Radionuclides are naturally occurring or artificially produced atoms with unstable nuclei. Therefore radionuclides undergo a radioactive decay, and emit gamma ray(s) and/or subatomic particles.
Radionuclides are used as agents in nuclear medicine and to destroy tumors in cancer therapy.

See Isotope and Radioactive Decay.

In radiology, the x-ray yield is the percentage of tube power transformed into radiation.
A high amount of the tube power is used to warm up the target. A higher tube voltage results in a linear increased x-ray yield. The transformation of tube power depends also on the atomic number of the target material. The higher the atomic number, the better the x-ray yield. Tungsten (the most common target material) in combination with a tube voltage of 100kv provides an x-ray yield of 0.7%.

Absorbers consist of material that stops ionizing radiation. For example, lead, steel and concrete attenuate x-rays. Alpha particles and most beta particles can be stopped or absorbed by a sheet of paper or thin metal.
The absorption depends on the atomic number, density, thickness, etc. of the used material.
The interactions between the radiation and the absorber are three major processes: photoelectric absorption, Compton scattering, and pair production.

See also Absorption.

The x-ray absorption is the uptake of energy or the decrease of the number of photons by the tissue or matter through which the radiation travels.
Absorption in nuclear reactions and particulate radiation is a process of taking up kinetic energy of particles or the combination of particles with an atom, a nucleus, or another particle.
Absorption characteristics of imaged tissues are represented by their linear attenuation coefficients.

See also Absorber.

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).