Radiation

For other uses, see Radiation (disambiguation).

This figure illustrates the relative abilities of three different types of ionizing radiation to penetrate solid matter.

In physics, radiation describes any process in which energy travels through a medium or through space, ultimately to be absorbed by another body. Non-physicists often associate the word with ionizing radiation (e.g., as occurring in nuclear weapons, nuclear reactors, and radioactive substances), but it can also refer to electromagnetic radiation (i.e., radio waves, infrared light, visible light, ultraviolet light, and X-rays) which can also be ionizing radiation, to acoustic radiation, or to other more obscure processes. What makes it radiation is that the energy radiates (i.e., it travels outward in straight lines in all directions) from the source. This geometry naturally leads to a system of measurements and physical units that are equally applicable to all types of radiation. Some radiations can't be hazardous.

Ionizing radiation

Main article: Ionizing radiation

Some types of radiation have enough energy to ionize particles. Generally, this involves an electron being 'knocked out' of an atom's electron shells, which will give it a (positive) charge. This is often disruptive in biological systems, and can cause mutations and cancer.

These types of radiation generally occur in radioactive decay and waste.

The three main types of radiation were discovered by Ernest Rutherford, The Alpha, Beta, and Gamma rays. They were found through simple experimentation, Rutherford used a radioactive source and found out that the rays produced hit three different areas: one of them being the positive, one of them being neutral, and one of them being negative. He figured out the charges by the place they hit. Using this data, Rutherford concluded that radiation consisted of three rays. He named them after the first three letters of the Greek alphabet alpha, beta, and gamma.

Alpha radiation

Main article: Alpha decay

Alpha (α) decay is a method of decay in large nuclei. An alpha particle (helium nucleus, He²⁺), consisting of two neutrons and two protons, is emitted. Because of the particle's relatively high charge, it is heavily ionizing and will cause severe damage if ingested. However, due to the high mass of the particle, it has little energy and a low range; typically, alpha particles can be stopped with a sheet of paper (or skin).

Beta(+/-) radiation

Main article: Beta decay

Beta-minus (β-) 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 (β+) 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

Main article: Gamma ray

Gamma (γ) 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 comprised 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.

Non-ionizing radiation

Main article: 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

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 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

Main article: 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 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,[2] 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

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

Main article: Light

Light, or visible light, is electromagnetic radiation of a wavelength that is visible to the human eye (about 400–700 nm), or up to 380–750 nm. More broadly, physicists refer to light as electromagnetic radiation of all wavelengths, whether visible or not.

Thermal radiation

Main article: 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 Planck’s law of radiation. Wien's law gives the most likely frequency of the emitted radiation, and the Stefan–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.

Discovery

Wilhelm Röntgen 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.

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

Main articles: Medical radiography and Medical radiation scientist

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 radioactive substance and monitoring the radiation given off as the substance moves through the body.gh

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

• Background radiation, which actually refers to the background ionizing radiation
• Black hole
• Čerenkov radiation
• Cosmic microwave background radiation, 3K blackbody radiation that fills the Universe
• Electromagnetic spectrum
• Hawking radiation
• Ionizing radiation
• Lists of nuclear disasters and radioactive incidents
• Non-ionizing radiation
• Radiant energy, radiation by a source into the surrounding environment.
• Radiation damage – adverse effects on materials and devices
• Radiation hardening – making devices resistant to failure in high radiation environments
• Radiation hormesis – dosage threshold damage theory
• Radiation poisoning – adverse effects on life forms
• Radioactive contamination
• Radioactive decay

External links

• Health Physics Society Public Education Website
• Physics for Future Presidents, by Prof. Richard Muller, Webcast.Berkeley