Ionizing radiation explained

Ionizing radiation (ionising radiation) consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them.[1] The particles generally travel at a speed that is greater than 1% of that of light, and the electromagnetic waves are on the high-energy portion of the electromagnetic spectrum.

Gamma rays, X-rays and the higher ultraviolet part of the electromagnetic spectrum are ionizing radiation, whereas the lower energy ultraviolet, visible light, nearly all types of laser light, infrared, microwaves, and radio waves are non-ionizing radiation. The boundary between ionizing and non-ionizing radiation in the ultraviolet area is not sharply defined, since different molecules and atoms ionize at different energies, but is between 10 electronvolts (eV) and 33 eV.

Typical ionizing subatomic particles due to radioactive decay include alpha particles, beta particles and neutrons and almost all are energetic enough to be ionizing. Secondary cosmic particles produced after cosmic rays interact with Earth's atmosphere include muons, mesons, and positrons.[2] [3] Cosmic rays may also produce radioisotopes on Earth (for example, carbon-14), which in turn decays and emits ionizing radiation. Cosmic rays and the decay of radioactive isotopes are the primary sources of natural ionizing radiation on Earth contributing to background radiation. Ionizing radiation is also generated artificially by such as X-ray tubes, particle accelerators, and nuclear fission.

Ionizing radiation is not detectable by human senses, so instruments such as Geiger counters must be used to detect and measure it. However, very high intensities can produce visible light such as in Cherenkov radiation. Ionizing radiation is used in a wide variety of fields such as medicine, nuclear power, research, and industrial manufacturing, but presents a health hazard if proper measures against excessive exposure are not taken. Exposure to ionizing radiation causes cell damage to living tissue and in high acute doses will result in radiation burns and radiation sickness, and lower level doses over a protracted time can cause cancer.[4] The International Commission on Radiological Protection (ICRP) issues guidance on ionizing radiation protection, and the effects of dose uptake on human health.

Directly ionizing radiation

See main article: article and Particle radiation. Ionizing radiation may be grouped as directly or indirectly ionizing.

Any charged particle with mass can ionize atoms directly by fundamental interaction through the Coulomb force if it carries sufficient kinetic energy. Such particles include atomic nuclei, electrons, muons, charged pions, protons, and energetic charged nuclei stripped of their electrons. When moving at relativistic speeds (near the speed of light, c) these particles have enough kinetic energy to be ionizing, but there is considerable speed variation. For example, a typical alpha particle moves at about 5% of c, but an electron with 33 eV (just enough to ionize) moves at about 1% of c.

Two of the first types of directly ionizing radiation to be discovered are alpha particles which are helium nuclei ejected from the nucleus of an atom during radioactive decay, and energetic electrons, which are called beta particles.

Natural cosmic rays are made up primarily of relativistic protons but also include heavier atomic nuclei like helium ions and HZE ions. In the atmosphere such particles are often stopped by air molecules, and this produces short-lived charged pions, which soon decay to muons, a primary type of cosmic ray radiation that reaches the surface of the earth. Pions can also be produced in large amounts in particle accelerators.

Alpha particles

See main article: article and Alpha particle. Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Alpha particle emissions are generally produced in the process of alpha decay.

Alpha particles are a strongly ionizing form of radiation, but when emitted by radioactive decay they have low penetration power and can be absorbed by a few centimeters of air, or by the top layer of human skin. More powerful alpha particles from ternary fission are three times as energetic, and penetrate proportionately farther in air. The helium nuclei that form 10–12% of cosmic rays, are also usually of much higher energy than those produced by radioactive decay and pose shielding problems in space. However, this type of radiation is significantly absorbed by the Earth's atmosphere, which is a radiation shield equivalent to about 10 meters of water.[5]


Beta particles

See main article: article and Beta particles. Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei, such as potassium-40. The production of beta particles is termed beta decay. They are designated by the Greek letter beta (β).There are two forms of beta decay, β and β+, which respectively give rise to the electron and the positron.[6] Beta particles are less penetrating than gamma radiation, but more penetrating than alpha particles.

High-energy beta particles may produce X-rays known as bremsstrahlung ("braking radiation") or secondary electrons (delta ray) as they pass through matter. Both of these can cause an indirect ionization effect. Bremsstrahlung is of concern when shielding beta emitters, as the interaction of beta particles with the shielding material produces Bremsstrahlung. This effect is greater with material of high atomic numbers, so material with low atomic numbers is used for beta source shielding.

Positrons and other types of antimatter

See main article: article, Positron and Antimatter. The positron or antielectron is the antiparticle or the antimatter counterpart of the electron. When a low-energy positron collides with a low-energy electron, annihilation occurs, resulting in their conversion into the energy of two or more gamma ray photons (see electron–positron annihilation). As positrons are positively charged particles they can directly ionize an atom through Coulomb interactions.


Charged nuclei



Indirectly ionizing radiation

Indirectly ionizing radiation is electrically neutral and does not interact strongly with matter, therefore the bulk of the ionization effects are due to secondary ionization.

Photon radiation

See also: Gamma rays and X-rays.

Even though photons are electrically neutral, they can ionize atoms indirectly through the photoelectric effect and the Compton effect. Either of those interactions will cause the ejection of an electron from an atom at relativistic speeds, turning that electron into a beta particle (secondary beta particle) that will ionize other atoms. Since most of the ionized atoms are due to the secondary beta particles, photons are indirectly ionizing radiation.[8]

Radiated photons are called gamma rays if they are produced by a nuclear reaction, subatomic particle decay, or radioactive decay within the nucleus. They are called x-rays if produced outside the nucleus. The generic term "photon" is used to describe both.[9] [10] [11]

X-rays normally have a lower energy than gamma rays, and an older convention was to define the boundary as a wavelength of 10−11 m (or a photon energy of 100 keV).[12] That threshold was driven by historic limitations of older X-ray tubes and low awareness of isomeric transitions. Modern technologies and discoveries have shown an overlap between X-ray and gamma energies. In many fields they are functionally identical, differing for terrestrial studies only in origin of the radiation. In astronomy, however, where radiation origin often cannot be reliably determined, the old energy division has been preserved, with X-rays defined as being between about 120 eV and 120 keV, and gamma rays as being of any energy above 100 to 120 keV, regardless of source. Most astronomical "gamma-ray astronomy" are known not to originate in nuclear radioactive processes but, rather, result from processes like those that produce astronomical X-rays, except driven by much more energetic electrons.


Definition boundary for lower-energy photons

See also: Ultraviolet. The lowest ionization energy of any element is 3.89 eV, for caesium. However, US Federal Communications Commission material defines ionizing radiation as that with a photon energy greater than 10 eV (equivalent to a far ultraviolet wavelength of 124 nanometers).[13] Roughly, this corresponds to both the first ionization energy of oxygen, and the ionization energy of hydrogen, both about 14 eV.[14] In some Environmental Protection Agency references, the ionization of a typical water molecule at an energy of 33 eV is referenced[15] as the appropriate biological threshold for ionizing radiation: this value represents the so-called W-value, the colloquial name for the ICRU's mean energy expended in a gas per ion pair formed,[16] which combines ionization energy plus the energy lost to other processes such as excitation.[17] At 38 nanometers wavelength for electromagnetic radiation, 33 eV is close to the energy at the conventional 10 nm wavelength transition between extreme ultraviolet and X-ray radiation, which occurs at about 125 eV. Thus, X-ray radiation is always ionizing, but only extreme-ultraviolet radiation can be considered ionizing under all definitions.











Physical effects

See main article: article and Radiation damage.

Nuclear effects


Chemical effects

See main article: article and Radiation chemistry. Ionization of molecules can lead to radiolysis (breaking chemical bonds), and formation of highly reactive free radicals. These free radicals may then react chemically with neighbouring materials even after the original radiation has stopped. (e.g., ozone cracking of polymers by ozone formed by ionization of air). Ionizing radiation can also accelerate existing chemical reactions such as polymerization and corrosion, by contributing to the activation energy required for the reaction. Optical materials deteriorate under the effect of ionizing radiation.

High-intensity ionizing radiation in air can produce a visible ionized air glow of telltale bluish-purple color. The glow can be observed, e.g., during criticality accidents, around mushroom clouds shortly after a nuclear explosion, or the inside of a damaged nuclear reactor like during the Chernobyl disaster.


Electrical effects


Proton radiation found in space can also cause single-event upsets in digital circuits. The electrical effects of ionizing radiation are exploited in gas-filled radiation detectors, e.g. the Geiger-Muller counter or the ion chamber.

Health effects

See main article: article and Radiobiology.

Most adverse health effects of exposure to ionizing radiation may be grouped in two general categories:

The most common impact is stochastic induction of cancer with a latent period of years or decades after exposure. For example, ionizing radiation is one cause of chronic myelogenous leukemia,[19] [20] [21] although most people with CML have not been exposed to radiation.[20] [21] The mechanism by which this occurs is well understood, but quantitative models predicting the level of risk remain controversial.

The most widely accepted model the Linear no-threshold model (LNT) holds that the incidence of cancers due to ionizing radiation increases linearly with effective radiation dose at a rate of 5.5% per sievert. If this is correct, then natural background radiation is the most hazardous source of radiation to general public health, followed by medical imaging as a close second. Other stochastic effects of ionizing radiation are teratogenesis, cognitive decline, and heart disease.


Measurement of radiation


Methods of radiation measurement
QuantityDetectorCGS unitsSI unitsOther units
Disintegration rate
Particle fluxcounts per minute, particles per cm per second
Energy fluence
Beam energy
Linear energy transferderived quantity
Kermagray roentgen
Absorbed doserep
Equivalent dosederived quantity sievertWR)
Effective dosederived quantity sievertWR × WT)BRET
Committed dosederived quantity banana equivalent dose

Uses of radiation



Sources of radiation



Type of radiation exposures
Public exposure
Natural Sources Normal occurrences Cosmic radiation
Terrestrial radiation
Enhanced sources Metal mining and smelting
Phosphate industry
Coal mining and power production from coal
Oil and gas drilling
Rare earth and titanium dioxide industries
Zirconium and ceramics industries
Application of radium and thorium
Other exposure situations
Man-made sources Peaceful purposes Nuclear power production
Transport of nuclear and radioactive material
Application other than nuclear power
Military purposes Nuclear tests
Residues in the environment. Nuclear fallout
Historical situations
Exposure from accidents
Occupational radiation exposure
Natural Sources Cosmic ray exposures of aircrew and space crew
Exposures in extractive and processing industries
Gas and oil extraction industries
Radon exposure in workplaces other than mines
Man-made sources Peaceful purposes Nuclear power industries
Medical uses of radiation
Industrial uses of radiation
Miscellaneous uses
Military purposes Other exposed workers
Source UNSCEAR 2008 Annex B retrieved 2011-7-4


Background radiation

See main article: article and Background radiation. Background radiation comes from both natural and man-made sources.



The background rate for natural radiation varies considerably with location, being as low as 1.5 mSv/a (1.5 mSv per year) in some areas and over 100 mSv/a in others. The highest level of purely natural radiation recorded on the Earth's surface is 90 µGy/h (0.8 Gy/a) on a Brazilian black beach composed of monazite.[24] The highest background radiation in an inhabited area is found in Ramsar, primarily due to naturally radioactive limestone used as a building material. Some 2000 of the most exposed residents receive an average radiation dose of 10 mGy per year, (1 rad/yr) ten times more than the ICRP recommended limit for exposure to the public from artificial sources.[25] Record levels were found in a house where the effective radiation dose due to external radiation was 135 mSv/a, (13.5 rem/yr) and the committed dose from radon was 640 mSv/a (64.0 rem/yr).[26] This unique case is over 200 times higher than the world average background radiation. Despite the high levels of background radiation that the residents of Ramsar receive there is no compelling evidence that they experience a greater health risk. The ICRP recommendations are conservative limits and may represent an over representation of the actual health risk. Generally radiation safety organization recommend the most conservative limits assuming it is best to err on the side of caution. This level of caution is appropriate but should not be used to create fear about background radiation danger. Radiation danger from background radiation may be a serious threat but is more likely a small overall risk compared to all other factors in the environment.

Cosmic radiation




External terrestrial sources


Internal radiation sources



See also: Health effects of radon. An important source of natural radiation is radon gas, which seeps continuously from bedrock but can, because of its high density, accumulate in poorly ventilated houses.

Radon-222 is a gas produced by the α-decay of radium-226. Both are a part of the natural uranium decay chain. Uranium is found in soil throughout the world in varying concentrations. Radon is the largest cause of lung cancer among non-smokers and the second-leading cause overall.[27]

Radiation exposure

See main article: article and Radiation protection. There are three standard ways to limit exposure:

  3. Shielding: Air or skin can be sufficient to substantially attenuate alpha and beta radiation. Barriers of lead, concrete, or water are often used to give effective protection from more penetrating particles such as gamma rays and neutrons. Some radioactive materials are stored or handled underwater or by remote control in rooms constructed of thick concrete or lined with lead. There are special plastic shields that stop beta particles, and air will stop most alpha particles. The effectiveness of a material in shielding radiation is determined by its half-value thicknesses, the thickness of material that reduces the radiation by half. This value is a function of the material itself and of the type and energy of ionizing radiation. Some generally accepted thicknesses of attenuating material are 5 mm of aluminum for most beta particles, and 3 inches of lead for gamma radiation.



Occupational exposure

Occupationally exposed individuals are controlled within the regulatory framework of the country they work in, and in accordance with any local nuclear licence constraints. These are usually based on the recommendations of the International Commission on Radiological Protection.The ICRP recommends limiting artificial irradiation. For occupational exposure, the limit is 50 mSv in a single year with a maximum of 100 mSv in a consecutive five-year period.


Examples of activities where occupational exposure is a concern include:


Public exposure







Air travel


Radiation hazard warning signs



See also


External links

Notes and References

  1. Web site: Ionizing radiation, health effects and protective measures. 29 April 2016. World Health Organization.
  2. Book: Woodside , Gayle . Environmental, Safety, and Health Engineering . John Wiley & Sons . 1997 . US . 476 . 978-0471109327 . live . . 2015-10-19.
  3. Book: Stallcup , James G. . OSHA: Stallcup's High-voltage Telecommunications Regulations Simplified . Jones & Bartlett Learning . 2006 . US . 133 . 978-0763743475 . live . . 2015-10-17.
  4. Ryan. Julie. 5 January 2012. Ionizing Radiation: The Good, the Bad, and the Ugly. The Journal of Investigative Dermatology. 132. 3 0 2. 985–993. 3779131. 22217743. 10.1038/jid.2011.411.
  5. One kg of water per cm squared is 10 meters of water
  6. Web site: Beta Decay. 9 August 2000. dead. 3 March 2016. 10 April 2014.
  7. [hdl:2060/19990051001|Contribution of High Charge and Energy (HZE) Ions During Solar-Particle Event of September 29, 1989]
  8. Web site: European Centre of Technological Safety. Interaction of Radiation with Matter. Radiation Hazard. 5 November 2012. dead. 12 May 2013.
  9. Book: Feynman , Richard . Robert Leighton . Matthew Sands . The Feynman Lectures on Physics, Vol.1 . registration . Addison-Wesley . 1963 . USA . 2–5 . 978-0-201-02116-5.
  10. Book: L'Annunziata , Michael . Mohammad Baradei . Handbook of Radioactivity Analysis . Academic Press . 2003 . 58 . 978-0-12-436603-9.
  11. Book: Grupen , Claus . G. Cowan . S. D. Eidelman . T. Stroh . Astroparticle Physics . Springer . 2005 . 109 . 978-3-540-25312-9.
  12. Book: Charles Hodgman, Ed. . CRC Handbook of Chemistry and Physics, 44th Ed. . Chemical Rubber Co. . 1961 . USA . 2850.
  13. Web site: Questions and Answers about Biological Effects and Potential Hazards of Radiofrequency Electromagnetic Fields . 2011-12-07 . live . . 2011-10-20 . Robert F. Cleveland, Jr. . Jerry L. Ulcek . OET (Office of Engineering and Technology) Federal Communications Commission . Washington, D.C. . 4th . August 1999.
  14. Web site: Jim Clark . 2000 . Ionisation Energy . 2011-12-07 . live . . 2011-11-26.
  15. Web site: Ionizing & Non-Ionizing Radiation . Radiation Protection . EPA . 2015-01-09 . dead . . 2015-02-12. 2014-07-16 .
  16. Journal of the ICRU . 11 . 1 . Fundamental Quantities and Units for Ionizing Radiation (ICRU Report 85) . 2011 . live . . 2012-04-20 .
  17. Book: . Gas Filled Detectors . dead . . 2012-06-17 . Hao Peng . MacMaster University, Department of Medical Physics and Radiation Sciences . Lecture notes for MED PHYS 4R06/6R03 – Radiation & Radioisotope Methodology.
  18. W.-M. Yao . etal . Particle Data Group Summary Data Table on Baryons . J. Phys. G . 33 . 1 . dead . . 2011-09-10 . 2007 . 2012-08-16.
  19. Book: Understanding pathophysiology . Elsevier . Huether . Sue E. . McCance . Kathryn L. . 9780323354097 . 6th . St. Louis, Missouri . 530 . 740632205. 2016-01-22 .
  20. Web site: Chronic myeloid leukemia (CML) . . 22 September 2019. 2015-02-26 .
  21. Web site: Chronic myelogenous leukemia (CML) Chronic myelogenous leukemia (CML) . Medline Plus Medical Encyclopedia . . 22 September 2019.
  22. Liebel F, Kaur S, Ruvolo E, Kollias N, Southall MD . Irradiation of skin with visible light induces reactive oxygen species and matrix-degrading enzymes . J. Invest. Dermatol. . 132 . 7 . 1901–1907 . 2012 . 22318388 . 10.1038/jid.2011.476. free .
  23. The Shoe-fitting Fluoroscope as a Radiation Hazard . California Medicine . Leon . Lewis . Paul E . Caplan . 72 . 1 . 15408494 . January 1, 1950 . 1520288 . 26–30 [27].
  24. Book: [[United Nations Scientific Committee on the Effects of Atomic Radiation]]. Sources and Effects of Ionizing Radiation. 2000. United Nations. 11 November 2012. 121. vol. 1. Annex B. dead. 4 August 2012.
  25. Mortazavi. S.M.J.. P.A. Karamb. Apparent lack of radiation susceptibility among residents of the high background radiation area in Ramsar, Iran: can we relax our standards?. Radioactivity in the Environment. 2005. 7. 1141–1147. 10.1016/S1569-4860(04)07140-2. 1569-4860. 9780080441375.
  26. Sohrabi. Mehdi. Babapouran, Mozhgan. New public dose assessment from internal and external exposures in low- and elevated-level natural radiation areas of Ramsar, Iran. Proceedings of the 6th International Conference on High Levels of Natural Radiation and Radon Areas. 2005. 1276. 169–174 . 10.1016/j.ics.2004.11.102.
  27. Web site: Health Risks . Radon . EPA . 2012-03-05 . dead . . 2008-10-20.
  29. 10.1088/0952-4746/16/4/004. Pattison JE, Bachmann DJ, Beddoe AH. Gamma Dosimetry at Surfaces of Cylindrical Containers. Journal of Radiological Protection. 16. 4. 249–261. 1996. 1996JRP....16..249P .
  30. 10.1097/00004032-199911000-00006. Pattison, J.E.. Finger Doses Received during Samarium-153 Injections. Health Physics. 77. 5. 530–5. 1999. 10524506.
  31. Web site: Superflares could kill unprotected astronauts. New Scientist. 21 March 2005. live. 27 March 2015.
  32. Web site: Effective Dose Rate . NAIRAS (Nowcast of Atmospheric Ionizing Radiation System) . live . . 2016-03-05 .
  33. Book: Fundamentals of Aerospace Medicine . Jeffrey R. Davis . Robert Johnson . Jan Stepanek . 2008 . 221–230 . Google Books. 9780781774666 .
  34. Web site: New Symbol Launched to Warn Public About Radiation Dangers . February 15, 2007 . International Atomic Energy Agency . live . . 2007-02-17 .