In order to define a more precise standard in the sub-nanometer range, we are planning to trace the wavelength of 57Fe Mößbauer radiation λM ≈ 0.086 nm (photon energy EM = 14.4125 keV) back to the SI system. The relative spectral width of the Mößbauer radiation and thus the relative uncertainty in the wavelength is 3 × 10-13. To achieve this objective, we are developing a combined, tunable Fabry-Perot interferometer (FPI) for Mößbauer radiation and visible light.
Mößbauer radiation can be generated in sufficient quantities by Nuclear Forward Scattering (NFS) at third-generation synchrotron sources such as, e.g., the Advanced Photon Source or the European Synchrotron Radiation Facility. The (1 3 -4 28) lattice planes of two plane-parallel sapphire crystals at 371.6 K serve as mirror in the Fabry-Perot resonator for Mößbauer radiation. Three additional vacuum-metallised metallic mirrors at the crystals' inner sides form the resonators for three optical FPIs. A variation of the mirror spacing in high vacuum makes a direct comparison of the optical wavelength λR ≈ 532 nm of an iodine-stabilised laser with the Mößbauer wavelength λM possible. The linear stage of the displacement mechanism must exhibit a very rectilinear movement, practically without angular deviation (< 3 nrad). The structure which is supposed to make this possible is composed of two flexure hinges which are linked to each other via a cylinder.
The sapphire crystals which are used for the interferometer must be free of dislocation in the zone used. It is thus necessary to proceed to a preliminary selection of suitable crystal areas by X-ray topography. In our Working Group, this can be achieved fast and effectively by means of Berg-Barrett topography.