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The simple nuclear Ramsauer model is extended to include explicitly the isovector potential. This model predicts large differences between proton and neutron total nuclear cross sections. When this model is applied to isotope/isotone total neutron cross section differences, it gives a good description of the measured data in the mass 140 region. Non-isospin-dependent differences in the isotopic imaginary potential are observed which totally obscure the contribution from the isovector imaginary potential. Additional measurements further from closed nuclear shells may yield useful information on the isovector imaginary potential.

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Differential cross sections for the (p, n) reaction to the ground-state and excited-state analogs of ' '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-Rip, 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 ' '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.PACS number(s): 25.40.Kv, 21.10.Hw, 27.60.+j the database on odd-even differences in analog states by measuring cross sections for ' Pd to compare with adjacent even isotopes [3]. At the same time, a careful examination of theoretical predictions is presented not just for spin Aip, but also higher multipoles, collective admixtures in the ground state, and higher-order (multistep) contributions to the analog cross section. Section II of this paper gives a description of our experiment and the methods used for data reduction. Section III contains the theoretical development, Sec. IV is a discussion of our results, and Sec. V presents a summary and our conclusions. 47 2077 1993 The American Physical Society 2078 V. A. MADSEN et al. 47