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

Abstract

Neutron time-of-flight spectra and $\ensuremath{\alpha}$-particle spectra were obtained in coincidence with $\ensuremath{\gamma}$ 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 $\ensuremath{\gamma}$ 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 $t\ensuremath{\sim}2$ 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 $t\ensuremath{\sim}1.8$ MeV) and a high-temperature ($t\ensuremath{\sim}6$ 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 $\ensuremath{\alpha}6n$ to $\ensuremath{\alpha}9n$ products. The fraction of $^{166\ensuremath{-}x}\mathrm{Er}$ products formed by $2p(x+2)n$ rather than $\ensuremath{\alpha}\mathrm{xn}$ 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 $\ensuremath{\alpha}$-particle spectra from ($^{12}\mathrm{C}$, $\ensuremath{\alpha}\mathrm{xn}$) for small $x$ exhibit a strong dependence on exit channel and on angle of emission, indicating clearly that these $\ensuremath{\alpha}$ particles are emitted from a nonequilibrated system. The angular correlations of the higher-energy $\ensuremath{\alpha}$ particles from ($^{12}\mathrm{C}$, $\ensuremath{\alpha}7n$) to ($^{12}\mathrm{C}$, $\ensuremath{\alpha}10n$) all have the same shape, suggesting that in these channels the $\ensuremath{\alpha}$ emission occurs first. The pre-equilibrium emission accounts for (16\ifmmode\pm\else\textpm\fi{}7)% and (28\ifmmode\pm\else\textpm\fi{}6)% of the total ($^{12}\mathrm{C}$, $\mathrm{xn}$) and ($^{12}\mathrm{C}$, $\ensuremath{\alpha}\mathrm{xn}$) cross sections, respectively. For these reactions the average multiplicities ${〈M〉}_{\mathrm{xn}}$ and ${〈M〉}_{\ensuremath{\alpha}\mathrm{xn}}$ from coincidences with neutrons and $\ensuremath{\alpha}$ particles of all energies saturate at \ensuremath{\sim}20.5 for $x\ensuremath{\le}9 \mathrm{and} \ensuremath{\le}8$, respectively, showing that the pre-equilibrium emission limits the angular momentum that can be imparted to the product nuclei prior to $\ensuremath{\gamma}$ decay. For ($^{20}\mathrm{Ne}$, $\ensuremath{\alpha}\mathrm{xn}$) evidence for such a limiting effect was found for $x\ensuremath{\le}7$ and the $\ensuremath{\alpha}$-particle spectra show some hardening for $x\ensuremath{\le}7$, but there is no evidence for pre-equilibirum emission in the ($^{20}\mathrm{Ne}$, $\mathrm{xn}$) reactions. The multiplicity from ($^{12}\mathrm{C}$, $\ensuremath{\alpha}8n$) associated with ${E}_{\ensuremath{\alpha}}\ensuremath{\ge}24$ MeV was found to be lower at forward $\ensuremath{\alpha}$-emission angles, apparently another manifestation of pre-equilibrium emission.NUCLEAR REACTIONS $^{158}\mathrm{Gd}(^{12}\mathrm{C}, \mathrm{xn}\ensuremath{\gamma})^{170\ensuremath{-}x}\mathrm{Yb}$, $x=7\ensuremath{-}11$, $^{158}\mathrm{Gd}(^{12}\mathrm{C}, \ensuremath{\alpha}\mathrm{xn}\ensuremath{\gamma})^{166\ensuremath{-}x}\mathrm{Er}$, $x=5\ensuremath{-}10$, $E=152.0$ MeV; $^{150}\mathrm{Nd}(^{20}\mathrm{Ne}, \mathrm{xn}\ensuremath{\gamma})^{170\ensuremath{-}x}\mathrm{Yb}$, $x=7\ensuremath{-}10$, $^{150}\mathrm{Nd}(^{20}\mathrm{Ne}, \ensuremath{\alpha}\mathrm{xn}\ensuremath{\gamma})^{166\ensuremath{-}x}\mathrm{Er}$, $x=6\ensuremath{-}9$, $E=174.8$ MeV; measured $\ensuremath{\sigma}({E}_{n}, {\ensuremath{\theta}}_{n})$, $\ensuremath{\sigma}({E}_{\ensuremath{\alpha}}, {\ensuremath{\theta}}_{\ensuremath{\alpha}})$, average $\ensuremath{\gamma}$-ray multiplicity; deduced temperatures, pre-equilibrium effects, $(2p+2n)\mathrm{xn}$ fraction. Enriched targets.