  X-ray back diffraction from monolithic two silicon crystal plates of 25--150
um thick and a 40--150 um gap using synchrotron radiation of energy resolution
deltaE=0.36 meV at 14.4388 keV shows clearly resonance fringes inside the
energy gap and the total-reflection range for the (12 4 0) reflection. This
cavity resonance results from the coherent interaction between the X-ray
wavefields generated by the two plates with a gap smaller than the X-ray
coherence length. This finding opens up new opportunities for high-resolution
and phase-contrast X-ray studies, and may lead to new developments in X-ray
optics.
