Structure of the N=Z+1 nucleus 69Se.

Abstract

Charged-particle-\ensuremath{\gamma} and charged-particle-neutron-\ensuremath{\gamma} coincidences have been used to identify in-beam \ensuremath{\gamma}-ray transitions in $^{69}\mathrm{Se}$ produced in the reaction $^{40}\mathrm{Ca}$${(}^{32}$S,2pn${)}^{69}$Se. Charged-particle-\ensuremath{\gamma}-\ensuremath{\gamma} coincidences, neutron-\ensuremath{\gamma}-\ensuremath{\gamma} coincidences, and neutron-gated \ensuremath{\gamma}-ray angular distributions were used to deduce the level scheme. A long-lived isomer attributed to the ${g}_{9/2}$ state was found; its half-life was measured with charged-particle-\ensuremath{\gamma} timing as 1023\ifmmode\pm\else\textpm\fi{}110 ns. The presence of strong \ensuremath{\Delta}J=1 and \ensuremath{\Delta}J=2 transitions in the band built on the \ensuremath{\nu}${g}_{9/2}$ neutron single-particle state and the measured sign of the E2/M1 mixing ratio for the ${(11/2}^{+}$\ensuremath{\rightarrow}${(9/2}^{+}$ transition are consistent with a theoretically predicted oblate shape for this nucleus.