Gamma Rays, a form of electromagnetic radiation. Of all forms of electromagnetic radiation, gamma rays have the shortest wavelengths and greatest energy. Gamma rays were discovered in 1900 by Paul Villard, a French physicist. They can be detected with photographic film or plates, or with such devices as Geiger-Mller counters and scintillation counters.

Gamma rays can be produced either as a result of a nuclear reaction or by the annihilation of matter by antimatter. Nuclear reactions that result in the emission of gamma rays include some types of radioactive decay and the fission (splitting) of a nucleus.

Gamma rays are very penetrating; even a thick sheet of a dense material such as lead will not block them entirely. When gamma rays pass through matter, they eject electrons from the atoms they strike. This process, called ionization, is harmful to living cells. A living thing exposed to intense or prolonged gamma radiation can become seriously ill and die.

Gamma rays are used in industry to inspect castings and welds. The gamma rays are passed through the object being inspected onto photographic film. The image formed on the film can reveal defects that are invisible to the eye or hidden from direct observation. In medicine, gamma rays are used to destroy certain types of cancer. Cobalt 60 is a substance that is commonly used in hospitals as a source of gamma rays for this purpose.

Artificial satellites have revealed that a variety of astronomical objects, including the sun, clouds of interstellar matter, and remnants of supernovae, are sources of gamma rays. They have also detected strong, random bursts of gamma rays from unknown distant sources.