Direct and Compound-Nucleus Neutrons from 14-18-MeV Protons onBe9,N14,Al27,Fe56,In115,Ta181, andPb208and from 33-MeV Bremsstrahlung onAl27,Pb206,Pb208, andBi209

Abstract

The angle-integrated experimental ($p,xn$) spectra, fitted with precompound and evaporation spectra from Griffin's statistical model of intermediate structure, produced $g$, the number of independent-particle states per MeV, and $f_c$, the compound fraction. For ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, ${}_{}{}^{56}{}_{}{}^{}\mathrm{Fe}$, ${}_{}{}^{115}{}_{}{}^{}\mathrm{In}$, and ${}_{}{}^{181}{}_{}{}^{}\mathrm{Ta}$, $g$ increased linearly with $A$, from 2 to 13, but decreased to 9±1 for ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, at 18-MeV proton energy $E_p$. At $E_p=14\mathrm{}$ MeV, $g=7\mathrm{}\pm 1$ for ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$, which may signify vanishing shell effects at large $E_p$. Monte Carlo-type intranuclear-cascade-plus-evaporation calculations yielded spectra and cross sections in agreement with the measurements, allowing for several calculational defects that were made obvious in the comparisons. The Monte Carlo calculations yielded $f_c$ values close to those derived by fitting the data with Griffin's calculations for targets heavier than ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$. "Nuclear temperatures," obtained from LeCouteur's evaporation analysis for multiple-nucleon emissions, agreed for ($p,xn$) and giant-resonance ($\gamma ,xn$) reactions, both via the intermediate nucleus ${}_{}{}^{209}{}_{}{}^{}\mathrm{Bi}$, but differed considerably for the target ${}_{}{}^{27}{}_{}{}^{}\mathrm{Al}$, where direct and evaporation neutrons are not well separated in mean energy. The three types of analyses utilized here are therefore much more applicable for heavy nuclei than for light. Energy and angle dependence of ($\gamma ,xn$) reactions indicate negligible direct effects, in sharp contrast to ($p,xn$) reactions, which display a strong forward anisotropy at the high-energy end of the spectra.