What causes some radionuclides to emit alpha particles?
Alpha-decay occurs mainly in the radioactive decay of the heavier elements, particularly in those members of the natural decay series heavier than lead (atomic number 82), such as uranium and thorium. Alpha-particles are emitted with one of a few discrete energies characteristic of the radionuclide from which they were emitted. These energies can be used to identify the radionuclide involved. Alpha-decay occurs when the ratio of neutrons to protons in the nucleus is low. For example: Polonium-210 has 126 neutrons and 84 protons, a ratio of 1.50 to 1. Following radioactive decay by the emission of an alpha particle, the ratio becomes 124 neutrons to 82 protons, or 1.51 to 1 (Figure 2).