Pionic double charge exchange on 1618O in the continuum region

Pion reactions with light nuclei at energies around the Δ-resonance (1232) are dominated by single quasi-free πN interactions. The pion double charge exchange reaction (DCX) requires the participation of at least two nucleons to conserve charge, and so provides a clean way of studying multiple pion-nucleon interactions experimentally. The reactions at 115, 163 and 240 MeV, on both 16O and 18O have been measured. This set of data allows one to compare the effects of varying the nuclear neutron number N on quasi-free single scattering, quasi-free double scattering and on DCX. Ratios of these cross-sections incompatible with sequential quasi-free pion interactions could be evidence of an anomalous mechanism contributing to at least one of these channels. (Auth.)