Ionizing radiation is the term given to forms of radiation that are energetic enough to displace orbiting electrons from the atoms in the absorbing medium, thus forming positive ions. The process of ionization is the principal means by which ionizing radiations dissipate their energy in matter and thus may cause biomolecular damage.

Indirect ionizing radiation involves uncharged particles. X-rays and gamma radiation are the commonest forms of indirect ionizing radiation. Occasionally ultraviolet rays are energetic enough to cause ionization. Most of the ionization associated with electromagnetic radiation is indirect via secondary electrons created secondarily to the primary electrons, which are formed by the initial Compton and photoelectric effects.

Neutrons may also indirectly ionize via their interactions with hydrogen nuclei.

Direct

Direct ionizing radiation involves charged particles. Alpha and beta particles from radioactive decay are examples of direct ionizing radiation.

Indeed any atom or subatomic particle with enough kinetic energy can be ionizing e.g. positrons.

Non-ionizing radiation

Most of the electromagnetic spectrum is non-ionizing, i.e. majority of the ultraviolet wavelengths, visible light, infrared, microwaves and radio waves.

Sound waves (used in ultrasound) are mechanical, as opposed to electromagnetic, waves. Ionization by ultrasound has been observed in laboratory settings but requires powers far exceeding those used in diagnostic imaging ⁴.

Ionizing radiation in radiation biology

Ionizing radiation dissipates energy in tissue via three processes:

ionization
excitation
thermal heating

The energy is deposited randomly and rapidly (in <10^-10 seconds) and occurs at the atomic and molecular level.

Only a fraction of the radiation energy deposited brings about chemical changes, the majority of energy is deposited as heat, which is of little biological significance.

However the energy deposited can damage important molecules such as DNA, inducing detrimental biological effects which may only become apparent after a period of time (latent period) that varies from minutes to weeks to years to a lifetime.