Pre-equilibrium particle emission from fusion of 12C + 158Gd and 20Ne + 150Nd

Neutron time-of-flight spectra and α-particle spectra were obtained in coincidence with γ rays characteristic of specific evaporation residues in the reactions 174.8 MeV 20Ne + 150Nd and 152.0 MeV 12C + 158Gd, producing 170Yb at 134.2 and 132.0 MeV of excitation. The average multiplicity of γ rays associated with such coincident events were also determined. The neutron spectra from the 20Ne reactions are characteristic of equilibrium evaporation processes. They show a temperature t approx. 2 MeV, in agreement with a prediction from level-density parameters based on data at far lower excitation energy. The neutron spectra from the 12C reactions show a low-temperature equilibrium component (average t approx. 1.8 MeV) and a high-temperature (t approx. 6 MeV) pre-equilibrium component. The latter observation may be interpreted as evidence for the transient existence of a hot spot involving no more than 25 nucleons. The estimated number of pre-equilibrium neutrons emitted varies from 1.8 to 0.6 for the 8n to 10n products and from 1.2 to 0.25 for the α6n to α9n products. The fraction of /sup 166-x/Er products formed by 2p (x + 2) n rather than αxn emission decreases with increasing x for both thee 20Ne and 12C reactions, and is roughly twice as large for 12C. The high-energy portions of the α-particle spectra from (12C,αxn) for small x exhibit a strong dependence on exit channel and on angle of emission, indicating clearly that these α particles are emitted from a nonequilibrated system. The angular correlations of the higher-energy α particles from (12C,α7n) to (12C,α10n) all have the same shape, suggesting that in these channels the α emission occurs first. The pre-equilibrium emission accounts for (16 +- 7) % and (28 +- 6) % of the total (12C,xn) and (12C,αxn) cross sections, respectively