Pre-equilibrium particle emission from fusion ofC12+Gd158andNe20+Nd150

Abstract

Neutron time-of-flight spectra and $\alpha$-particle spectra were obtained in coincidence with $\gamma$ rays characteristic of specific evaporation residues in the reactions 174.8 MeV ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ + ${}_{}{}^{150}{}_{}{}^{}\mathrm{Nd}$ and 152.0 MeV ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ + ${}_{}{}^{158}{}_{}{}^{}\mathrm{Gd}$, producing ${}_{}{}^{170}{}_{}{}^{}\mathrm{Yb}$ at 134.2 and 132.0 MeV of excitation. The average multiplicity of $\gamma$ rays associated with such coincident events were also determined. The neutron spectra from the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ reactions are characteristic of equilibrium evaporation processes. They show a temperature $t\sim 2$ MeV, in agreement with a prediction from level-density parameters based on data at far lower excitation energy. The neutron spectra from the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ reactions show a low-temperature equilibrium component (average $t\sim 1.8$ MeV) and a high-temperature ($t\sim 6$ 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 $\alpha 6n$ to $\alpha 9n$ products. The fraction of ${}_{}{}^{166-x}{}_{}{}^{}\mathrm{Er}$ products formed by $2p(x+2\mathrm{})n$ rather than $\alpha \mathrm{xn}$ emission decreases with increasing $x$ for both the ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$ and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$ reactions, and is roughly twice as large for ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$. The high-energy portions of the $\alpha$-particle spectra from (${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, $\alpha \mathrm{xn}$) for small $x$ exhibit a strong dependence on exit channel and on angle of emission, indicating clearly that these $\alpha$ particles are emitted from a nonequilibrated system. The angular correlations of the higher-energy $\alpha$ particles from (${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, $\alpha 7n$) to (${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, $\alpha 10n$) all have the same shape, suggesting that in these channels the $\alpha$ emission occurs first. The pre-equilibrium emission accounts for (16±7)% and (28±6)% of the total (${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, $\mathrm{xn}$) and (${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, $\alpha \mathrm{xn}$) cross sections, respectively. For these reactions the average multiplicities ${〈M〉}_{\mathrm{xn}}$ and ${〈M〉}_{\alpha \mathrm{xn}}$ from coincidences with neutrons and $\alpha$ particles of all energies saturate at ∼20.5 for $x\le 9 \mathrm{and} \le 8$, respectively, showing that the pre-equilibrium emission limits the angular momentum that can be imparted to the product nuclei prior to $\gamma$ decay. For (${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$, $\alpha \mathrm{xn}$) evidence for such a limiting effect was found for $x\le 7$ and the $\alpha$-particle spectra show some hardening for $x\le 7$, but there is no evidence for pre-equilibirum emission in the (${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$, $\mathrm{xn}$) reactions. The multiplicity from (${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, $\alpha 8n$) associated with $E_{\alpha }\ge 24$ MeV was found to be lower at forward $\alpha$-emission angles, apparently another manifestation of pre-equilibrium emission.