This figure illustrates the relative abilities of three different types of ionizing radiation
to penetrate solid matter. Alpha particles (î±) are stopped by a sheet of paper whilst beta particles (î²) halt to an aluminium plate. Gamma radiation (î³) is dampened when it penetrates matter.
In physics, radiation describes a process in which energetic particles 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.
 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 and X-Ray radiation, and Neutrons may all be accelerated to a high enough energy to ionize atoms.
 Alpha radiation
Main article: Alpha decay
An Alpha (î±) particle is commonly emitted during the decay of large nuclei. An alpha particle (identical to a helium nucleus, He2+) consists of two neutrons and two protons. This particle has a doubly positive charge and a relatively high atomic mass. It travels relatively slowly and is stopped quickly through ionizing interactions with other atoms. To a human being, material emitting Alpha radiation presents little to no external hazard, since it is unable to penetrate the dead skin layer. However, if ingested, Alpha emitting radioactive material can cause internal ionization. This can lead to cell damage, cell death or the development of cancer. Alpha particles can be stopped with a sheet of paper.
 Beta(+/-) radiation
Beta-minus (î²-) radiation consists of an energetic electron. It is less ionizing than alpha radiation, but more 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 (î²+) radiation is the emission of positrons. Because these are antimatter particles, they annihilate any matter nearby, releasing gamma photons.
Beta particles (which are electrons) can mostly be stopped by a piece of aluminum foil.
 Gamma radiation
Gamma (î³) radiation consists of photons with a frequency of greater than 1019 Hz. 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.
X-Rays are electromagnetic waves with a wavelength smaller than about 10 nanometers. A smaller wavelength corresponds to a higher energy according to the equation E=hâ‹…c/î». ("E" is Energy; "h" is Planck's Constant; "c" is the speed of light; "î»" 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. Depending on the structure of the atom and the energy of the X-ray photon, 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 in this way, 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. 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.
 Non-ionizing radiation
Non-ionizing (or non-ionising) forms of radiation on living tissue have only recently been studied. Instead of producing charged ions when passing through matter, the electromagnetic radiation has sufficient energy to change only the rotational, vibrational or electronic valence configurations of molecules and atoms. Nevertheless, different biological effects are observed for different types of non-ionizing radiation.
 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 readily react with the atomic nuclei of many elements, creating unstable isotopes and therefore inducing radioactivity in a previously non-radioactive material. This process is known as neutron activation.
 Electromagnetic radiation
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 spectrumâ€”high frequency ultraviolet, x-rays, and gamma raysâ€”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, 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. The electromagnetic spectrum (usually just spectrum) of an object is the characteristic distribution of electromagnetic radiation emitted by, or absorbed by, that particular object.
Light, or visible light, is electromagnetic radiation of a wavelength that is visible to the human eye (about 400â€“700 nm), or up to 380â€“750 nm. More broadly, physicists refer to light as electromagnetic radiation of all wavelengths, whether visible or not.
 Thermal radiation
Thermal radiation is the process by which the surface of an object radiates its thermal energy in the form of electromagnetic waves.Infrared radiation from a common household radiator or electric heater is an example of thermal radiation, as is the heat and light (IR and visible EM waves) emitted by a glowing incandescent light bulb. Thermal radiation is generated when heat from the movement of charged particles within atoms is converted to electromagnetic radiation. The emitted wave frequency of the thermal radiation is a probability distribution depending only on temperature, and for a genuine black body is given by Planckâ€™s law of radiation. Wien's law gives the most likely frequency of the emitted radiation, and the Stefanâ€“Boltzmann law gives the heat intensity.
 Black-body radiation
Black-body radiation is a common synonym for thermal radiation (see above). It is so-called because the ideal radiator of thermal energy would also be an ideal absorber of thermal energy: It would not reflect any light, and thus would appear, at cooler temperatures, to be absolutely black.
Wilhelm RÃ¶ntgen discovered and named X-Rays when experimenting with a vacuum and a tube, he noticed a phosphorescence on a nearby plate of coated glass. While working with various isotopes of hydrogen, namely tritium, he found a drastic change in photonic emissions when measuring electrical charges in a vacuum. When he took pictures of the tritium, he found that the state of one solid piece would deteriorate quickly. In one month, he discovered the main properties of X-rays that we understand to this day. Henri Becquerel found that uranium salts caused fogging of an unexposed photographic plate, and Marie Curie discovered that only certain elements gave off these rays of energy. She named this behavior radioactivity.
Alpha particles, Beta particles and Gamma ray radiation were discovered by Ernest Rutherford through simple experimentation. Rutherford used a generic radioactive source and determined that the rays produced by the source struck three distinct areas on a screen of reactive material: one of them corresponding to a positive charge (Alpha), one of them being negative (Beta), and one of them being neutral (Gamma). He calculated the magnitude of the charge by their location. Using this data, Rutherford concluded that this radiation consisted of three different types, and named them after the first three letters of the Greek alphabet alpha, beta, and gamma.
In December 1899, Marie Curie and Pierre Curie discovered radium in pitchblende. This new element was two million times more radioactive than uranium, as described by Marie.
 Uses of radiation
 In medicine
Radiation and radioactive substances are used for diagnosis, treatment, and research. X rays, for example, pass through muscles and other soft tissue but are stopped by dense materials. This property of X rays enables doctors to find broken bones and to locate cancers that might be growing in the body. Doctors also find certain diseases by injecting a radioactive substance and monitoring the radiation given off as the substance moves through the body.
 In communication
All modern communication systems use forms of electromagnetic radiation. Variations in the intensity of the radiation represent changes in the sound, pictures, or other information being transmitted. For example, a human voice can be sent as a radio wave or microwave by making the wave vary to correspond variations in the voice.
 In science
Researchers use radioactive atoms to determine the age of materials that were once part of a living organism. The age of such materials can be estimated by measuring the amount of radioactive carbon they contain in a process called radiocarbon dating. Environmental scientists use radioactive atoms known as tracer atoms to identify the pathways taken by pollutants through the environment.
Radiation is used to determine the composition of materials in a process called neutron activation analysis. In this process, scientists bombard a sample of a substance with particles called neutrons. Some of the atoms in the sample absorb neutrons and become radioactive. The scientists can identify the elements in the sample by studying the radiation given off.
 See also
 External links