Excitation of nuclei by absortion of π--mesons - III

Abstract

The results of two previous papers by Puppiet al. (*) are revised here. In those papers the authors examine the problem of the excitation of a medium heavy nucleus by a bound π--meson. They assume that the pion is absorbed by a nucleon pair; the subsequent disintegration is described as a two step process: a direct emission followed from an evaporation. In the present paper the evaporation process is revised, using a more refined formula. Level densities obtained from disintegration experiments are used; it is shown that the recoil of the nucleus is not negligible in the emission of α-particles; values of level densities differentiated for α-particles and nucleons are used. The results obtained in the case of absorption by a neutron-proton pair are in good agreement with experimental data. It is concluded that the bound π- are absorbed by neutron-proton pairs; other modes of absorption (by protoaproton pairs or by other nucleon groupings) seem to be quite improbable.