2ème Colloque du DRS
par Prof. Richard Caste, Wright Nuclear Structure Laboratory, Yale University (USA)
One of the overarching challenges facing nuclear structure physics is understanding the evolution of structure with the numbers of protons and neutrons. Since nuclei have a shell structure analogous to atoms, structure depends mostly on the numbers of valence protons and neutrons. In certain mass regions, extremely rapid changes in structure have been discovered and, in recent years, have been described in terms of quantum (equilibrium or zero energy) phase transitions (QPT) in the geometrical shape of the nucleus.
In nuclei the control parameter is nucleon number and the order parameter is either a quantity describing the shape or, preferably, an observable proxy for it. The best empirical evidence for QPTs occurs in the mass region around proton number 60-64 and neutron number N=90 - nuclei such as 150Nd and 152Sm. These QPTs have been modeled very successfully in terms of an extraordinarily simple, parameter-free, Critical Point Symmetry model, X(5).
We will then turn to the empirical evidence for QPTs in this region and the comparison with X(5). Areas of agreement and disagreement will be discussed and what they tell us will be analyzed. The talk will end with a discussion of the microscopic drivers of structural evolution in terms of the valence proton - neutron interaction and of extremely simple ways to estimate the structure of virtually any nucleus.