High spin structure of Ar39 and the FSU cross-shell interaction

Abstract

High-spin states in $^{39}\mathrm{Ar}$ were populated using the $^{27}\mathrm{Al}(^{14}\mathrm{C},\phantom{\rule{4pt}{0ex}}pn)$ reaction at 25.6 MeV. The de-exciting $\ensuremath{\gamma}$ rays were measured with the FSU $\ensuremath{\gamma}$ detector array along with evaporation protons in a Si $E\text{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}E$ telescope. The known high-spin level scheme was extended up to over 11 MeV with a dozen new levels above the neutron decay threshold. The decay pattern appears somewhat atypical for heavy-ion fusion-evaporation reactions. The structure of $^{39}\mathrm{Ar}$ is discussed in terms of the new FSU cross-shell $spsdpf$ interaction fitted to a wide range of nuclei. This interaction has proved quite successful in accounting for the level scheme of $^{39}\mathrm{Ar}$, including the previously suggested fully aligned $\ensuremath{\pi}{f}_{7/2}\ensuremath{\bigotimes}\ensuremath{\nu}{f}_{7/2}17/{2}^{+}$ state and previously discovered analogs of the lowest states in $^{39}\mathrm{Cl}$.