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

Fragment mass distributions of binary fission were measured for the re action Li-7 (43A MeV) + Th-232 in dependence on the linear momentum tr ansfer. The excitation energy (E) of the compound nuclei produced by incomplete fusion was deduced applying the massive transfer approach a nd amounted to 57-205 MeV. The analysis procedure avoids distortions d ue to the acceptance and error correlation. Total fragment masses and single fragment mass dispersions are discussed in terms of E, tempera ture and an effective parameter describing the stiffness against mass asymmetry. Assuming unchanged stiffness, the dispersions at E in exce ss of approximate to 70 MeV are found to be considerably smaller than expected. This behaviour is interpreted as a consequence of the remark able cooling down of the hot nuclei due to neutron emission before the fragment masses are formed in the course of fission. Agreement with t he data is achieved supposing a cooling time of (60 +/- 20) x 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. (C) 1998 Elsevier Science B.V.