Further Studies of Anomalous Inelastic Proton Scattering

Anomalous inelastic proton scattering was studied by (1) determining its dependence on bombarding energy, (2) measuring the energy distributions with greatly improved energy resolution, (3) extending the data to separated isotopes and to elements not previously investigated because of target preparation difficulties, and (4) observing the energy distribution of the de-excitation gamma rays following the reaction. In (1), the anomalous peak is found to stay at the same $Q$ value as the bombarding energy is varied. This indicates that the effect is not due to a giant resonance effect but rather, to the regular occurrence of a certain type of level in the final nucleus. In (2), many low-lying levels are resolved; a strong correlation is found between their cross sections for excitation in these reactions and in Coulomb excitation, and an anticorrelation is found between their cross sections for excitation in these reactions and in ($p, d$) reactions. This is interpreted as good evidence that the reactions used here strongly excite collective levels and only weakly excite single-particle transitions. The fine structures of the anomalous peaks differ greatly among neighboring nuclides reflecting even-versus-odd and closed-shell effects; however, the gross structure as obtained with poor resolution is very similar for neighboring elements. A few striking similarities between the spectra of several elements are pointed out. Several levels of known spin and parity are identified as contributing strongly to the anomalous peak. In (3), it is found that anomalous inelastic scattering ends by becoming irregular and weak between atomic numbers 54 and 64; it is apparently present in every isotope of every element between atomic numbers 30 and 53. In (4), gamma-ray transitions direct to the ground state following anomalous inelastic scattering are found to be relatively common.