Absolute thermal diffuse scattering measurements. I

Consideration is given to the quantities required to convert measured X-ray intensities into absolute units for comparison with theoretical predictions. Attention is focused on the instrumental resolution correction and the factors of incident beam spread and wavelength range, crystal mosaic spread, and detector size and efficiency that contribute to the spread of wavevectors necessarily sampled in a given measurement. A scheme is presented for handling each of these factors with confidence and generally without any assumptions as to their simple analytic form. A set of auxiliary measurements is described which enables a practical amount of data to be collected for the required convolutions. The scheme also provides a simple method for aligning the apparatus in a known way. Some numerical examples are given of the effect of the factors on smearing the scattering from KCl in our apparatus.