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On the interaction of low energy pions with nuclei Scherk, Leonard Raymond


The optical interaction of low energy( ≤30 MeV) pions with nuclei is discussed. In particular, it is shown that, since the nuclear density enters the low energy pion-nucleus interaction in a very direct manner, this interaction provides a sensitive means of investigating such properties of the nuclear density.as the diffuseness of the nuclear surface. A geometric discussion of the structure of the low energy pion-nucleus interaction is given which emphasizes the analogy between adding the scattered pion waves in the nuclear medium and adding electric potentials in a classical dielectric. The parameters of the optical potential which represents the interaction are taken to be those calculated by earlier authors who have used a multiple-scattering formalism to deduce the details of the-optical interaction from a microscopic point of view. The interaction is strongly momentum-dependent and the local part of the interaction is repulsive. It is shown that in optical scattering and absorption, the resonance aspects of the problem depend mainly only upon the height of the local potential barrier (~15 MeV) because of the long wavelength of the pion inside the nucleus. The optical absorption of low energy pions is shown to be sensitive to the diffuseness of the nuclear surface through the strong suppression of momentum-dependent absorption near the top of the potential barrier. It is argued that low energy pions can therefore be used to resolve the conflicts which presently exist in the information available from several experiments concerning the distribution of neutrons in the nuclear surface. The ideas developed in discussing the optical absorption of pions are extended to the excitation by pions of rotational states in strongly deformed nuclei. It is shown that, unlike the excitation cross sections obtained with more conventional interactions (such as the Coulomb interaction) , the excitation cross sections obtained from the pion-nucleus optical interaction depend strongly upon such characteristics of the nuclear density as its surface thickness. The rotational model of strongly deformed nuclei is assumed and an analysis of the excitation cross sections is made in the Distorted Wave Born Approximation. It is shown that the sensitivity to the nuclear surface thickness in the excitation cross sections arises from the suppression of the excitation processes due to the momentum-dependent interaction near the top of the potential barrier. It is suggested that the excitation of rotational levels in strongly deformed nuclei by pions can therefore be used to carefully examine the distribution of neutrons in the surfaces of strongly deformed nuclei.

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