Atomic screening of nuclear transitions

In the analysis of time-reversal and Mössbauer absorption experiments, it is important to consider atomic processes which interfere with the direct nuclear transition. Interaction of the photon with the atomic electrons causes the radiation to acquire a phase shift, specified by the interference parameter $\xi \left(L\pi \right)$. We present theoretical expressions for $\xi$ and compare our calculated values with experiment. Satisfactory agreement is obtained. In particular, an apparent violation of time-reversal invariance in the 129-keV transition of ${}_{}{}^{191}{}_{}{}^{}\mathrm{Ir}$ is fully explained by these effects.