Neutron-multiplicity distributions for (α, xnγ) reactions withEα=50−120MeV and the pre-equilibrium neutron deexcitation processes

Abstract

Pre-equilibrium and equilibrium deexcitation processes for ($\ensuremath{\alpha}, x\mathrm{n}\ensuremath{\gamma}$) reactions induced by 50-120 MeV $\ensuremath{\alpha}$ particles were studied. Reaction channels were identified by measuring rotational $\ensuremath{\gamma}$ 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 ${E}_{\ensuremath{\alpha}}=50\ensuremath{-}120$ 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.