GAMMA RAY

penetrating electromagnetic radiation of the same nature as an X ray, though usually of somewhat shorter wavelength, emitted spontaneously by some radioactive substances. Gamma rays were named (1903) by Ernest Rutherford when evidence showed they differed from beta rays (electrons ejected by radioactive nuclei). Gamma radiation originates in atomic nuclei during radioactive decay, sometimes in the decay of certain subatomic particles (as pions and sigma particles), and in particle-antiparticle annihilation. The absorption of gamma rays by nuclei can cause particles (as neutrons, alpha particles) to be ejected (photodisintegration) or can even split the nucleus (photofission). A high-energy gamma photon passing near a nucleus sometimes produces a pair of electrons, one negative (negatron), the other positive (positron). Gamma-ray photons lose energy by being scattered from free electrons (Compton effect) or are completely absorbed by ejecting electrons from atoms (photoelectric effect).