The process in which unstable atomic nuclei spontaneously lose some of their excess energy by disintegrating into more stable nuclei. This is accompanied by the emission of alpha particles, beta particles, or gamma rays. Several of the heavier radioactive elements decay through a series of unstable radioisotopes before reaching a stable end-product. It is possible, in some cases, to induce artificial radioactivity through bombardment of the nuclei with particles such as neutrons.