ABSFAC is a FORTRAN program that rapidly and efficiently calculates th e absorption factor, T, in multifaceted convex crystals and in other m aterials whose shape can be modeled by a convex polyhedron. The progra m also calculates derivatives of T with respect to small outward movem ents of the polyhedron faces and the mean path length for the scattere d rays. Although written for X-ray scattering, the programme is releva nt to other scattering provided the underlying assumptions are valid, The program uses a mathematically exact method for calculating quantit ies from given sample shape data, The size of any numerical error (typ ically of the order of 1 in 10(14)) that arises during the calculation is estimated by the program and is much less than the uncertainty in specifying the scattering geometry, Excellent accuracy is obtained for a very wide range of absorption coefficients. An option within the pr ogram permits storing of the calculated geometrical factors for any on e orientation of the sample and scattering vector, so that the absorpt ion can be rapidly recalculated for different values of the absorption constant. This is particularly relevant for energy dispersive scatter ing, The program can also calculate the absorption in samples that are only partly illuminated by the incident beam. (C) 1998 Published by E lsevier Science B.V.