Rapid Increase in Prescission GDR γ-ray Emission with Energy

A rapid increase in the emission of prescission giant dipole resonance (GDR) γ-rays with bombarding energy is observed in excited Th and Cf nuclei formed in the reactions ¹⁶O+ ²⁰⁸Pb and ³²S+ ^natW, ²⁰⁸Pb. This increase begins around E_exc = 40 MeV for the ¹⁶O+ ²⁰⁸Pb reaction and E_exc = 70 MeV for the ³²S-induced reactions. The excess γ-ray yield above these thresholds cannot be described within the standard statistical model. Statistical model calculations which include a temperature dependent nuclear dissipation are able to reproduce simultaneously the observed GDR γ-ray spectra and recently measured evaporation residue cross sections.