Shape coexistence in the light Po isotopes

The systematics of the even light Po isotopes (N$\le$126) are studied in theframework of the Particle-Core Model. The strong perturbation of thesystematics in the very light isotopes is interpreted as arising fromthe interaction between regular and intruder structures. Results ofPotential Energy Surface (PES) calculations and predictions of thePairing Vibration Model support this interpretation. The mixing between the regular and intruder structures is studiedwithin the IBM-2 and in a simple two-state mixingpicture. Matrix elements of the interaction and their spin-dependence areextracted. The 'reconstructed systematics' show the coexistence of a sphericalstructure, which varies little with the neutron number, with an intruder band,strongly lowered in energy as the neutron number approaches midshell.The crossing of the two configurations takes place over a few isotopes; theintruder band becomes the ground-state configuration in $^{192}$Po.