The breadth and shape of instrumental line profiles for the powder diffraction station 2.3 at the Daresbury Laboratory SRS

The Instrument Resolution Function (IRF) of station 2.3 diffractometer atthe Daresbury Laboratory SRS was measured from a standard powder sample. Besides the high flux and beam monochromaticity, the main feature concerns line profile width and shape. Narrow peaks (FWHM 0.04 to 0.08° 2theta) were observed over a wide angular range , whereas line widths increase significantly at higher angles for conventional diffractometers. The important characteristic of the 2.3 instrument is due to the long slit assembly in the path of the diffracted beam, which limits the peak width and at the same time increases the recorded intensity. However, any slight non-parallelism of the foils rleads to measurable extraneous scattering and a corresponding modification of the line-profile tails. By means of line-profile fitting, this effect can be modelled by means of two satellite peaks, about 0.1° - 0.2° on each side of the Bragg peak. The widths (FWHM) are about 0.2° and the intensity is a few percent of that of the main peak. It was found that these parameters are approximately constant over the range of 2theta considered. As with the main peak, the satellites can be quantified in order to correct data used for Line Profile Analysis (LPA) or in other applications