Ionizing (or ionising) radiation is radiation with sufficient energy to remove an electron from an atom or molecule. This ionization produces free radicals, atoms or molecules containing unpaired electrons, which tend to be especially chemically reactive.
The degree and nature of such ionization depends on the energy of the individual particles (including photons), not on their number (intensity). In the absence of heating a bulk substance up to ionization temperature, or multiple absorption of photons (a rare process), an intense flood of particles or particle-waves will not cause ionization if each particle or particle-wave does not carry enough individual energy to be ionizing (an example is a high-powered radio beam, which will not ionize if it does not cause high temperatures). Conversely, even very low-intensity radiation will ionize at low temperatures and powers, if the individual particles carry enough energy (e.g., a low-power X-ray beam). In general, particles or photons with energies above a few electron volts (eV) are ionizing, no matter what their intensity.
Examples of ionizing particles are alpha particles, beta particles, neutrons, and cosmic rays. The ability of an electromagnetic wave (photons) to ionize an atom or molecule depends on its frequency, which determines the energy of its associated particle, the photon. Radiation on the short-wavelength end of the electromagnetic spectrum—high-frequency ultraviolet, X-rays, and gamma rays—is ionizing, due to its composition of high-energy photons. Lower-energy radiation, such as visible light, infrared, microwaves, and radio waves, are not ionizing. The latter types of low-energy non-ionizing radiation may damage molecules, but the effect is generally indistinguishable from the effects of simple heating. Such heating does not produce free radicals until higher temperatures (for example, flame temperatures or “browning” temperatures, and above) are attained. In contrast, damage done by ionizing radiation produces free radicals, even at room temperatures and below, and production of such free radicals is the reason these and other ionizing radiations produce quite different types of chemical effects from (low-temperature) heating. Free radical production is also a primary basis for the particular danger to biological systems of relatively small amounts of ionizing radiation that are far smaller than needed to produce significant heating. Free radicals easily damage DNA, and ionizing radiation may also directly damage DNA by ionizing or breaking DNA molecules.
Ionizing radiation is ubiquitous in the environment, and also comes from radioactive materials, X-ray tubes, and particle accelerators. It is invisible and not directly detectable by human senses, so instruments such as Geiger counters are usually required to detect its presence. In some cases it may lead to secondary emission of visible light upon interaction with matter, such as in Cherenkov radiation and radioluminescence. It has many practical uses in medicine, research, construction, and other areas, but presents a health hazard if used improperly. Exposure to radiation causes damage to living tissue, and can result in mutation, radiation sickness, cancer, and death.