j-1 anomalous behaviour of the medium-mass odd-A Ag nuclei

Stefan Lalkovski (Sofia University, Bulgaria)

The Silver nuclei are three holes away from the magic Sn isotopes. Their structure should be well understood within the Shell model, where the three-particles or holes configurations are a straightforward application and among the textbook examples. As such, the structure of the positive-parity states in the semi-magic silver nuclei is well described by a πg_9/2^-3 configuration, split by the two-body interaction. Thus, the typical πg_9/2^-3 spectrum contains levels with Jπ from 3/2+ up to 21/2+, where the lowest-energy state is 9/2+, similar to the total angular momentum j. Away from the semi-magic silver nuclei 97Ag and 129Ag, 7/2+ becomes the lowest-lying positive-parity state. This energy swap of the 9/2+ and 7/2+ states is known as “the j-1 anomaly” and happens in all silver nuclei with more than 6 valence neutrons, or neutron holes. The phenomenon is observed already in the 60s and attributed to an unusually strong Q.Q interaction. Over the years various theoretical approaches were tested versus the Ag data, but with limited success. The present talk will outline the background of the problem and discuss the Ag features that emerge from the most recent data.