DETERMINATION OF NUCLEAR FRICTION IN STRONGLY DAMPED REACTIONS FROM PRESCISSION NEUTRON MULTIPLICITIES

Nonfusion, fissionlike reactions in collisions of four heavy systems ( well below the fusion extra-push energy threshold), Mr which Hinde and co-workers had measured the prescission neutron multiplicities, have been analyzed in terms of the deterministic dynamic model of Feldmeier coupled to a time-dependent statistical cascade calculation. In order to reproduce the measured prescission multiplicities and the observed (nearly symmetric) mass divisions, the energy dissipation must be dra matically changed with regard to the standard one-body dissipation: In the entrance channel, in the process of forming a composite system, t he energy dissipation has to be reduced to at least half of the one-bo dy dissipation strength (k(s)(in) less than or equal to 0.5), and in t he exit channel (from a mononucleus shape to scission) it must be incr eased by a factor ranging for the studied reactions from k(s)(out) = 4 to k(s)(out) = 12. These results are compared with the temperature de pendence of the friction coefficient, recently deduced by Hofman, Back , and Paul from data on the prescission giant dipole resonance emissio n in fusion-fission reactions. The combined picture of the temperature dependence of the friction coefficient, for both fusion-fission and n onfusion reactions, may indicate the onset of strong two-body dissipat ion already at a nuclear temperature of about 2 MeV.