Structure in theE2Quasicontinuum Spectrum ofDy154

The evolution of the $\gamma$ quasicontinuum spectrum with neutron number has been investigated in the sequence of isotopes ${}_{}{}^{152,154,156}{}_{}{}^{}\mathrm{Dy}$. The three nuclei display a pronounced collective $E2\mathrm{}$ component. In ${}_{}{}^{154}{}_{}{}^{}\mathrm{Dy}$ this component shows a splitting into two distinct parts, signifying a structural change along the $\gamma$ cascade above the yrast line. The $E2\mathrm{}$ and statistical components are reproduced in simple $\gamma$-cascade calculations; in ${}_{}{}^{152}{}_{}{}^{}\mathrm{Dy}$ and ${}_{}{}^{156}{}_{}{}^{}\mathrm{Dy}$ only rotational bands are included, whereas in ${}_{}{}^{154}{}_{}{}^{}\mathrm{Dy}$ additional vibration-like transitions are required to reproduce the two $E2\mathrm{}$ peaks.