Proton Strength Functions from (p, n) Reaction Cross Sections

The ($p, n$) reaction cross section has been measured for thick targets of thirty-one elements ranging in atomic weight from 37 to 133 with incident proton energies between 2.0 and 4.0 Mev. The reaction yields were divided by a penetrability factor to yield a "reduced cross section" closely related to the proton strength function. Peaks in the reduced cross section were observed at atomic weight 50 and 68. These were tentatively identified as the peaks in the $D$-wave and $S$-wave strength functions of the complex-potential model corresponding to effects observed in neutron experiments at atomic weights 40 and 51. Assuming these identifications, the difference between proton and neutron potentials was calculated. Within the validity of the assumptions, $V{r_0}^2$ for nuclei in the vicinity of atomic weight 70, was found after Coulomb corrections to be 6±6% larger for protons than for neutrons. The reduced cross section was observed to double in going from atomic weight 37 to 133, possibly because of the effect of spheroidal deformation on the barrier penetrability. Angular distribution measurements showed symmetry about 90° to better than 5%, indicating no measurable direct-interaction component in the reaction. The largest anisotropies observed were approximately 20%.