Radiation

In physics, radiation is a process in which energetic particles or energy or waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing. The word radiation is commonly used in reference to ionizing radiation only (i.e., having sufficient energy to ionize an atom), but it may also refer to non-ionizing radiation (e.g., radio waves or visible light). The energy radiates (i.e., travels outward in straight lines in all directions) from its source. This geometry naturally leads to a system of measurements and physical units that are equally applicable to all types of radiation. Both ionizing and non-ionizing radiation can be harmful to organisms and can result in changes to the natural environment.
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Ionizing radiation
Main article: Ionizing radiation

Radiation with sufficiently high energy can ionize atoms. Most often, this occurs when an electron is stripped (or 'knocked out') from an electron shell, which leaves the atom with a net positive charge. Because cells are made of atoms, this ionization can result in cancer. An individual cell is made of trillions of atoms. The probability of ionizing radiation causing cancer is dependent upon the dose rate of the radiation and the sensitivity of the organism being irradiated.

Alpha particles, beta particles, gamma rays, X-ray radiation, and neutrons may all be accelerated to an energy high enough to ionize atoms.
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Alpha
Main article: Alpha decay
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Beta(+/−)
Main article: Beta decay

Beta-minus (β−) radiation consists of an energetic electron. It is more ionizing than alpha radiation, but less than gamma. The electrons can often be stopped with a few centimeters of metal. It occurs when a neutron decays into a proton in a nucleus, releasing the beta particle and an antineutrino.

Beta-plus (β+) radiation is the emission of positrons. Because these are antimatter particles, they annihilate any matter nearby, releasing gamma photons.
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Neutron
Main article: Neutron

Neutrons are categorized according to their speed. High-energy (high-speed) neutrons have the ability to ionize atoms and are able to deeply penetrate materials. Neutrons are the only type of ionizing radiation that 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.

High-energy neutrons can travel great distances in air and typically require hydrogen rich shielding, such as concrete or water, to block them. A common source of neutron radiation occurs inside a nuclear reactor, where many feet of water is used as effective shielding.
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X-ray
Main article: X-ray

X-rays are electromagnetic waves with a wavelength smaller than about 10 nanometres. A smaller wavelength corresponds to a higher energy according to the equation E=h⋅c/λ. ("E" is Energy; "h" is Planck's Constant; "c" is the speed of light; "λ" is wavelength.) A "packet" of electromagnetic waves is called a photon. 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 very energetic, it may knock an electron from the atom altogether, causing the atom to ionize. Generally, a larger atom is more likely to absorb an X-ray photon, since larger atoms have greater energy differences between orbital electrons. Soft tissue in the human body is composed of smaller atoms than the calcium atoms that make up bone, hence 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.
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Gamma
Main article: Gamma ray

Gamma (γ) radiation consists of photons with a frequency of greater than 1019 Hz.[1] Gamma radiation occurs to rid the decaying nucleus of excess energy after it has emitted either alpha or beta radiation. 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 is composed of photons, and photons have neither mass nor electric charge. Gamma radiation penetrates much further through matter than either alpha or beta radiation.

Gamma rays, which are highly energetic photons, penetrate deeply and are difficult to stop. They can be stopped by a sufficiently thick layer of material with high atomic number, such as lead or depleted uranium.
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Non-ionizing radiation
Main article: Non-ionizing radiation

The energy of non-ionizing radiation is less and instead of producing charged ions when passing through matter, the electromagnetic radiation has only sufficient energy to change the rotational, vibrational or electronic valence configurations of molecules and atoms. The effect of non-ionizing forms of radiation on living tissue have only recently been studied. Nevertheless, different biological effects are observed for different types of non-ionizing radiation.[1][2]
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Neutron radiation
Main article: Neutron radiation

Neutron radiation is a kind of non-ionizing radiation that consists of free neutrons. These neutrons may be emitted during either spontaneous or induced nuclear fission, nuclear fusion processes, or from any other nuclear reactions. It does not ionize atoms in the same way that charged particles such as protons and electrons do (exciting an electron), because neutrons have no charge. However, neutrons react with the atomic nuclei of many elements upon collision with nuclei, creating unstable isotopes and therefore inducing radioactivity in a previously non-radioactive material. This process is known as neutron activation.
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Electromagnetic radiation
Main article: Electromagnetic radiation

The electromagnetic spectrum

Electromagnetic radiation (sometimes abbreviated EMR) takes the form of self-propagating waves in a vacuum or in matter. EM radiation has an electric and magnetic field component which oscillate in phase perpendicular to each other and to the direction of energy propagation. Electromagnetic radiation is classified into types according to the frequency of the wave, these types include (in order of increasing frequency): radio waves, microwaves, terahertz radiation, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays. Of these, radio waves have the longest wavelengths and gamma rays have the shortest. A small window of frequencies, called visible spectrum or light, is sensed by the eye of various organisms.

Ionizing radiation consists of subatomic particles or electromagnetic waves that are energetic enough to detach electrons from atoms or molecules, ionizing them. The occurrence of ionization depends on the energy of the individual particles or waves, and not on their number. An intense flood of particles or waves will not cause ionization if these particles or waves do not carry enough energy to be ionizing. Roughly speaking, particles or photons with energies above a few electron volts (eV) are ionizing.

Examples of ionizing particles are energetic alpha particles, beta particles, and neutrons. The ability of an electromagnetic wave (photons) to ionize an atom or molecule depends on its frequency. Radiation on the short-wavelength end of the electromagnetic spectrum—high frequency ultraviolet, X-rays, and gamma rays—is ionizing.

Ionizing radiation comes from radioactive materials, X-ray tubes, particle accelerators, and is present in the environment. 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, 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, resulting in skin burns, radiation sickness and death at high doses and cancer,[1] tumors and genetic damage at low doses.

EM radiation carries energy and momentum, which may be imparted when it interacts with matter.

The electromagnetic spectrum is the range of all possible electromagnetic radiation frequencies.[1] The electromagnetic spectrum (usually just spectrum) of an object is the characteristic distribution of electromagnetic radiation emitted by, or absorbed by, that particular object