A rapid increase with energy has been observed in the emission of prescission giant dipole resonance (GDR) γ rays in excited Th and Cf nuclei formed in the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{208}$Pb and ${}_{}{}^{32}{}_{}{}^{}\mathrm{S}$${+}^{\mathrm{nat}}$W${,}^{208}$Pb reactions, which is not explained by the normal reaction dynamics near the barrier. This increase occurs over a narrow excitation energy range of ${\mathit{E}}_{\mathrm{exc}}$=40–60 MeV for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{208}$Pb reaction and ${\mathit{E}}_{\mathrm{exc}}$=70–90 MeV for the ${}_{}{}^{32}{}_{}{}^{}\mathrm{induced}$ reactions. Below the transition energy the γ-ray spectra can be described by the standard statistical model, whereas inclusion of an increasingly strong nuclear dissipation is required to account for the data at higher excitation energies. For the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$${+}^{208}$Pb reaction a fit to the GDR γ-ray spectra and evaporation residue cross sections is used to extact the temperature dependence of the linear dissipation parameter.

• Received 3 November 1994

DOI:https://doi.org/10.1103/PhysRevC.51.2597