A Michelson interferometer for X-rays capable of high-order measurement

The first Michelson interferometer which was capable of working in the hard X-ray range featured geometric path-length differences of interfering beams of about 120 nm, corresponding to a scan range of more than 750 orders at a wavelength λ ≃ 0.15 nm. The new instrument uses the same principle, i.e. mixing of Laue- and Bragg-case beams diffracted by only one set of Bragg planes of two separate structures of perfect silicon crystals. Through modifying the geometry of beam paths, it has been possible to extend the scan range by a factor of 3.6 and to measure 2700 interference orders for the same wavelength. Six beat nodes of the Cu Kα₁–Kα₂ doublet were recorded in Fourier spectrometric mode. Allowing for realistic reflection curves of Laue and Bragg components, the output spectrum was calculated and found to be in good agreement with the measured spectrum. The influence of thermal, acoustic and vibrational disturbances was studied in detail and special means were developed to cope with them. Applying the know-how obtained in this study in scaling up the present design, it should be feasible to extend the scanning range to ∼40000 orders, which would allow hard X-ray Fourier spectroscopy at spectroscopic resolutions λ/Δλ ≃ 20000.