Quantitative determination of the effect of harmonic component in monochromatised synchrotron X-ray beam experiments

Full text: Harmonic contamination has limited many synchrotron experiments, often without the users realising the magnitude of the problem. A detailed understanding of the beamline optics, detection systems, and interaction cross-sections is necessary to quantitatively asses this effect. We demonstrate a multiple-foil method for the quantitative determination of the fraction of the (333) third-order harmonic in a synchrotron X-ray beam monochromatised by a monolithic silicon (111) channel-cut monochromator. The method can simultaneously be used to test the linearity of the detector response. The method is further illustrated by an investigation of the effect of tuning and detuning of the monochromator on the harmonic and a comparison with the monochromator in its natural, unstressed mode. Finally we show the effect of the harmonic content of the beam on an attenuation measurement of silicon and trace its energy dependence. Non-linearity of detector responses in attenuation measurements due to harmonic contamination, saturation, and inadequate estimation of dark current. The first two are the most common systematics whose effects on attenuation measurements can be significant. The last two are observable at high attenuation level in well-calibrated systems