First observation of a rotational band and the role of the proton intruder orbitalπ1/2+[431] in very neutron-rich odd-oddNb106

A rotational band was observed for the first time in $^{106}\mathrm{Nb}$ by means of \ensuremath{\gamma}-\ensuremath{\gamma}-\ensuremath{\gamma} and \ensuremath{\gamma}-\ensuremath{\gamma}-\ensuremath{\gamma}-\ensuremath{\gamma} measurements of prompt fission \ensuremath{\gamma} rays from $^{252}\mathrm{Cf}$ by using the Gammasphere multidetector array. Projected shell model and potential-energy surface calculations were performed and were compared to the experimental data of $^{106}\mathrm{Nb}$ and the previously reported $^{104}\mathrm{Nb}$. Configurations and spin-parity were assigned to the ground level, low-lying levels, and rotational bandheads of the two Nb isotopes. The new rotational band in $^{106}\mathrm{Nb}$ was interpreted as a ${K}^{\ensuremath{\pi}}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}{2}^{\ensuremath{-}}$ band with a configuration \ensuremath{\pi}1/2${}^{+}$[431] \ifmmode\times\else\texttimes\fi{} \ensuremath{\nu}5/2${}^{\ensuremath{-}}$[532]. The same spin-parity and configuration were assigned to the analogous band in $^{104}\mathrm{Nb}$. \ensuremath{\pi}1/2${}^{+}$[431] \ifmmode\times\else\texttimes\fi{} \ensuremath{\nu}5/2${}^{+}$[413], ${K}^{\ensuremath{\pi}}={3}^{+}$ were assigned to the 0.66-\ensuremath{\mu}s isomer and explained the $M$2 isomeric decay to the ground in $^{106}\mathrm{Nb}$. The proton intruder orbital \ensuremath{\pi}1/2${}^{+}$[431] plays an important role in shape evolution with regard to triaxial deformation in these neutron-rich Nb isotopes.