PION REACTIONS WITH LIGHT NUCLEI AT 3-3 RESONANCE

The reactions of positive and negative pions with light nuclei have been studied over the energy range 80 to 280 MeV by detecting the residual radio-activity. Charge exchange, (π+, π0), is shown to be closely related to the corresponding nucleon reaction, (p, n), but it is difficult to account for the ratio of the cross sections of 13C(π+, π0)13N and of 14N(π+, π0)14O in detail. Most of the reaction cross section of pions is made up by processes such as 12C →π 11C, i.e. single nucleon knockout. Previously this was accounted for quantitatively in terms of quasi-free scattering, which predicts a ratio of 3 : 1 for the 11C production by π- and π+ (influence of the 3-3 resonance of the π-nucleon interaction). The ratio observed is 1.0±0.1 for 12C → 11C, 14N → 13N and 16O → 15O. The simplest explanation is that the activity is the result of roughly equal amounts of quasi-free scattering and inelastic scattering exciting the giant dipole resonance of the target nucleus. Alternatively, a more complicated interaction of pions with nucleon pairs could account for almost any ratio. Cross sections for 22 π-reactions at 180 MeV are presented together with excitation functions for 5 of these reactions. © 1969.