Abstract
β− decay lifetimes are essential ingredients for r-process yield calculations. In N≈126 r-process waiting point nuclei first-forbidden and allowed β decays are expected to compete. Recent experiments performed at CERN/ISOLDE showed that 207,208Hg decay predominantly via first-forbidden decays. In addition, following on a high statistics study of the β+ /EC decay of 208At, it is suggested that the Z>82, N<126 nuclei provide an excellent testing ground for global calculations addressing the competition between first-forbidden and allowed β decays.
Highlights
Half of the nuclei heavier than iron were synthesised in the r process
Since heavy (A>150) r-process path nuclei still cannot be produced in laboratories, yield calculations have to rely on β decay half-lives predicted by theoretical calculations
We present results obtained in β-decay studies on nuclei around 208Pb in experiments performed at the ISOLDE Decay Station at CERN
Summary
Half of the nuclei heavier than iron were synthesised in the r process. This process is based on a succession of neutron captures and β− decays. An ideal atomic nucleus to study the competition between allowed and forbidden β decays should have a small number of both positive and negative parity levels which could be populated in β decay, and they should have simple and well-understood wave functions. None of the existing calculations can reproduce the experimental lifetimes of N≈126 nuclei in the region [12–16] This is not surprising, as even such state-of-the-art calculations are not expected to provide accurate single-particle energies, essential for nuclei close to closed shells where the β-decay strength is dominated by few transitions. It is suggested that the neutron-deficient region “north-west” of 208Pb provides an excellent testing ground for the competition between allowed and first forbidden β-decay calculations [19]. Presently very few global calculations for β+/EC decays exists on this
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