*2.2.1 RI-based QEGS spectroscopy: Rayleigh-scattering Mössbauer radiation*

Rayleigh-scattering Mössbauer radiation (RSMR) spectroscopy is a conventional QEGS spectroscopic technique that uses RI as the source of the gamma ray probe. RSMR spectroscopy has been used to study microscopic dynamics in glass formers, proteins and liquid crystals as summarised in a review by Champeney [8]. In this method, monochromatic Mössbauer gamma rays (e.g., from a radioactive 57Co source with an energy *E*<sup>0</sup> of 14.4 keV and an energy width of 4.67 neV) are sent to the sample. A broadening of the energy width of the quasi-elastically scattered

#### **Figure 3.**

*Energy-domain QEGS experimental setup and typical spectrum. (a) Schematic figures of QEGS experimental setups for energy-domain measurement and (b) energy spectra of the resolution function (solid line) and QEGS energy spectra in the presence of measurable dynamics (dashed line).*

*Synchrotron Radiation-Based Quasi-Elastic Scattering Using Mössbauer Gamma Ray… DOI: http://dx.doi.org/10.5772/intechopen.88898*

gamma rays from a sample is detected by an absorption spectroscopy method commonly used in Mössbauer spectroscopy (**Figure 3a**). A transmittance-type energy spectrum is obtained by scanning the velocity *v* of a movable 57Fe gamma ray absorber with a single-line excitation profile. The absorber acts as the energy analyser, since its velocity determines the relative energy shift *E* ¼ *E*0*v=*c via the Doppler effect, where c is the speed of light. RSMR measurements require ample measuring time (at least several weeks) to obtain a spectrum with enough statistics for analysis because the RI source emits gamma rays in all directions, and limited flux is introduced to the sample.
