An experiment of Rayleigh scattering of Moessbauer radiation on pyrolytic graphite

Due to the extraordinary high energy resolution of the Moessbauer effect, the Rayleigh scattering of Moessbauer radiation can be used to separate elastically and inelastically scattered radiation from crystals not containing Moessbauer isotope. In ordinary X-ray diffraction experiments there is no possibility to separate experimentally the contribution of the inelastic intensity and the corrections are performed using theoretical models based on different approximations. The equipment for Rayleigh scattering of Moessbauer radiation is similar to those used in X-ray diffractometer with a Moessbauer source instead of a X-ray tube. It consists of three main parts: a goniometer, a Moessbauer spectrometer and a personal computer. The personal computer controls the orientation of the sample, the Doppler movement and realizes the acquisition of the pulse height spectra and Moessbauer spectra. We present here the dependence on the order of reflection of the elastic and inelastic fractions of the Moessbauer radiation diffracted on pyrolytic graphite. All the measurements have been performed at room temperature in symmetric Bragg geometry. The diffractometer has been driven in (θ, 2θ) manner. Using the procedure described previously, the separation of the gamma rays which are elastically scattered by the sample from those which undergo inelastic scattering was performed. The diffraction curves of 14.4 keV radiation scattered on C(002) for θBRAGG1 7.381 angle, θBRAGG2 = 14.880 angle, θBRAGG3 = 22.660 angle, corresponding to the first three maxima of diffraction, have the same width but different intensities because, as the scattering angle increases, the ratio of coherent to non-coherent radiation diminishes. The Moessbauer spectra of 14.4 keV radiation scattered on C(002) oriented at Bragg angles for the three orders of reflection examined have the same width but different resonance effects. The corrected values (εC) are given. The effect measured without scatterer, in a usual transmission Moessbauer experiment, corrected for background, was ε0 = (63.5 ± 0.6)% and gives the fraction of recoilless quanta that undergo resonant absorption. Due to the fact that the effect is proportional to the fraction of recoilless gamma photons which are scattered elastically by the sample, one can see that the differences between εC and ε0 are due to the inelastic scattering of Moessbauer radiation (creation and annihilation of phonons in crystals). The quantities fel εC/ε0 and fin = 1 - εC/ε0 are the fractions of the incident radiation that are scattered elastically and inelastically, respectively, by the sample under investigation. The numerical values obtained for these quantities at θBRAGG corresponding to different orders of reflection for C(002) are given. We found an inelastic fraction for C(002) smaller than that reported for graphite, due probably to high energy optical phonons in pyrolytic graphite. (authors)