Shape coexistence and shape evolution of neutron-deficient bismuth nuclei

The neutron-deficient lead region contains the most well known examples of shape coexistence, to this day. As a result, this part of the Segre chart has been subject to extensive laser spectroscopy studies. In this thesis, the results from the in-source laser spectroscopy investigation of neutron-deficient Bi isotopes, taken at the ISOLDE, CERN facility, will be
presented.

During this experiment the hyperfine structures and isotope shifts of short lived α-decaying states in 187−189,191Bi were measured with the changes in mean-square charge radius and electromagnetic moments extracted for each nuclear state. These charge radii values reveal that unlike the early interpretation that the 9/2− ground states are spherical
in the odd-A Bi nuclei, a trend towards a weakly deformed ground state was observed in 187,189,191Bi. This is in contrast to the ground states of the Tl isotopes, which follow the near spherical trend of the Pb nuclei. The most spectacular result from this investigation was the strong shape staggering observed between the high-spin and low-spin isomeric states of 188Bi. This indicates that the low-spin isomer is much more deformed in shape in comparison to both the high-spin isomer in 188Bi and the 9/2− ground states of the neighbouring isotopes 187,189Bi. This observation implies a large difference in the structure of states in odd-odd and odd-A Bi nuclei near the N=104 midshell. This staggering effect between 188lsBi and 187gs,188hs,189gsBi resembles the long known staggering seen in the mercury isotopes 181−185Hg, with both starting at the same neutron number (N=105). In 2018, Monte Carlo Shell Model calculations were used to confirm the underlying proton and neutron configurations responsible for the shape staggering in the Hg isotopes. It was found that the shape staggering is correlated with the staggering in the shell-model states occupation. It is thought that these configurations are also producing this staggering effect in the Bi chain.