Neutron emission in central heavy-ion collisions ofHo165+Ne20at 11, 14.6, and 20.1 MeV/nucleon

The neutron emission from fusionlike reactions leading to evaporation residues and fission fragments was measured in the reaction ${}_{}{}^{165}{}_{}{}^{}\mathrm{Ho}$ + ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ at 220, 292, and 402 MeV ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ bombarding energies. Preequilibrium high-energy neutrons having up to twice the beam velocity were observed. The multiplicity of these neutrons increases with the bombarding energy from 0.4 to 2.3. This corresponds to 5 to 15% of the total number of neutrons emitted per fusionlike event. The measured energy spectra of the highly energetic neutrons cannot be described with the Fermi-jet mechanism (one body promptly emitted particles), especially at angles larger than 35°. Reasonable agreement with the modified Harp-Miller-Berne model can be obtained. In that case, however, it is necessary to treat the initial degree of freedom as a parameter which increases with bombarding energy from 20 to 28. The evaporative component in the energy spectra of fusion-fission events was used to deduce that the number of prefission neutrons is 5.6±0.5, 5.8±0.5, and 5.3±1.0 at bombarding energies of 220, 292, and 402 MeV, respectively, whereas the statistical model predicts a maximum number of 1.6 to 2.0 prefission neutrons. Although no conclusive explanation can be given for the unexpectedly large multiple chance fission probability, it is suggested that most of the additional prefission neutrons are emitted during the transition from saddle to scission.