The Monolithic Double-Crystal Spectrometer: Theory and Design Principles

The monolithic double-crystal spectrometer (MDCS) is a perfect-crystal device allowing X-ray spectroscopic measurements on an absolute energy scale with an accuracy of better than 1 in 10⁶. This paper presents a detailed analysis of its properties using the dynamical theory of X-ray propagation in perfect crystals. The transmitted wavelength, the transmission window profile, the energy dispersion and the integrated intensity of the transmitted radiation and their dependence on the scanning angle of rotation are derived. The polarization mixing is shown to have a subtle yet important effect on the transmission of the MDCS. An example of a specific MDCS, designed for measuring the Cu Kβ emission spectra, is discussed in detail. The results of the study highlight the advantages and limitations of this device and yield tools for optimizing the MDCS for a wide class of X-ray spectroscopic measurements and for correcting the inevitable, although minimal, distortions introduced by the finite instrumental window of the device.