Preequilibrium neutron emission in the reactionsZr90,Pb208(p,xn) with 80 MeV projectiles

The reactions ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$, ${}_{}{}^{208}{}_{}{}^{}(\mathit{p}$,xn) have been measured with 80.5 MeV protons in the neutron continuum $E_n$>20 MeV and the angular range 0°≤θ≤145°. Whereas the angle-integrated spectrum can be explained by semiclassical preequilibrium models, this nucleon-nucleon scattering approach fails at far backward angles. In contrast, the quantum statistical multistep calculations give good agreement. They are indicative of a decrease of the effective nucleon-nucleon interaction with increasing nucleon energy.