Octupole-Coupled States in 207Tl Populated via β Decay

Abstract

The nucleus 207Tl, a single proton hole away from the doubly magic 208Pb nucleus, was studied in two βγ decay spectroscopy experiments at the ISOLDE Decay Station (IDS), first in 2014 and again in 2016. The aim was to establish a level scheme below Q_β = 4.55(3) MeV, and then assign spin-parities and character to the observed states. From this work an improved level scheme has been established, with increased detail on the transitions between levels than had previously been reported. A total of 26 levels and 110 transitions are reported, including eleven newly observed levels lying at excitation energies 3014, 3197, 3431, 3494, 3570, 3581, 3634, 3644, 3800, 3850 and 3940 keV. Several different methods have been utilised to assert spin-parities for the observed states, including angular correlation analysis for the first time at IDS. States have been characterised by comparison with the results of state-of-the-art large-scale shell model calculations, in order to determine which result from coupling involving the highly collective octupole 3- vibration observed in a number of nuclei in the 208Pb region. Five octupole states (two confident, three tentative) have been assigned in this analysis for the first time. The KHM3Y shell model calculation is found to systematically underestimate the energy of the vibration by around 0.2 MeV, and also systematically underestimates its energy in other nuclei in the region, while overestimating the energies of other non-collective excited states. This is a significant limitation. The discrepancy is explained here as a consequence of omitting certain mixing considerations, and it is suggested that mixing with t=2 excitations is reduced for the collective coupled states. The detailed information on excited state energies, spin-parities, transition branching ratios and β-decay feeding intensities established here will be useful in future efforts to accurately describe nuclei in this region. Finally, during the investigation it was found that the π0g−1_7/2 3474(6) keV state appears not to be populated in the β decay from 207Hg: a clear indicator of the rarely-observed ∆n = 0 selection rule for allowed Gamow-Teller decay. A limit logft > 8.8 is determined from statistical limits of observation, and the result is evaluated in terms of the radius parameters of the orbitals involved and in terms of its potential impact on decay half-lives in neutron-rich nuclei of astrophysical interest.