Angular-momentum effects in pre-equilibrium processes

Abstract

Cross sections and the first three moments of the $\ensuremath{\gamma}$-ray multiplicity distribution were measured for many transitions corresponding to $\mathrm{xn}$ and $\mathrm{xn}\ensuremath{\alpha}$ products from bombardments of $^{166}\mathrm{Er}$ with $^{4}\mathrm{He}$, $^{150}\mathrm{Nd}$ with $^{20}\mathrm{Ne}$, and $^{130}\mathrm{Te}$ with $^{40}\mathrm{Ar}$. The $^{4}\mathrm{He}$ and $^{20}\mathrm{Ne}$ bombarding energies were chosen to produce $^{170}\mathrm{Yb}$ compound nuclei at similar excitation energies, approximately 63, 77, and 92 MeV. A Ge(Li)-NaI anti-Compton spectrometer was used in coincidence with nine 5.1-cm by 7.6-cm NaI(Tl) detectors. For the reactions ($^{20}\mathrm{Ne},xn$) and ($^{40}\mathrm{Ar},xn$), the average $\ensuremath{\gamma}$-ray multiplicity ${〈M〉}_{x}$ increases linearly with decreasing $x$ as expected from the statistical decay of compound nuclei. In the ($^{4}\mathrm{He},xn$) reactions, ${〈M〉}_{x}$ is similar for large $x$ but for small $x$ it decreases substantially with decreasing $x$. The latter corresponds to a shift in the population toward the low $J$ values of levels in $^{170\ensuremath{-}x}\mathrm{Yb}$ prior to $\ensuremath{\gamma}$ decay. Also, in the few-neutron channels the average multiplicity associated with a given gating transition in the residual nucleus is much smaller for transitions between low-lying levels. These features, absent in the heavy-ion bombardments, indicate a strong contribution from pre-equilibrium neutron emission. The measured ($^{4}\mathrm{He},xn$) cross sections are in reasonable agreement with those calculated from the geometry-dependent hybrid model of pre-equilibrium emission except for the smallest values of $x$; the latter results and the low ${〈M〉}_{x}$ values for $x\ensuremath{\le}4$ suggest that more than one pre-equilibrium neutron is emitted in these channels.[NUCLEAR REACTIONS $^{150}\mathrm{Nd}(^{20}\mathrm{Ne},xn\ensuremath{\gamma})$, ($^{20}\mathrm{Ne},xn\ensuremath{\alpha}\ensuremath{\gamma}$), $x=4\ensuremath{-}8$, $E=94.2, 110.7$ MeV; $^{130}\mathrm{Te}(^{40}\mathrm{Ar},xn\ensuremath{\gamma})$, $x=3\ensuremath{-}5$, $E=154.5$ MeV; $^{166}\mathrm{Er}(^{4}\mathrm{He},xn\ensuremath{\gamma})$, ($^{4}\mathrm{He},xn\ensuremath{\alpha}\ensuremath{\gamma}$), $x=2\ensuremath{-}8$, $E=67.6\ensuremath{-}95.3$ MeV; measured $\ensuremath{\sigma}(E,E\ensuremath{\gamma})$, $\ensuremath{\gamma}$-ray multiplicities; deduced $J$ distributions in evaporation residues. Enriched target, anti-Compton Ge(Li)-NaI spectrometer.]