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The Spin Orbit term in the Nuclear Shell Model

Quasi-nuclear systems, representing nuclei with variable size, are studied to investigate the occurrence of the spin-orbit term in the nuclear mean field in the transition from infinite nuclear matter to finite nuclei. Relativistic as well as non-relativistic mean field calculations based on models for the nucleon-nucleon ($NN$) interaction, which fit the $NN$ scattering data, are considered. A very strong correlation between the strength of the spin-orbit term and radius of the nuclear system is observed. The origin of the spin-orbit term is analyzed by inspecting the contributions of the different partial waves and various mesons in a One-Boson-Exchange model of the $NN$ interaction. The influence of correlation effects and the enhancement of the small component of Dirac spinors for nucleons in the nuclear medium is discussed.