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A monochromator for use at 13.84 keV with a calculated bandpass of 5.2 meV was designed, built, and tested. Tuning was performed by rotating the inner crystal of a pair of nested silicon channel cut crystals. The inner crystal employs the (884) reflection, and the outer crystal employs a collimating asymmetric (422) reflection (dynamical asymmetry factor, b, equal to -17.5). Tests were done with a double crystal Si(111) pre-monochromator situated upstream of the high resolution monochromator and a Si(777) backscattering crystal situated downstream. For this optical arrangement an ideal value of 6.3 meV as calculated by x-ray dynamical diffraction theory applies for the FWHM of the convolution of the net monochromator reflectivity function with that of the Si(777) reflection. This calculated value is to be compared to the value of 7.1 meV measured by tuning the high resolution monochromator. Measured efficiencies were less than ideal by a factor of 3.2 to 4.9, where the larger flux reduction factors were found with higher positron storage ring currents.

proceedings of spie, the international society for optical engineering/proceedings of spie

&lt;title&gt;High-resolution monochromator for inelastic scattering studies of high-energy phonons using undulator radiation at the Advanced Photon Source&lt;/title&gt;