Pion Capture in Complex Nuclei

The effect of the finite lifetime of pion-nucleon resonances (so-called "isobars") on intranuclear cascades is considered. The existence of these isobars inside the nucleus must be postulated if the impulse approximation is valid for intranuclear cascades involving pions. These isobars may interact with neighboring nucleons before decaying. One such interaction provides a mechanism for the capture of high-energy pions. Assuming the impulse approximation to be valid for isobar-nucleon interactions, the cross section for the capture of a high-energy pion by two nucleons inside the nucleus is estimated. Since this is a pion-nucleon cross section, an intranuclear cascade calculation must be made before the pion-nucleus capture cross section can be obtained and compared with experiment. However, the same mechanism should also hold in the capture of high-energy pions in deuterium. Here the calculated cross section is in satisfactory agreement with the experiment.