A FABRY-PEROT-INTERFEROMETER FOR HARD X-RAYS

Reflection and transmission coefficients are calculated of a sequence of N thin parallel crystals diffracting close to normal incidence and separated by thin non‐diffracting gaps. From the study of the particular case N = 2 a new Fabry‐Perot interferometer (FPI) is proposed for hard X‐rays (wavelengths of the order of Ångstroms). It is found that the FPI plates do not need to be carved from a single perfect crystal block but may be grown or cleaved by other methods. This interesting possibility is suggested by two facts. First, it is known that for incidence close to normal various features of dynamical diffraction are much less sensitive to crystal defects and crystal orientation. Second, the existence of a mismatch between the lattices of the two FPI plates may affect but does not qualitatively alter the performance of the FPI. It is shown that the optimal thickness of the plates is of the order of a few microns or less. The plates may be built from a wide variety of materials. The transmission of X‐rays is studied as a function of the energy and direction of the incident photons for various values of the plate thickness, of the gap, of the lattice mismatch, and also for different materials in the gap. The transmission profile exhibits a peak which is spectrally very sharp (about 10−3 eV for low order reflections and even less for higher ones) and which can be easily tuned by changing either the lattice mismatch or the temperature. FPI plates built of materials with low absorption such as graphite seem to be particularly convenient. Several possible applications are suggested. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA