Very high frequency electromagnetic radiation that is emitted spontaneously by certain radioactive elements in the course of a nuclear transition or can be produced in nuclear reactions, as in the annihilation of an elementary particle and its antiparticle. The wavelength of gamma rays emitted by radioactive substances is characteristic of the radioisotope involved and ranges from about 4 × 10−10 to 5 × 10−13 metre. Although gamma rays at one time lay at the extreme low-wavelength end of the electromagnetic spectrum, modern high-voltage generators can now produce X-rays of much shorter wavelength than that of most gamma rays.
Gamma rays undergo no deflection in electric and magnetic fields. Their depth of penetration is controlled by their energy, which depends on the wavelength. The energy of gamma rays is usually measured in electronvolts, and is determined by measuring the maximum energy of photoelectrons that they produce or their diffraction by certain crystal lattices.