Excited levels in the ${}^{109}\mathrm{Tc}$ nucleus, populated in the spontaneous fission of ${}^{248}\mathrm{Cm}$, have been studied using the EUROGAM2 array. In ${}^{109}\mathrm{Tc}$ we have found a new band corresponding to the $\pi 5$/$2^{-}$[303] configuration and a three-quasiparticle band with $I^{\pi }=(15/2^{-})$ band-head level at 1749.5 keV. The structure of the 1749.5-keV level probably involves the $\{\pi 5/2^{+}[422]\nu (5/2^{-}[532],5/2^{+}[413])\}{}_{{15/2}^{-}}$ prolate configuration. The quasiparticle-rotor model calculations performed in this work show that the odd-proton configurations observed in ${}^{107}\mathrm{Tc}$ and ${}^{109}\mathrm{Tc}$ are consistent with the scheme of proton excitations in a prolate triaxial potential. Significant differences between the degree of triaxiality observed for positive- and negative-parity excitations may be due to blocking of the triaxial shape by the odd proton populating negative-parity orbitals. The properties of the new $494.5-$, $1440.7-$, and $1748.8$-keV levels found in ${}^{109}\mathrm{Tc}$ may indicate a change toward an oblate shape in this nucleus.

• Received 12 January 2010

DOI:https://doi.org/10.1103/PhysRevC.82.064308