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

On the basis of data obtained by the incomplete fusion reactions Li-7(43A MeV) + Th-232 and N-14(34A MeV) + Au-197, the energy dependence of the variance (sigma(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 sigma(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 sigma(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* congruent to250 MeV, but the stochastic treatment does not reveal any increase in sigma(M)(2) at higher excitation energies in contradiction with the data. (C) 2002 MAIK "Nauka/Interperiodica".