Intensity of the beam passing through the cross-section during refraction on the side of the plate (for more details see refraction) is not equal owing to different length way in the substance till the exit of the plate. It will be fall down from almost 100% (the ray passing through silver) exponentially with the index –(mt/q), where m - linear coefficient of weakening x-radiation in the substance, t- interval from silver in the plane of cross-section, q - angle of the beam sliding inside the plate at approaching to refractive surface on the inside.
Effective section is a section (at the rate T) when index is -1. In that case we have the value T about 100 nm. Diffractive divergency of such a beam at least is half of the beam with sharp borders (sides) getting using diaphragm. It’s possible to change the beam T thick rotating the crystal (changing q angle).

A.G. Touryanskii, I.V. Pirshin, Instruments and Experimental Techniques, Vol.43,No.5, 2000, pp.664-670

"A metod for Formation of Ultrathin X-ray Beams"


0s27=Турьянский А.Г., Пиршин И.В. ПТЭ, № 5 (2000), с. 90-96.

"Метод получения сверхтонких рентгеновских пучков"