Low-lying level structures in the transuranic n -rich Z=93 nuclei Np243 and Np244

Abstract

Low-lying level structures of the neutron-rich transuranic $Z$=93 isotopes, namely, odd-mass $^{243}\mathrm{Np}$ and its odd-odd neighbor $^{244}\mathrm{Np}$, are investigated with a view to characterize the respective ground state (gs) and to identify any long-lived isomer (LLI). For $^{243}\mathrm{Np}$ levels, we critically examine all the available information from three sources, namely, experimental, systematics from isotopic and isotonic neighbor, and theoretical results from microscopic Nilsson model calculations. Our investigations conclude $^{243}\mathrm{Np}$ gs ${\mathrm{J}}^{\ensuremath{\pi}}$ and configuration to be 5/2${}^{\ensuremath{-}}[523\ensuremath{\downarrow}$] on all counts. For the odd-odd $^{244}\mathrm{Np}$, we employ our well-tested three-step two-quasiparticle rotor model (TQRM) to identify and evaluate excitation energies of low-lying (${E}_{x}\phantom{\rule{4pt}{0ex}}\ensuremath{\le}$ 400 keV) 2qp bands and respective I(I$+$1) rotational levels. These calculations yield $^{244}\mathrm{Np}$ gs to have ${\mathrm{J}}^{\ensuremath{\pi}}$ and 2qp configuration as 2${}^{+}${p:5/2${}^{\ensuremath{-}}[523]\ensuremath{\bigotimes}$ n:${9/2}^{\ensuremath{-}}$[734]} while confirming the earlier reported ${7}^{\ensuremath{-}}${p:5/2${}^{+}[642]\ensuremath{\bigotimes}$ n:${9/2}^{\ensuremath{-}}$[734]} 2.29 m high-spin isomer (HSI) to be an excited state placed around ${E}_{x}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}100$ keV. We also briefly discuss the earlier reported $^{244}\mathrm{Np}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-decay branches of the ${\mathrm{K}}^{\ensuremath{\pi}}$=7${}^{\ensuremath{-}}$ isomer and the admissible main $\ensuremath{\beta}$ branches of ${\mathrm{K}}^{\ensuremath{\pi}}$=2${}^{+}$ gs to the various levels in $^{244}\mathrm{Pu}$.