Theoretical treatment of analog (p,n) cross sections for odd nuclei: Application to measurements of 105Pd at 26 MeV.

Abstract

Differential cross sections for the (p,n) reaction to the ground-state and excited-state analogs of $^{105}\mathrm{Pd}$ have been measured at a proton bombarding energy of 26 MeV. Both the magnitude and the angular distribution of the cross sections for the analog states are found to follow the same general trend observed for the even-even palladium isotopes. Contributions to the analog transition on odd-A targets from spin-flip, higher multipoles, collective admixtures, and multistep processes are calculated and are found to be of order 1/(N-Z) or smaller, as compared to the (N-Z) scaling expected for the Fermi transition. This is in agreement with our experimental data which show that the analog cross section scales as (N-Z) relative to the neighboring nuclei. Among the excited analog states within the first MeV of excitation, the levels at 0.44 and 0.78 MeV were the most strongly populated. This is consistent with a two-step mechanism involving inelastic scattering and charge exchange, since these two states are also known to have the largest B(E2) values. Although the small predicted magnitude of the additional contributions for nonzero spin targets agrees nicely with the present measurements for $^{105}\mathrm{Pd}$, it leaves the puzzle as to why cross sections for odd isotopes for titanium and molybdenum measured in previous work were found to be larger than corresponding cross sections for adjacent even isotopes.