Most smoke detectors contain an isotope of Americium (241Am), the nuclei of which decay by emitting an alpha particle. When a nucleus of Americium emits an alpha particle the principle of conservation of momentum and the principle of conservation of energy are obeyed. Show that the ratio (KE of alpha particle)/(KE of resulting nucles) is about 60 and briefly discuss the origin of this energy. The answer given is: E = mc^2 (energy must have come from mass) Total mass after is a little less than before I don't get what this means??