Alpha particle emissions and beta particle emissions change the composition of the nucleus. Gamma rays are high energy electromagnetic radiation with short wavelengths. Gamma rays, unlike alpha and beta particles, do not change the composition of the nuclide. They have the highest penetrating power, being able to penetrate at least 30 centimetres of lead. They undergo radioactive decay.
Radioactivity is found in naturally occurring sources and in artificially produced ones.
People are constantly being exposed to radiation from a variety of natural and human-created sources. Exposure should be minimized, but it can never be reduced to zero. Special detectors are needed. Radiation has an effect on tissue and on genetic material. Several devices have been developed to detect radioactivity, with the earliest being an unexposed photographic plate placed in the vicinity of a source being detected.