Rotational and vibrational excitations in84Zrstudied through in-beam and84Nbβ-decay spectroscopy

Abstract

Low- and medium-spin states have been investigated in the even-even nucleus ${}^{84}\mathrm{Zr}$ by using in-beam \ensuremath{\gamma}-ray spectroscopy via the reactions ${}^{59}\mathrm{Co}{(}^{28}\mathrm{Si},p2n)$ and ${}^{58}\mathrm{Ni}{(}^{32}\mathrm{S},\ensuremath{\alpha}2p)$ at beam energies of 99 and 135 MeV, respectively. Several new levels have been identified, including a new even-spin sequence of the \ensuremath{\gamma}-vibrational band. The previously known sequence has been found to be built on a ${3}^{+}$ state at 1575.3 keV representing the odd-spin sequence. In addition, the ${\ensuremath{\beta}}^{+}/\mathrm{EC}$ decay of mass-separated ${}^{84}\mathrm{Nb}$ sources has been studied by \ensuremath{\beta}- and \ensuremath{\gamma}-ray spectroscopy following the bombardment of a thin ${}^{58}\mathrm{Ni}$ target with ${}^{28}\mathrm{Si}$ ions at 94 MeV. Some low-lying daughter states up to spin and parity of ${4}^{+}$ have been identified in ${}^{84}\mathrm{Zr},$ and a half-life of $9.5\ifmmode\pm\else\textpm\fi{}1.0\mathrm{s}$ has been measured for the ${}^{84}\mathrm{Nb}$ ground-state decay. The systematics of \ensuremath{\gamma}-vibrational states in even-even Zr nuclei and a comparison with model predictions indicate increasing $\ensuremath{\gamma}$ softness and triaxiality as the spherical neutron shell closure at $N=50$ is approached. Using an empirical estimate, a triaxiality of about $25\ifmmode^\circ\else\textdegree\fi{}$ has been extracted for ${}^{84}\mathrm{Zr}.$ Furthermore, a negative-parity structure of ${4}^{\ensuremath{-}}$ and ${6}^{\ensuremath{-}}$ states which is similar to other $N=44$ isotones has been identified in ${}^{84}\mathrm{Zr}$ as a two-quasineutron excitation.