PNNL

Abstract

The previously observed 247-keV state in 54Sc, populated in the ß decay of 54Ca, is reported here as a new nanosecond isomer with a half-life of 26.0(22) ns. The state is interpreted as the 1+ member of the pf7/2 ? ?f5/2 spin-coupled multiplet, which decays to the 3+, pf7/2 ? ?p1/2 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 active protons. Notably, it is roughly four times larger than the B(E2; 1/2- ? 5/2-) value in 55Ca. The results, as compared to semi-empirical and ab initio shell-model calculations, indicate (1) a weak N = 34 sub-shell gap relative to N = 32, (2) a large E2 enhancement of Sc over Ca from 1p-1h proton excitations across Z = 28, and (3) empirical effective proton and neutron charges, ep = 1.30(8)e and en = 0.452(7)e, respectively, that are in contrast to reports of ep ˜ 1.1 - 1.15e and en ˜ 0.6 - 0.8e for fp-shell nuclei near N = Z. We demonstrate that these previous reports are erroneous and that, in fact, a universal set of effective charges can be used across the sd and fp shells.