Nuclear Physics A$$3 (1993) 543c-552c North-Holland, Amsterdam RADIATIVE PROTON CAPTURE AND EXCHANGE CURRENTS IN LIGHT NUCLEI NUCLEAR PHYSICS A B. H6istad, S. Isaksson, E. Nilsson and J. Thun Department of Radiation Sciences, Uppsala University, S-75121 Uppsala, Sweden G. S. Adams and C. Landberg Physics Department, Rensselaer Polytechnic Institute, Troy, New York 12180 T. B. Bright and S. R. Cotanch Department of Physics, North Carolina State University, Raleigh, North Carolina 27695-8202 Abstract A new type of high resolution pair spectrometer for medium energy photons has been used for studies of the exclusive (P,7) and (p,e"e-) reactions on light nuclei in the energy region 100 to 200 MeV. From the current experiments we present angular distributions of the differential cross section for transitions to the ground state and first three excited states in 12C. The distributions from the ground state and first excited state are compared with predictions from a microscopic con- tinuum shell model calculation which uses a realistic finite-range effective interac- tion with tensor components. Based on preliminary data, the salient features of the (P, 7) reaction on 12C and the (p,e÷e -) reaction on lib are also discussed. 1. INTRODUCTION The identification of non-nucleonic degrees of freedom in the nucleus continues to be a topic of great urgency in nuclear physics. The presence of non- nucleonic particles in the nucleus is generally incorporated in the Hamiltonian for the many-body system. Mesons and resonances are thus present in the nucleus to the extent that they are explicitly used to generate the nuclear interaction. The occurrence of mesons in the nucleus is then manifested in the meson exchange part of the total nuclear electromagnetic current. These exchange currents 1, generated by the strong interaction, are generally detected by the electromagnetic interaction in photo-nuclear or electro-nuclear reactions. By choosing a suitable region with respect to energy and momentum transfer for these reactions, it should be possible to isolate the effects due to exchange currents in the experimental data. The radiative proton capture reaction at intermediate energies, which has been a subject of great interest 2-7, is expected to be partic~jlarly sensitive to these two-body currents, since this reaction always involves a large momentum transfer to the residual nucleus. This large momentum is more likely to be absorbed on a correlated nucleon pair allowing momentum sharing, rather than on a single nucleon. The photon can then couple not only to the individual nucleon, but also to the mesons being exchanged. At intermediate energies delta 0375-9474/931506.00 © 1993 - Elsevier Science PublishersB.V. All rights reserved.