Observation of higher order parametric X-ray spectra in mosaic graphite and single silicon crystals

We have observed up to 8 orders ([ital n]) in the spectra of parametric x-radiation (PXR) in the range 5--40 keV, produced by the interaction of a 90 MeV electron beam with mosaic graphite and single silicon crystals. The measured yields and intensity ratios, [ital I]([ital n][ge]2)/[ital I]([ital n]=1), in graphite are not in agreement with the theory of PXR for mosaic crystals. In comparison, the ratios of intensities in silicon are close to the predictions of PXR theory for perfect crystals. The bandwidths of spectral lines measured in both silicon and graphite are in good agreement with theoretical predictions.