Exclusive quasi-deuteron absorption of pions in 16O and 18O at 116 MeV.

Abstract

The reaction /sup 16/O(..pi../sup +/,2p)/sup 14/N was measured at T/sub ..pi../ = 116 MeV with an excitation energy resolution between 1.6 and 2.5 MeV with one proton angle fixed at 50/sup 0/ 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 ..pi..pn..-->..pp process. Absorption leading to the T = 1 state at 2.3 MeV is <5% as strong as absorption to the T = 0 groundmore » state. The /sup 18/O(..pi../sup +/,2p)/sup 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.« less