Novel features of the fragment mass variance in fission of hot nuclei

On the basis of data obtained by the incomplete fusion reactions 7Li(43A MeV)+232Th and 14N(34A MeV)+197Au, the energy dependence of the variance (σ M 2 ) of the fragment mass in fission of highly heated nuclei has been investigated for total excitation energies E tot * ranging from 50 up to 350 MeV. The dependence σ M 2 E tot * shows some unexpected features when E tot * exceeds a value of about 70 MeV. After this value, the steady increase of σ M 2 expected from its temperature dependence changes to some kind of plateau between 100 and 200 MeV. Further on, at E tot * in excess of about 250 MeV, the variance is found to increase again sharply. In order to analyze this behavior quantitatively, a dynamical stochastic model has been developed. The model employs the one-body dissipation mechanism and describes the decay of highly excited and rotating nuclei by fission and light-particle evaporation. It satisfactorily explains the measured prior-to-scission neutron multiplicities and the experimental mass variances up to E tot * ≅250 MeV, but the stochastic treatment does not reveal any increase in σ M 2 at higher excitation energies in contradiction with the data.