Abstract
The 247-keV state in 54Sc, populated in the π½ decay of 54Ca, 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 πβ’π7/2βπβ’π5/2 spin-coupled multiplet, which decays to the 3+,πβ’π7/2βπβ’π1/2 ground state. The new half-life corresponds to a pure πΈβ’2 transition with a strength of 1.93(16) W.u., providing the most precise, unambiguous π΅β‘(πΈβ’2) value in the neutron-rich πβ‘π region to date for a nucleus with valence protons above π =20. Notably, it is roughly 4 times larger than the π΅β‘(πΈβ’2;1/2ββ5/2β) value in 55Ca. The results, as compared to emiempirical and ab initio shell-model calculations, indicate (1) a weak π =34 subshell gap relative to π =32, (2) a large πΈβ’2 enhancement in Sc as compared to Ca due to 1β’π β1β’β proton excitations across π =28, and (3) empirical effective proton and neutron charges ππ=1.30β’(8)β’π and ππ=0.452β’(7)β’π, respectively, that are in contrast to reports of ππ β1.1β1.15β’π and ππβ0.6β0.8β’π for πβ‘π-shell nuclei near π =π. We demonstrate that these reports are erroneous and that, in fact, a universal set of effective charges can be used across the π β’π and πβ‘π shells.
