Illustration of the relative abilities of three different types of ionizing radiation to penetrate solid matter. Alpha particles (α) are stopped by a sheet of paper while beta particles (β) are stopped by an aluminium plate. Gamma radiation (γ) is dampened when it penetrates matter.

In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. Two types of radiation are commonly differentiated in the way they interact with normal chemical matter: ionizing and non-ionizing radiation. The word radiation is often colloquially used in reference to ionizing radiation (i.e., radiation having sufficient energy to ionize an atom), but the term radiation may correctly also refer to non-ionizing radiation (e.g., radio waves, heat or visible light). The particles or waves radiate (i.e., travel outward in all directions) from a source. This aspect leads to a system of measurements and physical units that are 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. In general, however, ionizing radiation is far more harmful to living organisms per unit of energy deposited than non-ionizing radiation, since the ions that are produced by ionizing radiation, even at low radiation powers, have the potential to cause DNA damage. By contrast, most non-ionizing radiation is harmful to organisms only in proportion to the thermal energy deposited, and is conventionally considered harmless at low powers which do not produce significant temperature rise. Ultraviolet radiation in some aspects occupies a middle ground, in having some features of both ionizing and non-ionizing radiation. Although nearly all of the ultraviolet spectrum of radiation is non-ionizing, at the same time ultraviolet radiation does far more damage to many molecules in biological systems than is accounted for by heating effects (an example is sunburn). These properties derive from ultraviolet's power to alter chemical bonds, even without having quite enough energy to ionize atoms.

The question of harm to biological systems due to low-power ionizing and non-ionization radiation is not settled. Controversy continues about possible non-heating effects of low-power non-ionizing radiation, such as non-heating microwave and radio wave exposure. Non-ionizing radiation is usually considered to have no completely safe lower limit, although at some energy levels, new exposures do not add appreciably to background radiation. The evidence that small amounts of some types of ionizing radiation might confer a net health benefit in some situations is called radiation hormesis.

Ionizing radiation[link]

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 and more importantly the DNA can be damaged, this ionization can result in an increased chance of cancer. An individual cell is made of trillions of atoms. The probability of ionizing radiation causing cancer is dependent upon the absorbed dose of the radiation, as adjusted for the damaging tendency of the type of radiation (equivalent dose) and the sensitivity of the organism or tissue being irradiated (effective dose).

Roughly speaking, photons and particles with energies above about 10 electron volts (eV) are ionizing. Alpha particles, beta particles, cosmic rays, gamma rays, and X-ray radiation all carry energy high enough to ionize atoms. In addition, free neutrons are also ionizing, since their interactions with matter are inevitably more energetic than this threshold.

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.

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 electromagnetic radiation is that for which the photons making up the radiation have energies larger than about 10 electron volts. The ability of an electromagnetic wave (photons) to ionize an atom or molecule thus depends on its frequency, which determines the energy of a photon of the radiation. An energy of 10 eV is about 1.6×10⁻¹⁸ joules, which is a typical binding energy of an outer electron to an atom or organic molecule.^[2] This corresponds with a frequency of 2.4×10¹⁵ Hz, and a wavelength of 125 nm (this is in far ultraviolet).^[3] Radiation on the short-wavelength end of the electromagnetic spectrum, and above 125 nm, is ionizing. This includes extreme ultraviolet, X-rays, and gamma rays.

Most of the ultraviolet spectrum (which begins above energies of 3.1 eV (400 nm) is non-ionizing, but is still biologically hazardous due 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 formation of pyrimidine dimers in DNA. 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.

Alpha[link]

Alpha particles are the same as a helium-4 nucleus (two protons and two neutrons) and travel at speeds in excess of 5% of the speed of light, but they interact with matter very heavily, and thus at their usual velocities only penetrate a few centimeters of air, or a few millimeters 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 skin and cause no damage to 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 ingested (breathed or swallowed). This brings the radioisotope close enough to tissue for the alpha radiation to damage cells. Per unit of energy, alpha particles are at least 20 times more effective at cell-damage as gamma rays and X-rays. See relative biological effectiveness for a discussion of this. Examples of highly poisonous alpha-emitters are radium, radon, and polonium.

Beta[link]

Beta-minus (β−) radiation consists of an energetic electron. It is less ionizing than alpha radiation, but more than gamma. Beta radiation from radioactive decay can be stopped with a few centimeters of plastic or a few millimeters 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 antimatter electrons. When a positron slows down to speeds similar to those of electrons in the material, the positron will annihilate an electron, releasing two gamma photons in the process. Those two gamma photons will be traveling in (approximately) opposite directions.

Neutron[link]

Neutrons are categorized according to their speed. Neutron radiation 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.

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. Even comparatively low speed thermal neutrons, which do not carry enough kinetic energy individually to be ionizing, will cause neutron activation (in fact, they cause it more efficiently). Such neutrons are "indirectly ionizing."

Neutrons do 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, both slow and fast neutrons react with the atomic nuclei of many elements upon collision with those nuclei, creating unstable isotopes and therefore inducing radioactivity in a previously non-radioactive material. This is neutron activation.

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 meters) and moderate distances (several meters) in common solids. They typically require hydrogen rich shielding, such as concrete or water, to block them within distances of less than a meter. A common source of neutron radiation occurs inside a nuclear reactor, where meters-thick water layer is used as effective shielding.

X-ray[link]

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=hc/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.

Gamma[link]

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, which 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, where stopping power of the material per given area depends mostly (but not entirely) on its total mass, whether the material is of high or low density. However, as is the case with X-rays, materials with 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).

Non-ionizing radiation[link]

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 has only recently been studied. Nevertheless, different biological effects are observed for different types of non-ionizing radiation.^[1][4]

Even "non-ionizing" radiation is capable of causing thermal-ionization if it deposits enough heat to raise temperatures to ionization energies. These reactions occur at far higher energies than with non-ionization radiation, which requires only single particles to ionize. A familiar example of thermal ionization is the flame-ionization of a common fire, and the browning (chemical process) reactions in common food items induced by infrared radiation, during broiling-type cooking.

Non-ionizing electromagnetic radiation[link]

The electromagnetic spectrum

The non-ionizing portion of electromagnetic radiation consists of electromagnetic waves that (as individual quanta or particles, see photon) are not energetic enough to detach electrons from atoms or molecules, ionizing them. These include radio waves, microwaves, infrared, and (sometimes) visible light. (Ultraviolet light, X-rays and gamma-rays are regarded as ionizing.) 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, unless they raise the temperature of a body to a point high enough to ionize small fractions of atoms or molecules by the process of thermal-ionization (this requires relatively extreme radiation energies, however).

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.

Visible light[link]

Light, or visible light, is a very narrow range of electromagnetic radiation of a wavelength that is visible to the human eye (about 400–700 nm), or up to 380–750 nm.^[1] More broadly, physicists refer to light as electromagnetic radiation of all wavelengths, whether visible or not.

Infrared[link]

Infrared (IR) light is electromagnetic radiation with a wavelength between 0.7 and 300 micrometers, which equates to a frequency range between approximately 1 and 430 THz. IR wavelengths are longer than that of visible light, but shorter than that of terahertz radiation microwaves. Bright sunlight provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation.^[1]

Microwave[link]

Microwaves are electromagnetic waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz. This broad definition includes both UHF and EHF (millimeter waves), and various sources use different boundaries.^[1] In all cases, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3mm).

Radio waves[link]

Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by lightning, or by astronomical objects. Artificially generated radio waves are used for fixed and mobile radio communication, broadcasting, radar and other navigation systems, satellite communication, computer networks and innumerable other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves may cover a part of the Earth very consistently, shorter waves can reflect off the ionosphere and travel around the world, and much shorter wavelengths bend or reflect very little and travel on a line of sight.

Very low frequency (VLF)[link]

Very low frequency or VLF refers to radio frequencies (RF) in the range of 3 to 30 kHz. Since there is not much bandwidth in this band of the radio spectrum, only the very simplest signals are used, such as for radio navigation. Also known as the myriameter band or myriameter wave as the wavelengths range from ten to one myriameter (an obsolete metric unit equal to 10 kilometers)

Extremely low frequency (ELF)[link]

Extremely low frequency (ELF) is a term used to describe radiation frequencies from 3 to 30 Hz. In atmosphere science, an alternative definition is usually given, from 3 Hz to 3 kHz.^[1] In the related magnetosphere science, the lower frequency electromagnetic oscillations (pulsations occurring below ~3 Hz) are considered to lie in the ULF range, which is thus also defined differently from the ITU Radio Bands.

Thermal radiation (heat)[link]

Thermal radiation, a common synonym for infra-red when it occurs at temperatures often encountered on Earth, 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 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.

Parts of the electromagnetic spectrum of thermal radiation may be ionizing, if the object emitting the radiation is hot enough (has a high enough temperature). A common example of such radiation is sunlight, which is thermal radiation from the Sun's photosphere and which contains enough ultraviolet light to cause ionization in many molecules and atoms. An extreme example is the flash from the detonation of a nuclear weapon, which emits a large number of ionizing X-rays purely as a product of heating the atmosphere around the bomb to extremely high temperatures.

As noted above, even low-frequency thermal radiation may cause temperature-ionization whenever it deposits sufficient thermal energy to raises temperatures to a high enough level. Common examples of this are the ionization (plasma) seen in common flames, and the molecular changes caused by the "browning" in food-cooking, which is a chemical process that begins with a large component of ionization.

Black body radiation[link]

Black body radiation is radiation from an idealized radiator that emits at any temperature the maximum possible amount of radiation at any given wavelength. A black body will also absorb the maximum possible incident radiation at any given wavelength. The radiation emitted covers the entire electromagnetic spectrum and the intensity (power/unit-area) at a given frequency is dictated by Planck's law of radiation. A black body at temperatures at or below room temperature would thus appear absolutely black as it would not reflect any light. Theoretically a black body emits electromagnetic radiation over the entire spectrum from very low frequency radio waves to x-rays. The frequency at which the black body radiation is at maximum is given by Wien's displacement law.

Discovery[link]

Wilhelm Röntgen discovered on 8 November, 1895 and named X-rays when experimenting with a vacuum and a tube, he noticed a fluorescence on a nearby plate of coated glass. 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 (both in 1899) and gamma ray radiation (1903) 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 Madam Curie.

Uses of radiation[link]

In medicine[link]

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^[5] . Doctors also find certain diseases by injecting a radioactive substance and monitoring the radiation given off as the substance moves through the body^[6] . Radiation used for cancer treatment is called ionizing radiation because it forms ions in the cells of the tissues it passes through as it dislodges electrons from atoms. This can kill cells or change genes so the cells cannot grow. Other forms of radiation such as radio waves, microwaves, and light waves are called non-ionizing. They don't have as much energy and are not able to ionize cells.

In communication[link]

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[link]

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 emitted radiation.

See also[link]

References[link]

External links[link]

• Radiation on In Our Time at the BBC. (listen now)
• Health Physics Society Public Education Website
• Ionizing Radiation and Radon from World Health Organization