A potential well
The alpha decay of polonium-212 is the one that releases the most important quantity of energy, 8.95 MeV. This decay is forbidden by classical mechanics. It is impossible for an alpha particle to go from inside the nucleus in A to outside in B. It finds itself trapped at the bottom of a "well" as shown by the curve (gray) that represents the potential energy of interaction between the particle and the rest of the nucleus. To get from A to B, the particle must pass through a restricted area where its kinetic energy would be negative. The permitted areas are the well where nuclear attraction dominates, and outside the well where the repulsion due to the nuclear charge prevails.