Radioactive light signs. 2 of 2. These historical signs have self-powered lighting, where they emit light without requiring an external power source. They work by using a radioactive source that undergoes beta decay. Beta decay is a process where unstable isotopes (atoms that vary in their neutron number) transform into more stable isotopes. During this process, they release energy in the form of beta particles. The emitted beta particles are used as the energy source for these signs. The two main sources of beta lighting were radium and tritium. See v900/142 for an image of these signs in daylight.