ionizing radiation ionizing radiation ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. some particles can travel up to 99% of the speed of light, and the electromagnetic waves are on the high-energy portion of the electromagnetic spectrum. gamma rays, x-rays, and the higher energy 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, as different molecules and atoms ionize at different energies. the energy of ionizing radiation starts between 10 electronvolts (ev) and 33 ev. typical ionizing subatomic particles include alpha particles, beta particles, and neutrons. these are typically created by radioactive decay, and almost all are energetic enough to ionize. there are also secondary cosmic particles produced after cosmic rays interact with earth's atmosphere, including muons, mesons, and positrons. cosmic rays may also produce radioisotopes on earth (for example, carbon-14), which in turn decay and emit 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 x-ray tubes, particle accelerators, and nuclear fission. ionizing radiation is not immediately detectable by human senses, so instruments such as geiger counters are used to detect and measure it. however, very high energy particles can produce visible effects on both organic and inorganic matter (e.g. water lighting in cherenkov radiation) or humans (e.g. acute radiation syndrome). 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 organ damage. in high acute doses, it will result in radiation burns and radiation sickness, and lower level doses over a protracted time can cause cancer. 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 alpha (α) radiation consists of a fast-moving helium-4 (4he) nucleus and is stopped by a sheet of paper. beta (β) radiation, consisting of electrons, is halted by an aluminium plate. gamma (γ) radiation, consisting of energetic photons, is eventually absorbed as it penetrates a dense material. neutron (n) radiation consists of free neutrons that are blocked by light elements, like hydrogen, which slow and/or capture them. not shown: galactic cosmic rays that consist of energetic charged nuclei such as protons, helium nuclei, and high-charged nuclei called hze ions. cloud chambers are used to visualise ionizing radiation. this image show the tracks of particles, which ionise saturated air and leave a trail of water vapour. 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 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. the alpha particle was named by ernest rutherford after the first letter in the greek alphabet, α, when he ranked the known radioactive emissions in descending order of ionising effect in 1899. the symbol is α or α2+. because they are identical to helium nuclei, they are also sometimes written as he2+ or 42he2+ indicating a helium ion with a +2 charge (missing its two electrons). if the ion gains electrons from its environment, the alpha particle can be written as a normal (electrically neutral) helium atom 42he. 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. 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 some shielding materials produces bremsstrahlung. the effect is greater with material having high atomic numbers, so material with low atomic numbers is used for beta source shielding. positrons and other types of 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. positrons can be generated by positron emission nuclear decay (through weak interactions), or by pair production from a sufficiently energetic photon. positrons are common artificial sources of ionizing radiation used in medical positron emission tomography (pet) scans. charged nuclei charged nuclei are characteristic of galactic cosmic rays and solar particle events and except for alpha particles (charged helium nuclei) have no natural sources on the earth. in space, however, very high energy protons, helium nuclei, and hze ions can be initially stopped by relatively thin layers of shielding, clothes, or skin. however, the resulting interaction will generate secondary radiation and cause cascading biological effects. if just one atom of tissue is displaced by an energetic proton, for example, the collision will cause further interactions in the body. this is called "linear energy transfer" (let), which utilizes elastic scattering. let can be visualized as a billiard ball hitting another in the manner of the conservation of momentum, sending both away with the energy of the first ball divided between the two unequally. when a charged nucleus strikes a relatively slow-moving nucleus of an object in space, let occurs and neutrons, alpha particles, low-energy protons, and other nuclei will be released by the collisions and contribute to the total absorbed dose of tissue. 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