FRAGMENT MASS DISTRIBUTION OF FISSION AFTER INCOMPLETE FUSION IN THE REACTION 7LI (43A MEV) + 232TH

Fragment mass distributions of binary fission were measured for the reaction 7Li (43A MeV) + 232Th in dependence on the linear momentum transfer. The excitation energy (E*) of the compound nuclei produced by incomplete fusion was deduced applying the massive transfer approach and amounted to 57-205 MeV. The analysis procedure avoids distortions due to the acceptance and error correlation. Total fragment masses and single fragment mass dispersions are discussed in terms of E*, temperature and an effective parameter describing the stiffness against mass asymmetry. Assuming unchanged stiffness, the dispersions at E* in excess of ~ 70 MeV are found to be considerably smaller than expected. This behaviour is interpreted as a consequence of the remarkable cooling down of the hot nuclei due to neutron emission before the fragment masses are formed in the course of fission. Agreement with the data is achieved supposing a cooling time of (60-1-20) z 10 -21 s based on the neutron emission time taken from the statistical model. It is concluded that fission remains a relatively slow process up to excitation energies of about 200 MeV. @ 1998 Elsevier Science B.V.