Mossbauer transmission spectra at pure nuclear dynamical diffraction in Bragg geometry

Abstract

Mossbauer spectra of the reflection from and the transmission through a thin nearly perfect ⁵⁷FeBO₃ single-crystal platelet were measured at different angular positions in the vicinity of a Bragg angle. In the transmission spectra an angular-dependent shift of the nuclear resonance dips was observed in common with a shift of the resonance peaks in the reflection spectra. In addition, a pronounced asymmetry of the transmitted intensity above and below the resonances was observed. The angular dependence of the transmission spectra reveals that in transmission under Bragg reflection conditions the coherent channel (diffraction) becomes as important as the incoherent channels (internal conversion and incoherent scattering). The Borrmann effect induced by nuclear resonance diffraction was observed for the first time.