Thick-Absorber Line-Shape Analysis and Interference Effects in Mössbauer Absorption Spectra for theE2M177.3-keV Transition ofAu197

A dispersion term which is attributed to interference between internal conversion following nuclear resonance absorption of $\gamma$ rays and the photoelectric effect has been observed in ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ Mössbauer absorption spectra measured as a function of Au-metal-absorber thickness. From line-shape analysis which accounts for absorber thickness, the amplitude of the dispersion term is found to be $\xi =(-4.14\mathrm{}\pm 0.17)\times {10}^{-3\mathrm{}}$. Neglect of the dispersion term leads to an apparent thickness-dependent isomer shift, whereas inclusion of the dispersion term gives isomer shifts properly independent of thickness.