Transitional Nuclei. II. Higher Vibrational States in NucleusGd154

Isotopically separated ${}_{}{}^{154}{}_{}{}^{}\mathrm{Eu}$ has been used in conjunction with sources prepared from neutron irradiation of isotopically separated ${}_{}{}^{153}{}_{}{}^{}\mathrm{Eu}$ to study the $\gamma$-ray spectra and concomitant level structure of ${}_{}{}^{154}{}_{}{}^{}\mathrm{Gd}$. Several levels which have been identified as collective levels are found to have two-phonon character. A collective $K=2\mathrm{}$ vibrational state has been proposed at 1531.39 keV and can be described as a coupled ($\beta \gamma$) two-phonon state. Members of the rotational band built on this state have been identified at 1531.39±0.07, 1660.94±0.07, and 1790.4±0.3 keV with spin and parity of $2^{+}$, $3^{+}$, and $4^{+}$, respectively. A possible second band at 1292.7 keV has been found to possess two ($\beta$) phonon characteristics. Evidence is found for the breakdown of the simple $E2\mathrm{}$ selection rules for transitions from higher phonon bands.