p-wave size resonances observed by the (p, n) reaction for 2.6- to 7-MeV protons incident on isotopes of Sn

The ${}_{}{}^{117,118,119,120,122,124}{}_{}{}^{}\mathrm{Sn}(p, n)$ cross sections were measured with ∼ 100-keV resolution for 2.6- to 7-MeV protons with systematic uncertainties of less than ± 1%. An exact sequence of broad maxima in the strength functions is observed with the energies at the peaks ranging from 5.45 MeV for ${}_{}{}^{124}{}_{}{}^{}\mathrm{Sn}$ to 6.15 MeV for ${}_{}{}^{117}{}_{}{}^{}\mathrm{Sn}$. These are interpreted in terms of the $3p$ quasibound proton state in the optical-model potential. Values of $V{R}^2$ for the real well at the resonance energies are determined unambiguously to ± 1% and range from 2120 MeV ${\mathrm{fm}}^2$ for ${}_{}{}^{117}{}_{}{}^{}\mathrm{Sn}$ to 2215 MeV ${\mathrm{fm}}^2$ for ${}_{}{}^{124}{}_{}{}^{}\mathrm{Sn}$. For an assumed Woods-Saxon real-well geometry of radius $1.2A^{\frac{1}{3}}$ fm and diffuseness 0.73 fm, the volume integral per nucleon is 513 ± 1 MeV ${\mathrm{fm}}^3$ at each resonant maximum. The isotopic sequence of resonances appears to be unrelated to the asymmetry potential. The volume integral per nucleon of the surface absorptive potential is 80 ± 4 MeV ${\mathrm{fm}}^3$ and the diffuseness is 0.4 ± 0.1 fm, where the uncertainties include ambiguities in the other model parameters. Comparisons with scattering at higher energies give evidence for an $l$ dependence in both the real and absorptive potentials. Comparison with the neutron $3p$ size resonance near $A=100\mathrm{}$ is consistent with a 24-MeV asymmetry term in the real potential and a 15-MeV asymmetry term in the absorptive potential. Statistical model analyses of $\gamma$-ray and neutron emission from the compound nuclei suggest that the compound temperatures are about a factor of 2 higher than expected. Evidence is found for broad structures at 1 to 1.5 MeV excitation in${}_{}{}^{118,120}{}_{}{}^{}\mathrm{Sb}$. The Appendix includes an evaluation of levels below 2 MeV in ${}_{}{}^{117}{}_{}{}^{}\mathrm{Sb}$ and ${}_{}{}^{119}{}_{}{}^{}\mathrm{Sb}$.