Ionizing radiation
Ionizing radiation, also spelled ionising radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle 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, infrared, microwaves, and radio waves are non-ionizing radiation. Nearly all types of laser light are non-ionizing radiation. The boundary between ionizing and non-ionizing radiation in the ultraviolet area cannot be sharply defined, as different molecules and atoms ionize at different energies. The energy of ionizing radiation starts around 10 electronvolts.
Ionizing subatomic particles include alpha particles, beta particles, and neutrons. These particles are 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, 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 or humans.
Ionizing radiation is used in a wide variety of fields such as medicine, nuclear power, research, and industrial manufacturing, but is 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 issues guidance on ionizing radiation protection, and the effects of dose uptake on human health.
Directly ionizing 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 has enough 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 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 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: a helium-4 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 from radioactive decay and pose shielding problems in space. However, this type of radiation is significantly absorbed by 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 ionizing effect in 1899. The symbol is α or α. Because they are identical to helium nuclei, they are also called He or indicating helium with a charge of +2 e. If the ion gains electrons from its environment, the α particle can be written as a normal helium atom.
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 β particles is termed beta decay. There are two forms of β decay, β and β, which respectively give rise to the electron and the positron. Beta particles are much less penetrating than gamma radiation, but more penetrating than alpha particles.High-energy beta particles may produce X-rays known as bremsstrahlung or secondary electrons 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. As positrons are positively charged particles they can directly ionize an atom through Coulomb interactions.Positrons can be generated by positron emission nuclear decay, or by pair production from a sufficiently energetic photon. Positrons are common artificial sources of ionizing radiation used in medical positron emission tomography scans.
Charged nuclei
Charged nuclei are characteristic of galactic cosmic rays and solar particle events and except for alpha particles have no natural sources on 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", 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.Contribution of High Charge and Energy Ions During Solar-Particle Event of September 29, 1989 Kim, Myung-Hee Y.; Wilson, John W.; Cucinotta, Francis A.; Simonsen, Lisa C.; Atwell, William; Badavi, Francis F.; Miller, Jack, NASA Johnson Space Center; Langley Research Center, May 1999.
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
Even though photons are electrically neutral, they can ionize atoms indirectly through the photoelectric effect and the Compton effect. Either of those interactions cause the ejection of an electron from an atom at relativistic speeds, turning that electron into a 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.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.
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. 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-rays" are known not to originate from radioactivity but, rather, result from processes like those that produce astronomical X-rays, except driven by much more energetic electrons.
Photoelectric absorption is the dominant mechanism in organic materials for photon energies below 100 keV, typical of classical X-ray tube originated X-rays. At energies beyond 100 keV, photons ionize matter increasingly through the Compton effect, and then indirectly through pair production at energies beyond 5 MeV. The accompanying interaction diagram shows two Compton scatterings happening sequentially. In every scattering event, the gamma ray transfers energy to an electron, and it continues on its path in a different direction and with reduced energy.