Preequilibrium neutron emission in fusion of ¹⁶⁵Ho+¹²C at 25 MeV per nucleon

Neutrons were measured in coincidence with evaporation residues from the reaction ¹⁶⁵Ho+(300 MeV) ¹²C. The evaporation residue velocity distribution is indicative of an average transfer of 80% of the full linear momentum in this reaction. The energy spectra of the coincident neutrons exhibit evaporative and preequilibrium components associated with integral multiplicities of M_EV=(9.5±0.5) and M_{PE=(1.7±0.3)}, respectively. The experimental neutron energy and angular distributions are analyzed in terms of multiple-source parametrizations, assuming two or three emitters. The results are compared to those obtained from other inclusive and exclusive associated-particle data. It is observed that the emission patterns of the preequilibrium neutrons are in accord with the predictions of a Fermi-jet model, for neutron angles forward of 35°, while this model fails to reproduce the data at angles in the vicinity of 90° and beyond. Various different nucleon momentum distributions have been employed in the model comparison. The insufficiency of the Fermi-jet model to reproduce the data is attributed to the neglect of two-body collisions in this one-body theory. In contrast, the shape of the angle-integrated preequilibrium-neutron energy spectrum is well reproduced with the Harp-Miller-Berne preequilibrium model, if an initial exciton number of n₀=15 is adopted. This value, as well as the preequilibrium neutron multiplicity, is at variance with systematics established previously.