Stephanie Lyons

In an experiment performed at the Facility for Rare Isotope Beams (FRIB) using the FRIB Decay Station initiator, 15 new half-lives of isotopes near ^{54}Ca were measured. A new method of extracting lifetimes from experimental data, taking into account the unknown β-delayed neutron emission branches of very neutron-rich nuclei, was developed to enable systematic uncertainty analysis. The experiment observed a dramatic change in the half-life systematics for the isotopes with neutron number N=34. Beyond N=34, the decline of nuclear lifetime is much slower, leading to longer than anticipated lifetimes for near-dripline nuclei. State-of-the-art shell-model calculations can explain the experimental results, revealing the imprint of shell effects and the need for modification of single-particle neutron states. The results from a newly developed quasirandom-phase approximation model with potential for making global predictions were also tested against the experimental results and good agreement was found.

The 247-keV state in ^{54}Sc, populated in the β decay of ^{54}Ca, is reported here as a nanosecond isomer with a half-life of 26.0(22) ns. The state is interpreted as the 1^{+} member of the πf_{7/2}⊗νf_{5/2} spin-coupled multiplet, which decays to the 3^{+},πf_{7/2}⊗νp_{1/2} ground state. The new half-life corresponds to a pure E2 transition with a strength of 1.93(16) W.u., providing the most precise, unambiguous B(E2) value in the neutron-rich fp region to date for a nucleus with valence protons above Z=20. Notably, it is roughly 4 times larger than the B(E2;1/2^{-}→5/2^{-}) value in ^{55}Ca. The results, as compared to semiempirical and ab initio shell-model calculations, indicate (1) a weak N=34 subshell gap relative to N=32, (2) a large E2 enhancement in Sc as compared to Ca due to 1p-1h proton excitations across Z=28, and (3) empirical effective proton and neutron charges e_{π}=1.30(8)e and e_{ν}=0.452(7)e, respectively, that are in contrast to reports of e_{π}≈1.1-1.15e and e_{ν}≈0.6-0.8e for fp-shell nuclei near N=Z. We demonstrate that these reports are erroneous and that, in fact, a universal set of effective charges can be used across the sd and fp shells.