Rw. Cheary et Aa. Coelho, Axial divergence in a conventional X-ray powder diffractometer. I. Theoretical foundations, J APPL CRYS, 31, 1998, pp. 851-861
A general model for the axial divergence aberration function has been devel
oped for a conventional X-ray powder diffractometer equipped with either in
cident-beam and/or diffracted-beam Seller slits. In this model, the axial d
ivergence function is generated from the geometrical dimensions of the diff
ractometer system, which include the axial lengths of the X-ray source, sam
ple and receiving slit as well as the angular apertures of the incident-bea
m and diffracted-beam Seller slits. The paper describes how the diffractome
ter design and the extent of the axial divergence influence the shape of th
e axial divergence aberration function. The model is developed in three sta
ges starting with analytical profile shapes for a single-ray incident X-ray
beam. The single-ray results are then generalized to represent a line X-ra
y source and a specimen with axial length. In the third stage, the effects
of the Seller slits in the beam path are incorporated by considering them a
s angular selective filters in the incident and diffracted beams. A general
discussion is presented on the axial divergence aberration function with 2
theta for a range of standard instrument configurations. This work forms t
he basis of an accurate fundamental parameters approach to profile analysis
and Rietveld refinement.