USING LINE-SHAPE TO PRECISELY DETERMINE RECOIL-FREE FRACTION - APPLICATION TO TUNGSTEN

The analytic description of Mössbauer line shape based on the Fourier-transform method [1] makes it possible to precisely determine the recoil-free fraction, f, of a single absorber over a wide temperature range with a single measurement at each temperature. Most measurements of recoil-free fraction have been done with a Lorentzian representation of the Mössbauer line with semi empirical correction factors of uncertain precision. Our method, based on line shape rather than area, is exact for absorbers of any thickness, limited only by the number of terms taken in the expansion. In fact, our exact line-shape description makes it advantageous to use very thick absorbers. As an example, we report values for an absorber of effective thickness 12. The resonance thickness number of the absorber is an intrinsic parameter, so that a fit yields this quantity directly. The correlations that arise between certain variables may be reduced by fitting, simultaneously and with known constraints, spectra taken with different absorbers or with one absorber at various temperatures. Using this technique, we report precise values for f(T) in tungsten using the 46.5 keV transition in183W at T=297 K and 1067 K; these values are 0.300±0.009 and 0.0123±0.0002, respectively. © 1990 J.C. Baltzer A.G. Scientific Publishing Company.