Compound-nucleus mechanism for the (p,np̃) reaction near the neutron threshold

A compound-nucleus mechanism is considered for the reaction where the isobaric analogue resonance (IAR) is populated by a (p, n) stage and decays later emitting a proton p?. It is shown that this mechanism leads to an enhancement in the p? spectra near the IAR energy of the type expected from t · T charge exchange. This phenomenon is general, and will occur whenever the analogue state is not populated selectively in the entrance channel. For the case ⁹¹Zr(p, np?)⁹⁰Zr(g.s.) we estimate the contribution to the cross section due to this mechanism. It is found that the model reproduces the shape of the measured cross section and a normalization consistent with the uncertainties of the data. In order to handle fine-structure properties of the IAR, we develop a version of the external-mixing model particularly adapted for these purposes.