X-ray diffraction order selection with a prism in DEI (abstract)

Abstract

The refraction angle of x rays as they pass through a refractive prism is proportional to the square of the x-ray wavelength. This was used to discriminate between different harmonic orders, which have different energies, created by a two-crystal 111 monochromator [Z. Zhong, J. Appl. Crystallogr 33, 1082 (2000)]. Refractive prisms inserted between the two monochromator crystals allowed selection, among the 111, 333, 444, and 555 diffractions, in our diffraction enhanced imaging program at National Synchrotron Light Source’s X15A beamline, and facilitated convenient change of sensitivity in our efforts to optimize DEI system for mammography and other radiographic applications. The selection of 111 and 333 refractions were straightforward since the necessary angle of refraction is small. The challenge of selecting 444 and 555 reflections was met by designing an acrylic prism that offered an angle of refraction much larger than 10 μrad and was compact enough to be manipulated in the small space between the crystals. Examples are given on use of the prism for studying the effects of diffraction orders on DEI’s refraction and extinction contrasts. Its modes of operation and limitations are discussed. The principle of harmonics selection through prism and the specific design of the device could be relevant for other synchrotron monochromators that have a harmonics concern or can benefit from change of energy, through selection of harmonic orders, without changing crystals.