Gamma rays (γ-rays) are electromagnetic waves with the smallest wavelengths in the electromagnetic spectrum. They were discovered in 1900 by Paul Villard, and named in 1903 by Ernest Rutherford.
Gamma rays are like x-rays, but the waves are smaller in wavelength. Both gamma rays and x-rays are photons with very high energies, and gamma have even more energy. They are also a type of ionizing radiation. Gamma rays can travel through thicker materials than x-rays can.
Gamma rays are produced by some types of radioactive atoms. Cobalt-60 and potassium-40 are two isotopes that emit gamma rays. Cobalt-60 is created in accelerators and is used in hospitals. Potassium-40 occurs naturally. Small amounts of potassium-40 are in all plants and animals. Gamma rays from potassium-40 each have an energy of 1460 thousand electron volts (keV).
Gamma rays and X-rays can also be distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.