Neutron-multiplicity distributions for (xn) reactions with E±=50-120 MeV and the pre-equilibrium neutron deexcitation processes

Pre-equilibrium and equilibrium deexcitation processes for (±xn) reactions induced by 50-120 MeV particles were studied. Reaction channels were identified by measuring rotational 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±=50-120 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. NUCLEAR REACTIONS Dy162,164(xn)Er166,168-x, x=3-12, and Yb174,176(xn)Hf178,180- x, x=3-12, E±=50, 70, 90, and 120 MeV, measured (x); properties of PEQ decay process and analysis by the exciton model. Enriched targets.