Determination of the Spreading Width for the Collective Transition Strength at Finite Temperature

The experimentally observed structure of the $E2\mathrm{}$ transitions in the quasicontinuum for ${}_{}{}^{168}{}_{}{}^{}\mathrm{Yb}$ is analyzed. The valley ridge structure along the $E_{\gamma 1}=E_{\gamma 2}$ diagonal in $\gamma$-energy correlation spectra has been reproduced by a model simulation in which a spreading of the rotational decay strength ${\Gamma }_{\mathrm{rot}}$ is introduced in regions of nuclear excitation energy $U>\mathrm{}{U}_0$ where the density of bands with slightly differing moments of inertia and alignment is high. The values of ${\Gamma }_{\mathrm{rot}}$ and $U_0$ vary from 75 to 110 keV and from 1.25 to 1.0 MeV, respectively, in the energy range $780<\mathrm{}{E}_{\gamma }<1160\mathrm{}$ keV.