Interpretation of the Optical Model in Nuclear Reactions

Abstract

The theoretical basis of the nuclear optical model is investigated for the elastic scattering of neutrons at low energies. It is argued in connection with statistical mechanics of irreversible processes that the characteristic feature of the optical model can be revealed by means of wave packets whose energy spread is wide enough to cover many compound levels. The giant resonance of the Lorentzian shape is found to result owing to fluctuations in the interaction potential which is experienced by an incident nucleon in a target nucleus. The imaginary part of the optical potential or the half width of a giant resonance is expressed by the product of the second moment and the correlation time. The latter is closely related to the concept of portee introduced by Bloch, with whom we share the same opinion in many respects. The correlation time is shown by our method to reveal a feature of the fluctuations and can be evaluated, if the fourth moment is known. (auth)