NEUTRONS IN COINCIDENCE WITH FISSION OF U-238 INDUCED BY STOPPED ANTIPROTONS

Neutron multiplicities after antiproton-induced fission have been dete rmined as a function of the total mass of the fission fragments. Pre- and postscission neutron multiplicities of the reaction p(-) + U-238 h ave been deduced using the moving-source parametrization of the neutro n energy spectra by Maxwellian distributions. A postscission multiplic ity of 6.3(6) neutrons out of 20.0(10) in total emitted neutrons in fi ssion indicates that fission is a rather slow process. The increase of the evaporative prescission neutron multiplicity is about 80% of the total increase of evaporative neutrons as a measure of excitation ener gy. The postscission multiplicity gets only 20% of the total increase, thus indicating that the nucleus has only a moderate excitation energ y at the moment of scission. The results are compared with standard ca scade-evaporation calculations predicting a much lower number of evapo rated prescission neutrons as the experiment shows. This is a strong i ndication of the importance of dissipative effects in the fission proc ess. Application of dissipative effects like a saddle-to-scission dela y time and nuclear viscosity in the model calculation describes the ex perimental results much better.