Odd-even effects in pre-equilibrium (p,n) reactions

Measurements of the ($p,n$) continuum spectra for targets of ${}_{}{}^{103}{}_{}{}^{}\mathrm{Rh}$, ${}_{}{}^{104,105,106,108,110}{}_{}{}^{}\mathrm{Pd}$, and ${}_{}{}^{107,109}{}_{}{}^{}\mathrm{Ag}$ at proton energies of 18, 22, and 25 MeV are compared with the predictions of the hybrid model for pre-equilibrium reactions. The systematics of the odd-even differences in spectral shape are found to be much clearer than in a previous measurement at lower energy with targets near a closed shell. Most of the variation in spectral shape between even- and odd-$A$ targets is attributed to energy shifts produced by pairing effects. Comparisons between the various even-$A$ or odd-$A$ targets show little change in shape and magnitude of the highest energy portion of the measured ($p,n$) spectra over this range in $A$; this is consistent with the predictions of the model. The density of one-proton one-neutron-hole states at low excitations is calculated from a single-particle basis; the calculated values show more structure than is implied by the data, but do have the correct energy dependence when averaged over a wide enough interval.