Mossbauer effect overlap

Before the discovery of the Mossbauer effect, many attempts had been made to improve the chances for y ray resonance absorption. The idea was to make the overlap between the emission and absorption lines larger by setting the emitter/absorber nuclei in motion (in order to compensate for the energy loss due to recoil) either mechanically (Moon 1950) or thermally (Malmfors 1953). 

The nuclear y-resonance effect in ° Ni was first observed in 1960 by Obenshain and Wegener [2]. However, the practical application to the study of nickel compounds was hampered for several years by (1) the lack of a suitable single-line source, (2) the poor resolution of the overlapping broad hyperfine lines due to the short excited state lifetime, and (3) the difficulties in producing and handling the short-lived Mossbauer sources containing the Co and Cu parent nuclides, respectively. 

How important is this effect for micas It is apparent from calculations presented by Rancourt (1989) and data in Hargraves et al. (1990) that small spectral contributions are always overestimated when thickness effects are considered. This conclusion has considerable implications for Mossbauer spectra of micas because the Fe peaks are often small relative to those of Fe, and therefore vulnerable to exaggeration by thickness effects. For example, consider a sample with an average value (for micas, cf Rancourt et al. 1994a) off= 0.5, and an ideal absorber thickness of 3.3 x 10 Fe/cm. If a doublet in this sample has an area of 10% of the total area, then its true area when corrected for thickness is 7% (see Fig. 5 in Rancourt 1989). An apparent area of 30% would have a true area of 26%, and so on. These error estimates are only approximations, as they apply to fits using Lorentzian lines that are well-separated the problems are aggravated in situations where peak overlap occurs (as is frequently the case with micas ). 

The Mossbauer spectrum, a plot of the relative transmission as a function of Doppler velocity, shows maximum resonance and therefore minimum relative transmission at relative velocities where emission and absorption lines overlap ideally (cf. Fig. 4). At high positive or negative velocities the overlap of emission and absorption lines is negligible, the resonance effect being practically zero, i.e., the relative transmission yields the base line. 

The interpretation of in-field Mossbauer spectra which exhibit rather broadened and overlapped lines, remains rather ambiguous because the presence of structural defects in the core of the particle cannot be clearly distinguished from the surface effects. 

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