Speaker
Description
Studies of close-to-magic nuclei are of fundamental importance for our understanding of nuclear structure. In addition, the properties of N=126 semi-magic nuclei are important in predicting the abundances of elements in the A~195 r-process peak [1].
Neutron-rich nuclei around $^{208}$Pb can be populated in fragmentation and deep-inelastic (multi-nucleon transfer) reactions, with dedicated campaigns being planned at DESPEC (GSI), KISS (RIKEN), AGATA (GANIL), ISOLDE (CERN). Here we report results form a range of recent experiments performed at ISOLDE aimed at this mass region:
(i) $^{208}$Tl was populated from the beta decay of $^{208}$Hg [2]. This single proton-hole single neutron-particle nucleus provides information on proton-neutron interaction. More importantly, it is a unique testing ground for the study of the competition between allowed and first-forbidden beta decay. Both positive and negative parity states are predicted at low excitation energies in $^{208}$Tl. While the population of negative parity states proceeds via first-forbidden beta decay, the population of the positive-parity states is hindered by the very small overlap of the wave-functions (it proceeds via a very small mixing of core-excited configurations).
(ii) information on octupole collectivity was obtained from studies of $^{207}$Tl [3] and $^{20}$Hg [4]. Several collective octupole states coupled to a proton-hole were observed in $^{207}$Tl populated in beta decay. Octupole (and quadrupole) collectivity was studied in $^{206}$Hg using Coulomb excitation. Comparison with shell model calculations give a deeper insight into the composition of the octupole phonon, and consequently a better understanding of the nuclear structure in this mass region.
The talk will present recent results obtained on heavy neutron-rich nuclei, together with further plans for measuring the most exotic N=126 nuclei within the DESPEC project at GSI/FAIR, as well as their astrophysical relevance [1].
[1] N. Nishimura, Zs. Podolyák, D.-L. Fang, T. Suzuki, Phys.Lett. B 756, 273 (2016).
[2] R. Carroll et al., to be published.
[3] T. Berry et al., to be published.
[4] L. Morrison et al., to be published.
