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 antiparticle is a subatomic particle similar in mass but with opposite electromagnetic properties. Most particles have their antiparticles, electron - positron, etc. Antimatter is composed of antiparticles. Colliding of a particle with its respective antiparticle leads in case of low-energy particles mostly to photon production, in case of high-energy particles to exotic heavy particles.

An atom is the basic particle of matter, the smallest (~10^-8cm) particle of a chemical element. Atoms consist of a nucleus with neutrons, positive charged protons and orbiting negative charged electrons. The chemical properties of elements are defined largely by the number of protons in the nucleus. The number of electrons is similar to the number of protons. An atom is ionized when the number of electrons is not equal to the number of protons; the resulting electrical charge depends on the difference between the number of protons and the number of electrons.

See Rutherford-Bohr Atom Model, Ionization.

The attenuation of radiation is a decrease in intensity as a result of interactions by transmission through matter. X-ray beams attenuate due to photon absorption by the material or scattering. Both effects are energy dependent. The probability of absorption or scattering is a function of the photon energy. The photoelectric absorption is much more energy dependent than the Compton scatter effect.

See also Attenuation Correction, Linear Energy Transfer, Broad Beam and Ion Beam.

In radiology dose is the term for radiation related to the amount of energy absorbed in matter (for example absorbed dose, acute dose, effective dose, external dose, personal dose and committed dose equivalent). The SI standard unit of radiation dose is the gray.
Dose refers also to the amount of medication, radiopharmaceutical or contrast medium used in diagnostic imaging.

See also Deep Dose Equivalent, Critical Organ, Eye Dose Equivalent, Collective Effective Dose, Medical Internal Radiation Dose Committee, and Chronic Dose.