Proton Reaction Cross Sections and Strength Functions

The average yields of neutrons from proton-induced reactions in thick targets have been measured for about 33 elements in the mass region $90\lesssim A<~209$ at incident proton energies between 5.5 and 9.5 MeV. In this region of energies and masses, the ($p, n$) reaction overwhelms competing reactions so that, except in a few cases, the average ($p, n$) cross sections deduced from the measured yields are equal to the total reaction cross sections for protons. Reduced cross sections, which are directly related to the proton strength function, were obtained from the measurements. These results, as well as the results of previous measurements for $A<\mathrm{}90\mathrm{}$, were compared with optical-model calculations. Proton reaction cross sections calculated from the optical potential of Perey are in remarkably good quantitative agreement with the measurements. The data are consistent with a resonance in the $P$-wave proton strength function at $A\approx 105$, and with dominant contributions from $S$- and $D$-wave protons at $A\approx 65$ and for $155<~A<~200$. The measured angular distributions for some of the nuclei showed symmetry about 90° to about 2%, indicating no measurable direct-interaction component. The maximum anisotropy observed was about 7%.