Survey of shell model two and four nucleon correlations

Shell model strong and weak bonds in nuclei are discussed. This problem concerns in particular the recent fragmentation experiments of medium and heavy nuclei. They are induced by the scattering of high energy projectiles (p,π,α...) or the atomic capture of pions or kaons. Nucleons in space symmetric states have greatly enhanced interactions because of the exchange and short range nature of nuclear forces. This fact leads not only to cluster models in light nuclei but also to specific 2 and 4 nucleon sub-structures in the shell model of medium and heavy nuclei. These correlations are approached by examining the following two points: 1. Strong and weak bonds between core nucleons described by a spherical j-j coupling Hartree-Fock state. 2. Strong and weak bonds between valence nucleons as shown by (a) the fine structure of the mass curve; (b) shell model configuration mixing calculations; (c) aligned or pairing schemes. Core and valence nucleons must be distinguished because in the latter case the pairing and alignment effects of nuclear forces can fully play in paired or aligned structures. The way these effects yield stable sub-structures in closed j shells require a special treatment. It is convenient to define as a ''quartet'' any group of two protons and two neutrons whose mutual interactions are much stronger than their interactions with the remaining nucleons of the system, whatever their space correlation or relative quantum numbers