Shape coexistence in the light Po isotopes

The systematics of the even light Po isotopes (N less than or equal to 126) are studied in the framework of the Particle-Core Model. The strong pertur bation of the systematics in the very light isotopes is interpreted as aris ing from the interaction between regular and intruder structures. Results o f Potential Energy Surface (PES) calculations and predictions of the Pairin g Vibration Model support this interpretation. The mixing between the regul ar and intruder structures is studied within the IBM-2 and in a simple two- state mixing picture. Matrix elements of the interaction and their spin dep endence are extracted. The 'reconstructed systematics' show the coexistence of a spherical structure, which varies little with the neutron number, wit h an intruder band, strongly lowered in energy as the neutron number approa ches midshell. The crossing of the two configurations takes place over a fe w isotopes; the intruder band becomes the ground-state configuration in Po- 192. (C) 1999 Elsevier Science B.V.