Entrance and exit channel phenomena ind- andHe3-induced preequilibrium decay

Abstract

Activation techniques were used to measure more than 30 excitation functions for single and multiple nucleon and/or $\ensuremath{\alpha}$ particle emission for $d$+$^{64,66}\mathrm{Zn}$, $^{89}\mathrm{Y}$ with ${E}_{d}=9\ensuremath{-}26$ MeV and $^{3}\mathrm{He}$+$^{63,65}\mathrm{Cu}$, $^{93}\mathrm{Nb}$ with $E(^{3}\mathrm{He})=10\ensuremath{-}44$ MeV. The excitation functions are generally in agreement with the results of a combined equilibrium and preequilibrium hybrid model calculation applying initial exciton numbers ${n}_{0}=3$ for $d$ and ${n}_{0}=4$ for $^{3}\mathrm{He}$ reactions. The composite system $^{66}\mathrm{Ga}$ has been produced via $d$+$^{64}\mathrm{Zn}$ and $^{3}\mathrm{He}$+$^{63}\mathrm{Cu}$ at excitation energies between 22 and 36 MeV. An entrance channel dependence shows up in the yields for single $p$- and $n$-emission when compared in the double ratio $R=\frac{[\frac{\ensuremath{\sigma}(^{3}\mathrm{He}, p)}{\ensuremath{\sigma}(^{3}\mathrm{He}, n)}]}{[\frac{\ensuremath{\sigma}(d, p)}{\ensuremath{\sigma}(d, n)}]}$. It approaches a value of about 2, indicating enhanced $p$ emission for the $^{3}\mathrm{He}$-induced reaction. This value disagrees with the equilibrium isospin formalism and is best reproduced by initial particle exciton numbers ${n}_{0p}={n}_{0n}=1.5$ for $d$ and ${n}_{0p}=2.5$, ${n}_{0n}=1.5$ for $^{3}\mathrm{He}$ projectiles, indicating conservation of charge asymmetry in the entrance channel. Isomeric ratios have been measured for $^{89}\mathrm{Y}(d, 2n)^{89}\mathrm{Zr}$ and $^{93}\mathrm{Nb}(^{3}\mathrm{He}, xn)^{96\ensuremath{-}x}\mathrm{Tc}$ ($x=1, 2, 3$). Calculations with a full statistical model fail to reproduce $\frac{{\ensuremath{\sigma}}_{g}}{{\ensuremath{\sigma}}_{m}}$ as well as ${\ensuremath{\sigma}}_{g}$ and ${\ensuremath{\sigma}}_{m}$ for reasonable values of the spin cutoff parameter. Inclusion of a preequilibrium decay mode improves the fit, in particular if the angular momentum depletion of the composite system due to preequilibrium decay is increased over that of the equilibrium decay at the same channel energy.NUCLEAR REACTIONS $^{64,66}\mathrm{Zn}$, $^{89}\mathrm{Y}(d, \mathrm{xnypz}\ensuremath{\alpha})$, ${E}_{d}=9\ensuremath{-}26$ MeV, $^{63,65}\mathrm{Cu}$, $^{93}\mathrm{Nb}(^{3}\mathrm{He}, \mathrm{xnypz}\ensuremath{\alpha})$, ${E}_{^{3}\mathrm{He}}=10\ensuremath{-}44$ MeV, $x<~4$, $y<~1$, $z<~2$; measured $\ensuremath{\sigma}(E)$ by activation, enriched targets. Statistical model analysis including preequilibrium decay, deduced reaction mechanism, charge asymmetry conservation, spin depletion.