Mesonic exchange contributions to nuclear photo reactions

In the present investigation we discuss the importance of mesonic effects in processes like (γ,n), (γ,p), (γ,pn), (γ,pp) and (γ,nn). The description of these reactions in the shell model, as well as in extended shell models (Jastrow-, Brueckner- Goldstone-correlations), has not been satisfactory. We show that a systematic treatment of the exchange currents, in the simplest approach, is able to explain the standing discrepancy between experiment and pure shell model. The reason for the simular behaviour of the reactions (γ,p) and (γ,n) is explained, as well as the suppression of the reactions (γ,pp) and (γ,nn) relative to (γ,pn). Numerical examples are given in the case of 4He for the reactions (γ,p) and (γ,n). The presented results have been obtained in collaboration with H. Hebach and will be published in Physics Letters. (orig.)