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(2 theta)=1/(1+K(mon1)s(Kmon2)), with s=(2 sin theta)/lambda, where K-mon1 and K-mon2 are constants which have been refined by means of a Rietveld analysis, using a physically modeled background (Riello et al., J. Appl. Crystallogr. 28, 115-120). We have investigated two polycrystalline powders: alpha-Al2O3, and a mixture of alpha and beta-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 S-Pert-Philips diffractometer with a graphite focusing manochromator). 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.
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