Cauchois x-ray spectrometer sensitivity variation as 1/R with distance from the source

High resolution molybdenum K-shell spectra were recorded by a Cauchois x-ray spectrometer as a function of the distance from the source. The intensities of the Mo Kα1 and Kα2 spectral lines near 17.4 keV energy were found to vary inversely with distance (1/R) from the crystal to the x-ray source at 0.5, 1.0, 1.5, and 2.0 m. The spectrometer consists of a cylindrically bent transmission crystal and an image plate detector placed on the Rowland circle where the spectral lines are focused. Ray tracing indicates the 1/R intensity variation results from the collection and focusing of the spectral lines on the Rowland circle detector and the divergence of rays in the direction perpendicular to the plane of the Rowland circle. This enhanced signal, compared to non-focusing instruments (1/R2), results in higher signal levels that are particularly important in applications where the spectrometer is far from the source, such as at pulsed-power and laser-driven fusion energy facilities where debris mitigation is an issue.