It is the decaying or breaking down of an atom (mostly unstable isotopes of heavy elements), which causes it to give off radiation, (alpha, beta, gamma). Alpha, is the weakest radiation and is composed of 2 protons and 2 neutrons, and are stopped by a sheet of paper. Beta, is composed of electrons, and it travels over half the speed of light and stopped by a thin sheet of aluminum. Gamma radiation, the strongest, travels at the speed of light (c) and is composed of electromagnetic waves. it can be stopped by thick lead