When radioactive rays are incident on the materials, electrons from molecules may be excited as the rays move through. When these excited electrons decay to their ground states, electromagnetic radiation is emitted. The frequency of the radiation depends on the energy of the rays and the type of the material excited.

Bremstrahlung [local] radiation is released when free electrons decelerate as they pass by a nucleus and feel the effect of its positive charge. Bremstrahlung radiation can be produced from beta rays [local].