ENHANCED NUCLEAR RESONANCE SCATTERING IN DYNAMICAL DIFFRACTION OF GAMMA-RAYS

Abstract

ibstract. The angular and energy dependence of the dynamical diffraction of gamma rays was studied experimentally in the symmetric Brag case. Mossbauer diffraction spectra were measured with high angular resolution in a pure nuclear Bragg reflection of a nearly perfect 57FeB0, single crystal. In the spectrum taken at the exact Bragg position, very broad hyperfine resonance peaks were observed. In the spectra just above and below the Bragg angle the reflection peaks were much smaller and were shifted to energy positions above and below resonance respectively. The results were interpreted by means of the dynamical theory and compared with computer calculations. The measurements represent the first direct observation of the enhancement of nuclear resonance scattering in the case of a thick crystal reflection. Furthermore the angular dependence of the resonance scattering by a perfect single crystal has been measured for the first time. Soon after the discovery of the Mossbauer effect the dynamical diffraction of gamma radiation by a regular ensemble of resonant nuclei was considered theoretically. First the diffraction of gamma rays incident on a thin perfect single-crystal film near the Bragg angle was studied (Trammell 1961). It was found that in this case the halfwidth of the resonance scattering is larger than for an isolated nucleus and increases with the thickness of the crystal film. The energy broadening of the resonance scattering was called the enhancement effect. A general treatment of the dynamical diffraction of resonant gamma rays in thick perfect single crystals was given for the hue case (Afanas’ev and Kagan 1965) and also for the Bragg case (Kagan et al1968). In the latter paper it was shown that in the case of a Bragg reflection by an infinitely thick perfect crystal, under certain conditions, an angular position can be found where the reflection becomes total in spite of the presence of a strong inelastic channel and independent of the energy distance from resonance. In the region around this angular position the reflectivity falls off rapidly. Thereby the reflectivity remains unchanged when the energy distance from resonance and the angular deviii:ion are both changed in sign. Also the Bragg reflection by thin perfect crystals was treated in a general way (Hannon and Trammell 1969). The contributions of other authors may be found summarised in review articles (Belyakov 1975, Champeney 1979). Recently the problem of filtering of