Calibration of the monochromator bandpass function for the X-ray Rietveld analysis

Abstract

In this paper we propose a fitting procedure to describe the bandpass effect on all x radiation that passes through a focusing graphite monochromator used on the diffracted beam. The proposed bandpass function is:M(2θ)=1/(1+Kmon1sKmon2), withs=(2sinθ)/λ, whereKmon1andKmon2are constants which have been refined by means of a Rietveld analysis, using a physically modeled background (Rielloet al., J. Appl. Crystallogr.28, 115–120). We have investigated two polycrystalline powders: α-Al2O3and a mixture of α and β-Si3N4. The so-obtained bandpass functions for these materials are close enough to conclude that they depend only on the used experimental setup (in the present case the X-Pert-Philips diffractometer with a graphite focusing monochromator). Knowledge of the bandpass function is important to suitably model the Compton scattering, which is a component of the background scattering. The present procedure allows one to avoid the direct experimental determination of the bandpass function, which requires the use of another monochromator (analyzer) and another tube with an intense white spectrum.