When an electron collides with its antiparticle, a positron, causing both elements to be completely destroyed and in most cases resulting in the emission of gamma rays.

See also Electron, Positron and Antiparticle.

(F) The number of cycles of a periodic process per unit time. Frequency and wavelength are inversely related. The higher the frequency the smaller the wavelength. The frequency of for example ultrasound is expressed in units of hertz (Hz), where 1 Hz = 1 cycle per second.
Usual frequencies in medical imaging are e.g., for ultrasound in the range of 1-20 MHz, for x-rays above 3x10¹⁶ Hz and for gamma rays less than 10²⁰ Hz.

Gamma radiation is electromagnetic radiation emitted in decay of radionuclides. Also called gamma ray and sometimes shortened to gamma (e.g., gamma-emitting radionuclides).

Radiation can ionize matter caused by the high energy which displaces electrons during interactions with atoms. In the electromagnetic spectrum higher frequency ultraviolet radiation begins to have enough energy to ionize matter.
Examples of ionizing radiation include alpha particles, beta particles, gamma rays, x-rays, neutrons, high-speed electrons, high-speed protons, and other particles capable of producing ions by direct or secondary processes in passage through tissues.
Damage of living tissue results from the transfer of energy to atoms and molecules in the cellular structure. Ionized cells have to repair themselves to remain alive. Generally, healthy cells have a higher capability to repair themselves than cancer cells.

Biological effects of ionizing radiation exposure:
Ionizing radiation are used in a wide range of facilities, including health care, research institutions, nuclear reactors and their support facilities, and other manufacturing settings. These radiation sources can pose a serious hazard to affected people and environment if not properly controlled.

See also Radiation Safety, Controlled Area, Radiotoxicity and As Low As Reasonably Achievable.

If a nucleus still has excess energy after attempts of stabilization, it can emit energy without changing the number of protons or neutrons. This process is named isomeric transition. One way of isomeric transition is the emission of a gamma rays, the other competing way is internal conversion, where the excess energy of the nucleus must exceed the binding energy of an electron, which then will be ejected from the atom.

See also Decay, Gamma Radiation and Internal Conversion.