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The last term in the Weizsäcker mass formula deals with a particular residual interaction, originating from pairing correlations of two nucleons inside a nucleus. Two-body correlations are of significant influence in various areas of many-body and condensed matter physics and can even be of immense practical importance. The best example is superconductivity mediated by electron Cooper pairs. Astonishingly, very little is known about the structure and origin of the pairing force in nuclei. Frequently, only a schematic ansatz for the pairing interaction is used and superimposed in an artificial manner on top of microscopic calculations. This approach may be justified in regions close to stability where pairing is a minor effect. Off stability, pairing correlations are of increasing importance and finally are decisive for the exact location of the driplines. One way to study the pairing correlations inside a nucleus is by measuring the production cross sections of residues formed in fragmentation and low-energy fission. The distributions of light residues after violent heavy-ion collisions reveal an even-odd staggering of similar magnitude as in the case of low-energy reactions. This was observed in the mass yields of light residues in different reactions with different target-projectile combinations and at different beam energies. Except for the N=Z nuclei, the observed even-odd staggering in fragmentation reactions can be explained by the available phase space at the end of the evaporation process and the number of available levels in the mother nucleus. A possible origins of the enhanced even-odd effect for N=Z nuclei could be connected with Wigner energy, alpha clustering, neutron-proton pairing, or mean-field contributions to pairing effects. It is a challenge to quantitatively interpret these results with theoretical models in order to better understand the complex nuclear-structure phenomena behind. On the other hand, in case of low-energy fission, from the even-odd structure observed in fission-fragment yields the viscosity of cold nuclear matter can be deduced, as well as information on the superfluid - Fermi liquid phase transition. More information: |
