Observation of a Time-Reversal—Like Phase Shift between Competing Multipoles in a Nuclear Gamma Transition

The ratio of the matrix elements for two competing multipoles ($E2\mathrm{}$ and $M1\mathrm{}$) for the 129-keV transition in ${}_{}{}^{191}{}_{}{}^{}\mathrm{Ir}$ was found to have an imaginary part with a phase angle of (-4.7±0.3)×${10}^{-3\mathrm{}}$ rad. This time-reversal-like phase shift may be attributed, at least in part, to quantum electrodynamical effects in the atomic final state. The observed phase shift, however, is somewhat larger than the atomic final-state phase calculated by Goldwire and Hannon.