Atomic final state effects and a limit for time-reversal invariance in 191Ir

An experiment has been performed to test the time-reversal symmetry in the emission of the mixed E2 and M1 129 keV γ transition in 191Ir. The phase angle associated with the imaginary part of the ratio of multipole transition amplitudes was measured through observation of the angular distribution of the linear polarization of the γ ray from a 191Os source polarized at low temperature (20 to 30 mK obtained with a dilution refrigerator). A Compton polarimeter was used to measure linear polarization. Interaction of the γ ray with the surrounding atomic electrons is expected to give rise to a phase shift xi indistinguishable from the time-reversal phase eta. In the present experiment we have observed a phase angle (eta+xi) = (-4.8 +- 0.2) x 10-3 for the 129 keV γ ray. This value is in good agreement with the most recent calculations of the atomic final state interaction xi = (-4.3 +- 0.4) x 10-3. Assuming the validity of the calculated xi, a limit Vertical BaretaVertical Bar<10-3 is deduced, consistent with time-reversal invariance