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

An accelerator uses electrostatic or electromagnetic fields to increase the kinetic energy of charged particles (see alpha particle, beta particle) in order to produce ionization or a nuclear reaction in a target.
Accelerators (see cyclotron, linear accelerator) are used for the production of radionuclides (see Fluorine-18, Molybdenum, Technetium-99m) or directly for radiation therapy. Accelerator-produced radioactive material (ARM) is any radioactive substance that is produced by a particle accelerator. The accelerators used for radiation therapy generate gamma rays (also called Bremsstrahlung) with continuous energy by collision of high energy electrons on materials with high density (also referred as 'high z' - chemical elements with a high atomic number (Z)).
Electron accelerators with energies above 10 MeV can also produce neutrons induced by photons in the accelerator head material (mainly caused by photo nuclear reaction).

Activation is the production of radionuclides (instable atoms) by bombarding atomic nuclei (stable atoms) with radiation (e.g., photons, neutrons, alpha particles).
With the activation of an atom its decay starts.

See Neutron Activation, Neutron Activation Analysis.

Activity is the rate of radioactive decay or the emission rate of a radiation from radioactive material. The cumulated activity is useful for nuclides that emit multiple radiations per disintegration.
The units for activity are the curie (Ci) and the becquerel (Bq).

(ALI) The annual limit on intake refers to the body intake of radionuclides by inhalation, ingestion or through the skin in one year, which would result in a committed dose equal to the relevant dose limit.