radiation can be detected in a
cloud chamber. Radiation with sufficiently high energy can
ionize atoms; that is to say it can knock
electrons off atoms, creating
ions. Ionization occurs when an electron is stripped (or "knocked out") from an electron shell of the atom, which leaves the atom with a net positive charge. Because living
cells and, more importantly, the DNA in those cells can be damaged by this ionization, exposure to ionizing radiation increases the risk of
cancer. Thus "ionizing radiation" is somewhat artificially separated from particle radiation and electromagnetic radiation, simply due to its great potential for biological damage. While an individual cell is made of
trillions of atoms, only a small fraction of those will be ionized at low to moderate radiation powers. The probability of ionizing radiation causing cancer is dependent upon the
absorbed dose of the radiation and is a function of the damaging tendency of the type of radiation (
equivalent dose) and the sensitivity of the irradiated organism or tissue (
effective dose). If the source of the ionizing radiation is a radioactive material or a nuclear process such as
fission or
fusion, there is
particle radiation to consider. Particle radiation is
subatomic particles accelerated to
relativistic speeds by nuclear reactions. Because of their
momenta, they are quite capable of knocking out electrons and ionizing materials, but since most have an electrical charge, they do not have the penetrating power of ionizing radiation. The exception is neutron particles; see below. There are several different kinds of these particles, but the majority are
alpha particles,
beta particles,
neutrons, and
protons. Roughly speaking, photons and particles with energies above about 10
electron volts (eV) are ionizing (some authorities use 33 eV, the ionization energy for water). Particle radiation from radioactive material or cosmic rays almost invariably carries enough energy to be ionizing. Most ionizing radiation originates from radioactive materials and space (cosmic rays), and as such is naturally present in the environment, since most rocks and soil have small concentrations of radioactive materials. Since this radiation is invisible and not directly detectable by human senses, 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 its interaction with matter, as in the case of
Cherenkov radiation and radio-luminescence. Ionizing radiation has many practical uses in medicine, research, and construction, but presents a health hazard if used improperly. Exposure to radiation causes damage to living tissue; high doses result in
Acute radiation syndrome (ARS), with skin burns, hair loss, internal organ failure, and death, while any dose may result in an increased chance of cancer and
genetic damage; a particular form of cancer,
thyroid cancer, often occurs when nuclear weapons and reactors are the radiation source because of the biological proclivities of the radioactive iodine fission product,
iodine-131.
Ultraviolet radiation Ultraviolet, of wavelengths from 10 nm to 200 nm, ionizes air molecules, causing it to be strongly absorbed by air and by ozone (O3) in particular. Ionizing UV therefore does not penetrate Earth's atmosphere to a significant degree, and is sometimes referred to as
vacuum ultraviolet. Although present in space, this part of the UV spectrum is not of biological importance, because it does not reach living organisms on Earth. There is a zone of the atmosphere in which ozone absorbs some 98% of non-ionizing but dangerous UV-C and UV-B. This
ozone layer starts at about and extends upward. Some of the ultraviolet spectrum that does reach the ground is non-ionizing, but is still biologically hazardous due to the ability of single photons of this energy to cause electronic excitation in biological molecules, and thus damage them by means of unwanted reactions. An example is the formation of
pyrimidine dimers in DNA, which begins at wavelengths below 365 nm (3.4 eV), which is well below ionization energy. This property gives the ultraviolet spectrum some of the dangers of ionizing radiation in biological systems without actual ionization occurring. In contrast, visible light and longer-wavelength electromagnetic radiation, such as infrared, microwaves, and radio waves, consists of photons with too little energy to cause damaging molecular excitation, and thus this radiation is far less hazardous per unit of energy.
X-rays X-rays are electromagnetic waves with a wavelength less than about 10−9 m (greater than and ). A smaller wavelength corresponds to a higher energy according to the equation
E =
hc/
λ. (
E is Energy;
h is the Planck constant;
c is the speed of light;
λ is wavelength.) When an X-ray photon collides with an atom, the atom may absorb the energy of the photon and boost an electron to a higher orbital level, or if the photon is extremely energetic, it may knock an electron from the atom altogether, causing the atom to ionize. Generally, larger atoms are more likely to absorb an X-ray photon since they have greater energy differences between orbital electrons. The soft tissue in the human body is composed of smaller atoms than the calcium atoms that make up bone, so there is a contrast in the absorption of X-rays. X-ray machines are specifically designed to take advantage of the absorption difference between bone and soft tissue, allowing physicians to examine structure in the human body. X-rays are also totally absorbed by the thickness of the earth's atmosphere, resulting in the prevention of the X-ray output of the sun, smaller in quantity than that of UV but nonetheless powerful, from reaching the surface.
Gamma radiation cloud chamber. Gamma (γ) radiation consists of photons with a wavelength less than (greater than 1019 Hz and 41.4 keV). Gamma radiation emission is a nuclear process that occurs to rid an unstable
nucleus of excess energy after most nuclear reactions. Both alpha and beta particles have an electric charge and mass, and thus are quite likely to interact with other atoms in their path. Gamma radiation, however, is composed of photons, which have neither mass nor electric charge and, as a result, penetrates much further through matter than either alpha or beta radiation. Gamma rays can be stopped by a sufficiently thick or dense layer of material, where the stopping power of the material per given area depends mostly (but not entirely) on the total mass along the path of the radiation, regardless of whether the material is of high or low density. However, as is the case with X-rays, materials with a high atomic number such as lead or
depleted uranium add a modest (typically 20% to 30%) amount of stopping power over an equal mass of less dense and lower atomic weight materials (such as water or concrete). The atmosphere absorbs all gamma rays approaching Earth from space. Even air is capable of absorbing gamma rays, halving the energy of such waves by passing through, on the average, .
Alpha radiation detected in an
isopropanol cloud chamber Alpha particles are
helium-4 nuclei (two protons and two neutrons). They interact with matter strongly due to their charges and combined mass, and at their usual velocities only penetrate a few centimetres of air, or a few millimetres of low density material (such as the thin mica material which is specially placed in some Geiger counter tubes to allow alpha particles in). This means that alpha particles from ordinary
alpha decay do not penetrate the outer layers of dead skin cells and cause no damage to the live tissues below. Some very high energy alpha particles compose about 10% of
cosmic rays, and these are capable of penetrating the body and even thin metal plates. However, they are of danger only to astronauts, since they are deflected by the Earth's magnetic field and then stopped by its atmosphere. Alpha radiation is dangerous when alpha-emitting
radioisotopes are inhaled or ingested (breathed or swallowed). This brings the radioisotope close enough to sensitive live tissue for the alpha radiation to damage cells. Per unit of energy, alpha particles are at least 20 times more effective at cell-damage than gamma rays and X-rays. See
relative biological effectiveness for a discussion of this. Examples of highly poisonous alpha-emitters are all isotopes of
radium,
radon, and
polonium, due to the amount of decay that occur in these short half-life materials.
Beta radiation (beta radiation) detected in an
isopropanol cloud chamber Beta-minus (β−) radiation consists of an energetic electron. It is more penetrating than alpha radiation but less than gamma. Beta radiation from
radioactive decay can be stopped with a few centimetres of plastic or a few millimetres of metal. It occurs when a neutron decays into a proton in a nucleus, releasing the beta particle and an
antineutrino. Beta radiation from
linac accelerators is far more energetic and penetrating than natural beta radiation. It is sometimes used therapeutically in
radiotherapy to treat superficial tumors. Beta-plus (β+) radiation is the emission of
positrons, which are the
antimatter form of electrons. When a positron slows to speeds similar to those of electrons in the material, the positron will annihilate an electron, releasing two gamma photons of 511 keV in the process. Those two gamma photons will be traveling in (approximately) opposite directions. The gamma radiation from positron annihilation consists of high energy photons, and is also ionizing.
Neutron radiation Neutrons are categorized according to their speed/energy. Neutron radiation consists of
free neutrons. These neutrons may be emitted during either spontaneous or induced nuclear fission. Neutrons are rare radiation particles; they are produced in large numbers only where
chain reaction fission or fusion reactions are active; this happens for about 10 microseconds in a thermonuclear explosion, or continuously inside an operating nuclear reactor; production of the neutrons stops almost immediately in the reactor when it goes non-critical. Neutrons can make other objects, or material, radioactive. This process, called
neutron activation, is the primary method used to produce radioactive sources for use in medical, academic, and industrial applications. Even comparatively low speed
thermal neutrons cause neutron activation (in fact, they cause it more efficiently). Neutrons do not ionize atoms in the same way that charged particles such as protons and electrons do (by the excitation of an electron), because neutrons have no charge. It is through their absorption by nuclei which then become unstable that they cause ionization. Hence, neutrons are said to be "indirectly ionizing". Even neutrons without significant kinetic energy are indirectly ionizing, and are thus a significant radiation hazard. Not all materials are capable of neutron activation; in water, for example, the most common isotopes of both types atoms present (hydrogen and oxygen) capture neutrons and become heavier but remain stable forms of those atoms. Only the absorption of more than one neutron, a statistically rare occurrence, can activate a hydrogen atom, while oxygen requires two additional absorptions. Thus water is only very weakly capable of activation. The sodium in salt (as in sea water), on the other hand, need only absorb a single neutron to become Na-24, a very intense source of beta decay, with a half-life of 15 hours. In addition, high-energy (high-speed) neutrons have the ability to directly ionize atoms. One mechanism by which high energy neutrons ionize atoms is to strike the nucleus of an atom and knock the atom out of a molecule, leaving one or more electrons behind as the
chemical bond is broken. This leads to production of chemical
free radicals. In addition, very high energy neutrons can cause ionizing radiation by "neutron spallation" or knockout, wherein neutrons cause emission of high-energy protons from atomic nuclei (especially hydrogen nuclei) on impact. The last process imparts most of the neutron's energy to the proton, much like one
billiard ball striking another. The charged protons and other products from such reactions are directly ionizing. High-energy neutrons are very penetrating and can travel great distances in air (hundreds or even thousands of metres) and moderate distances (several metres) in common solids. They typically require hydrogen rich shielding, such as concrete or water, to block them within distances of less than 1 m. A common source of neutron radiation occurs inside a
nuclear reactor, where a metres-thick water layer is used as effective shielding. == Cosmic radiation ==