TESTS FOR EQUILIBRATION OF TB-149-ASTERISK COMPOSITE NUCLEI PRODUCED IN THE REACTIONS 337 MEV AR-40+AG-NAT AND 640 MEV KR-86+CU-63

Experimental results are compared for the two matched heavy ion reactions 337 MeV Ar-40 + (nat)Ag and 640 MeV Kr-86 + Cu-63 as a test of the extent of equilibration of the intermediate composite nucleus Tb-149* (E* = 194 MeV). Spin zones associated with the evaporation residues (ER) are found to agree well for these entrance channels. Agreement is not as close for the fission class due to the inclusion of deeply inelastic reactions (DIR) along with fusion-fission (FF) for the Kr-86 reaction. Inclusive and coincidence measurements of energy spectra and angular distributions for the light charged particles indicated that these reactions produce composite nuclei with similar temperatures, spins, and moments of inertia. The energy spectra for both reactions are identically shifted relative to statistical model calculations. This suggests that the composite nuclei are highly distended prior to their cooling by particle evaporation. Multiplicities also agree for H-1 and He-4 production in association with the ER and FF reaction classes. It is noteworthy that prescission H-1 and He-4 multiplicities are in such good agreement, because the magnitude of these multiplicities seems to be dictated by the dynamical time scale of the fission process. That these prescission multiplicities are large suggests that the fission time scale is slow relative to particle evaporation; that they match for these reactions indicates that this is an equilibrium property of the intermediate composite nuclei. We conclude that extensive and rapid thermalization and shape equilibration have occurred despite estimates that the nuclear relaxation time and decay lifetime are nearly the same.