Analysis of Two-Neutron (L=0) Transfer Cross Sections for Calcium and Nickel

We have tried to determine whether experimental data on $L=0\mathrm{}$ ($t,p$) and ($p,t$) reactions on the even calcium and nickel isotopes are consistent with a reasonable amount of configuration mixing and a distorted-wave Born-approximation treatment of the two-neutron transfer process. Our calcium calculation involves the six shells from $2s_{\frac{1}{2}}$ through $1f_{\frac{5}{2}}$; our nickel calculation involves the five shells from $1f_{\frac{7}{2}}$ through $1g_{\frac{9}{2}}$. We have simulated the configuration mixing by diagonalizing a pairing force between seniority-zero states. The calculated eigenstates are consistent with one-particle transfer data, and give a satisfactory account of the ratios of observed ($t,p$) and ($p,t$) cross sections for ground-state transitions. However, certain excited calcium states seen in the ($t,p$) reaction are predicted to be populated with about twice the observed cross section. These states must involve more complicated degrees of freedom than the seniority-zero components we have included. Calculated ($t,p$) angular distributions are in good agreement with the experimental data. Calculated ($p,t$) angular distributions are about 5° out of phase with the experimental data.