Pre-equilibrium and equilibrium deexcitation processes for ($\alpha , x\mathrm{n}\gamma$) reactions induced by 50-120 MeV $\alpha$ particles were studied. Reaction channels were identified by measuring rotational $\gamma$ rays characteristic of the reaction residues. The branching of the reaction channels gave neutron multiplicity distributions. Features characteristic of the pre-equilibrium process were seen in the reaction channels with small neutron multiplicity $x$. An exciton model calculation code was developed so as to incorporate both multiparticle emission at the fast pre-equilibrium stage and multiparticle evaporation at the slow equilibrium stage. The calculation reproduced the neutron multiplicity distributions in the whole range of $E_{\alpha }=50-120\mathrm{}$ MeV. The pre-equilibrium fractions and the entry lines from the pre-equilibrium stage to the equilibrium one were deduced. The pre-equilibrium fractions were found to be approximately 40-60%, being rather independent of the individual reaction channel. The entry lines slowly increase from 25 to 35 MeV with increasing projectile energy.

NUCLEAR REACTIONS ${}_{}{}^{162,164}{}_{}{}^{}\mathrm{Dy}(\alpha ,x\mathrm{n}\gamma ){}_{}{}^{166,168-x}{}_{}{}^{}\mathrm{Er}$, $x=3-12\mathrm{}$, and ${}_{}{}^{174,176}{}_{}{}^{}\mathrm{Yb}(\alpha , x\mathrm{n}\gamma ){}_{}{}^{178,180-x}{}_{}{}^{}\mathrm{Hf}$, $x=3-12\mathrm{}$, $E_{\alpha }=50, 70, 90, \mathrm{and} 120$ MeV, measured $\sigma \mathrm{}\left(x\right)\mathrm{}$; properties of PEQ decay process and analysis by the exciton model. Enriched targets.

• Received 21 July 1982

DOI:https://doi.org/10.1103/PhysRevC.28.635