ARE PREEQUILIBRIUM REACTIONS MAINLY DIRECT

Cross sections for the (p, xn) reactions for proton energies from 9 to 27 MeV on targets from titanium to thulium are consistently explained by statistical direct reactions accompanied by gradual absorption of protons into successive compound states of increasing complexity which evolve towards the compound nucleus. It is shown that, in the absence of strong collective excitations, pre-equilibrium compound reactions prevail over direct processes. Physical parameters of the multistep di rect reaction model are obtained, which when applied in analyses of (n , xn) data may reveal the extent of collective enhancement of the scat tering cross section.