Exclusive quasi-deuteron absorption of pions inO16andO18at 116 MeV

The reaction ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(${\pi }^{+}$,2p${)}^{14}$N was measured at $T_{\pi }$=116 MeV with an excitation energy resolution between 1.6 and 2.5 MeV with one proton angle fixed at 50° and for a wide range of the second angle. Proton angular correlations, energy sharing distributions, and recoil momentum distributions for four strongly populated states are consistent with a model of pion absorption on quasi-deuteron pairs with angular momentum transfer to the recoil nucleus of L=0 (3.9 MeV state), L=2 (7.0 and 11.0 MeV states), or a mixture of L=0 and L=2 (ground state). Integration of the measured absorption cross section over angle and up to 20 MeV of excitation shows that 19±4% of the total absorption cross section can be found in two-proton final states near quasifree kinematics, with substantial additional two-nucleon strength between 20 and roughly 50 MeV of excitation. With estimated corrections for final-state interactions, about half of the absorption cross section below 20 MeV excitation proceeds via the direct quasifree πpn→pp process. Absorption leading to the T=1 state at 2.3 MeV is <5% as strong as absorption to the T=0 ground state. The ${}_{}{}^{18}{}_{}{}^{}\mathrm{O}$(${\pi }^{+}$,2p${)}^{16}$N reaction was measured under the same kinematic conditions. States of less than 0.4 MeV excitation, which correspond to absorption on nucleons in different shells, account for at least 6.1±0.6% of the estimated two-nucleon absorption cross section below 20 MeV excitation; no unexpected suppression of cross-shell absorption was found.