New type of nuclear fission discovered: These facilities enable physicists to work with pure beams of highly unstable heavy elements and collect their reaction products and analyze them. They began with a beam of the highly unstable thallium-180, which has 81 protons and 99 neutrons, and which decayed primarily by the capture of an electron to convert one of the protons into a neutron, giving the 80 protons and 100 neutrons of mercury-180. This should then theoretically split symmetrically.
Instead, the mercury isotope split into ruthenium-100, with 44 protons and 56 neutrons, and krypton-80, with 36 protons and 44 neutrons. These are isotopes with incompletely filled energy levels.