Observation of oscillations and near-barrier suppression in proton capture by 20O

The existence and character of low-angular momentum quasi-bound states in 21 F was probed using a (p,n) reaction. Using an active target detector, the capture excitation function for 20 O + 1 H was measured for the first time. Near the barrier, the capture cross section manifests an oscillatory behavior with broad peaks ∼ 50-100 keV wide. The presence of these peaks reflects the existence of one or more low-angular-momentum states in the quasi-bound regime. One-channel coupled channel (CC) calculations, which account only for the ground state density distributions of the colliding nuclei, show reasonable agreement with the excitation function. However, they overpredict the experimental cross section at the lowest energies measured and fail to reproduce the experimentally observed oscillations. Inclusion of the first excited 2 + state in 20 O in R-matrix (CC) calculations does produce an oscillatory behavior that roughly reproduces the observed oscillations both in magnitude and position. This result suggests that the quasi-bound state in 21 F may share the vibrational character of the 2 + state in 20 O and possibly be coupled to it.