The defocussed X-ray interferometer

Abstract

The influence of a different distance between beam splitterS and mirror crystalM on one hand and analyser crystalA on the other (“defocus” Δx) on the structure and the contrast of the fringe pattern of a Laue-case interferometer is investigated both experimentally and theoretically. A nonzero Δx generates a typical additional pattern which is superimposed on any ordinary interference pattern to be studied with the instrument. Δx≠0 also impairs the fringe contrast. In order not to loose all contrast ¦Δx¦ must be kept below 25 to 100 microns, the exact value depending on the wave length and the reflection used. The influence of other misalignments on the pattern is also assessed. The agreement between theory and the experiments is very good.