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Radioactivity

Radioactivity

Radioactivity originates from extraterrestrial sources and terrestrial geologic sources. All elements with more than 83 protons (i.e., an atomic number greater than 83) are radioactive. Some radioactive isotopes also occur in elements with lower atomic numbers.

Atoms that are radioactive emit radioactivity during spontaneous transformation from an unstable isotope to a more stable one. Natural radioactive decay provides a source of heating in Earth's interior that drives mantle dynamics and plate tectonics . Both natural and man-made sources of radioactivity at certain levels may represent a significant health risk to humans and other organisms. Radioactive materials must be isolated from the environment until their radiation level has decreased to a safe level, a process which requires thousands of years for some materials.

Radiation is classified as being ionizing or nonionizing. Both types can be harmful to humans and other organisms. Nonionizing radiation is relatively long-wavelength electro-magnetic radiation, such as radio waves, microwaves, visible radiation, ultraviolet radiation, and very low-energy electro-magnetic fields. Nonionizing radiation is generally considered less dangerous than ionizing radiation. However, some forms of nonionizing radiation, such as ultraviolet radiation, can damage biological molecules and cause health problems. Scientists do not yet fully understand the longer-term health effects of some forms of nonionizing radiation, such as that from very low-level electromagnetic fields (e.g., high-voltage power lines), although the evidence to date suggests that the risks are extremely small.

Ionizing radiation is the short wavelength radiation or particulate radiation emitted by certain unstable isotopes during radioactive decay. There are about 70 radioactive isotopes, all of which emit some form of ionizing radiation as they decay. A radioactive isotope typically decays through a series of intermediate isotopes until it reaches a stable isotope state. As indicated by its name, ionizing radiation can ionize the atoms or molecules with which it interacts. In other words, ionizing radiation can cause other atoms to release their electrons. These free electrons can damage many biochemicals, such as proteins, lipids, and nucleic acids (including DNA). In intense radioactivity, this damage can cause severe human health problems, including cancers, and death.

Ionizing radiation can be either short-wavelength electromagnetic radiation or particulate radiation. Gamma radiation and × radiation are short-wavelength electromagnetic radiation. Alpha particles, beta particles, neutrons, and protons are particulate radiation. Alpha particles, beta particles, and gamma rays are the most commonly encountered forms of radioactive pollution. Alpha particles are simply ionized helium nuclei, and consist of two protons and two neutrons. Beta particles are electrons, which have a negative charge. Gamma radiation is high-energy electromagnetic radiation.

Scientists have devised various units for measuring radioactivity. A Curie (Ci) represents the rate of radioactive decay. One Curie is 3.7 × 1010 radioactive disintegrations per second. A rad is a unit representing the absorbed dose of radioactivity. One rad is equal to an absorbed energy dose of 100 ergs per gram of radiated medium. A rem is a unit that measures the effectiveness of radioactivity in causing biological damage. One rem is equal to one rad times a biological weighting factor. The weighting factor is 1.0 for gamma radiation and beta particles, and it is 20 for alpha particles. The radioactive half-life is a measure of the persistence of radioactive material. The half-life is the time required for one-half of an initial quantity of atoms of a radioactive isotope to decay to a different isotope.

In the United States, people are typically exposed to about 350 millirems of ionizing radiation per year. On average, 82% of this radiation comes from natural sources and 18% from anthropogenic sources (i.e., those associated with human activities). The major natural source of radiation is radon gas, which accounts for about 55% of the total radiation dose. The principal anthropogenic sources of radioactivity are medical x rays and nuclear medicine. Radioactivity from the fallout of nuclear weapons testing and from nuclear power plants make up less than 0.5% of the total radiation dose, i.e., less than 2 millirems. Although the contribution to the total human radiation dose is extremely small, radioactive isotopes released during previous atmospheric testing of nuclear weapons will remain in the atmosphere at detectable levels for the next 100 to 1000 years.

People who live in certain regions are exposed to higher doses of radiation. For example, residents of the Rocky Mountains of Colorado receive about 30 millirems more cosmic radiation than people living at sea level. This is because the atmosphere is thinner at higher elevations, and therefore less effective at shielding the surface from cosmic radiation. Exposure to cosmic radiation is also high while people are flying in an airplane, so pilots and flight attendants have an enhanced, occupational exposure. In addition, residents of certain regions receive higher doses of radiation from radon-222, due to local geological anomalies. Radon-222 is a colorless and odorless gas that results from the decay of naturally occurring, radioactive isotopes of uranium. Radon-222 typically enters buildings from their ground level.

Personal lifestyle also influences the amount of radioactivity to which people are exposed. For example, miners, who spend a lot of time underground, are exposed to relatively high doses of radon-222 and consequently have relatively high rates of lung cancer. Cigarette smokers expose their lungs to high levels of radiation, because tobacco plants contain trace quantities of polonium-210, lead-210, and radon-222. These radioactive isotopes come from the small amount of uranium present in fertilizers used to promote tobacco growth. Consequently, the lungs of a cigarette smoker are exposed to thousands of additional millirems of radioactivity, although any associated hazards are much less than those of tar and nicotine.

The U.S. Nuclear Regulatory Commission has strict requirements regarding the amount of radioactivity that can be released from a nuclear power reactor. In particular, a nuclear reactor can expose an individual who lives on the fence line of the power plant to no more than 10 millirems of radiation per year. Actual measurements at U.S. nuclear power plants have shown that a person who lived at the fence line would actually be exposed to much less than 10 millirems.

Thus, for a typical person who is exposed to about 350 millirems of radiation per year from all other sources, much of which is natural background, the proportion of radiation from nuclear power plants is extremely small. In fact, coal- and oilfired power plants, which release small amounts of radioactivity contained in their fuels , are responsible for more airborne radioactive pollution in the United States than are nuclear power plants.

By far, the worst nuclear reactor accident occurred in 1986 in Chernobyl, Ukraine. An uncontrolled build-up of heat resulted in a meltdown of the reactor core and combustion of graphite moderator material in one of the several generating units at Chernobyl, releasing more than 50 million Curies of radioactivity to the ambient environment. The disaster killed 31 workers and resulted in the hospitalization of more than 500 other people from radiation sickness. According to

Ukrainian authorities, during the decade following the Chernobyl disaster an estimated 10,000 people in Belarus, Russia, and Ukraine died from cancers and other radiationrelated diseases caused by the accident. In addition to these relatively local effects, the atmosphere transported radioactive fallout from Chernobyl into Europe and throughout the Northern Hemisphere.

The large amount of radioactive waste generated by nuclear power plants is another important problem. This waste will remain radioactive for many thousands of years, so technologists must design systems for extremely long-term storage. One obvious problem is that the long-term reliability of the storage systems cannot be fully assured, because they cannot be directly tested for the length of time they will be used (i.e., for thousands of years). Another problem with nuclear waste is that it will remain extremely dangerous for much longer than the expected lifetimes of existing governments and social institutions. Thus, future societies of the following millennia, however they may be structured, will be responsible for the safe storage of nuclear waste that is being generated today.

See also Atmospheric chemistry; Atmospheric pollution; Atomic mass and weight; Atomic theory; Atoms; Atomic theory; Carbon dating; Cosmic microwave background radiation; Environmental pollution; Geochemistry; Radioactive waste storage (geological considerations); Radon production, detection and elimination; Ultraviolet rays and radiation

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radioactivity

radioactivity, spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation. The energy produced by radioactivity has important military and industrial applications. However, the rays emitted by radioactive substances can cause radiation sickness, and such substances must therefore be handled with extreme care (see radioactive waste).

Radioactive Emissions

Natural radioactivity is exhibited by several elements, including radium, uranium, and other members of the actinide series, and by some isotopes of lighter elements, such as carbon-14, used in radioactive dating. Radioactivity may also be induced, or created artificially, by bombarding the nuclei of normally stable elements in a particle accelerator. Essentially there is no difference between these two manifestations of radioactivity.

The radiation produced during radioactivity is predominantly of three types, designated as alpha, beta, and gamma rays. These types differ in velocity, in the way in which they are affected by a magnetic field, and in their ability to penetrate or pass through matter. Other, less common, types of radioactivity are electron capture (capture of one of the orbiting atomic electrons by the unstable nucleus) and positron emission—both forms of beta decay and both resulting in the change of a proton to a neutron within the nucleus—an internal conversion, in which an excited nucleus transfers energy directly to one of the atom's orbiting electrons and ejects it from the atom.

Alpha Radiation

Alpha rays have the least penetrating power, move at a slower velocity than the other types, and are deflected slightly by a magnetic field in a direction that indicates a positive charge. Alpha rays are nuclei of ordinary helium atoms (see alpha particle). Alpha decay reduces the atomic weight, or mass number, of a nucleus, while beta and gamma decay leave the mass number unchanged. Thus, the net effect of alpha radioactivity is to produce nuclei lighter than those of the original radioactive substance. For example, in the disintegration, or decay, of uranium-238 by the emission of alpha particles, radioactive thorium (formerly called ionium) is produced. The alpha decay reduces the atomic number of the nucleus by 2 and the mass number by 4:

Beta Radiation

Beta rays are more penetrating than alpha rays, move at a very high speed, and are deflected considerably by a magnetic field in a direction that indicates a negative charge; analysis shows that beta rays are high-speed electrons (see beta particle; electron). In beta decay a neutron within the nucleus changes to a proton, in the process emitting an electron and an antineutrino (the antiparticle of the neutrino, a neutral particle with a small mass). The electron is immediately ejected from the nucleus, and the net result is an increase of 1 in the atomic number of the nucleus but no change in the mass number. The thorium-234 produced above experiences two successive beta decays:

Gamma Radiation

Gamma rays have very great penetrating power and are not affected at all by a magnetic field. They move at the speed of light and have a very short wavelength (or high frequency); thus they are a type of electromagnetic radiation (see gamma radiation). Gamma rays result from the transition of nuclei from excited states (higher energy) to their ground state (lowest energy), and their production is analogous to the emission of ordinary light caused by transitions of electrons within the atom (see atom; spectrum). Gamma decay often accompanies alpha or beta decay and affects neither the atomic number nor the mass number of the nucleus.

Radioactive Decay

The nuclei of elements exhibiting radioactivity are unstable and are found to be undergoing continuous disintegration (i.e., gradual breakdown). The disintegration proceeds at a definite rate characteristic of the particular nucleus; that is, each radioactive isotope has a definite lifetime. However, the time of decay of an individual nucleus is unpredictable. The lifetime of a radioactive substance is not affected in any way by any physical or chemical conditions to which the substance may be subjected.

Half-Life of an Element

The rate of disintegration of a radioactive substance is commonly designated by its half-life, which is the time required for one half of a given quantity of the substance to decay. Depending on the element, a half-life can be as short as a fraction of a second or as long as several billion years.

Radioactive Disintegration Series

The product of a radioactive decay may itself be unstable and undergo further decays, by either alpha or beta emission. Thus, a succession of unstable elements may be produced, the series continuing until a nucleus is produced that is stable. Such a series is known as a radioactive disintegration, or decay, series. The original nucleus in a decay series is called the parent nucleus, and the nuclei resulting from successive disintegrations are known as daughter nuclei.

There are four known radioactive decay series, the members of a given series having mass numbers that differ by jumps of 4. The series beginning with uranium-238 and ending with lead-206 is known as the 4n+2 series because all the mass numbers in the series are 2 greater than an integral multiple of 4 (e.g., 238=4×59+2, 206=4×51+2). The 4n+1 series, which begins with neptunium-237, is not found in nature because the half-life of the parent nucleus (about 2 million years) is many times less than the age of the earth, and all naturally occurring samples have already disintegrated. The 4n+1 series is produced artificially in nuclear reactors.

Because the rates of disintegration of the members of a radioactive decay series are constant, the age of rocks and other materials can be determined by measuring the relative abundances of the different members of the series. All of the decay series end in a stable isotope of lead, so that a rock containing mostly lead as compared to heavier elements would be very old.

Discovery of Radioactivity

Natural radioactivity was first observed in 1896 by A. H. Becquerel, who discovered that when salts of uranium are brought into the vicinity of an unexposed photographic plate carefully protected from light, the plate becomes exposed. The radiation from uranium salts also causes a charged electroscope to discharge. In addition, the salts exhibit phosphorescence and are able to produce fluorescence. Since these effects are produced both by salts and by pure uranium, radioactivity must be a property of the element and not of the salt. In 1899 E. Rutherford discovered and named alpha and beta radiation, and in 1900 P. Villard identified gamma radiation. Marie and Pierre Curie extended the work on radioactivity, demonstrating the radioactive properties of thorium and discovering the highly radioactive element radium in 1898. Frédéric and Irène Joliot-Curie discovered the first example of artificial radioactivity in 1934 by bombarding nonradioactive elements with alpha particles.

Bibliography

See Sir James Chadwick, Radioactivity and Radioactive Substances (rev. ed. 1962); A. Romer, ed., Radiochemistry and the Discovery of Isotopes (1970).

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Radioactivity

Radioactivity


In 1896 Henri Bequerel, a French physicist, was studying the fluorescence of uranium compounds. He placed crystals of potassium uranyl sulfate on top of photographic film wrapped in dark paper and exposed the crystals to sunlight. Bequerel interpreted darkening of the film to be the result of penetration of the paper by the fluorescence of the uranium. A second experiment on this uranyl fluorescence was delayed for some days due to cloudy, wintry weather in Paris, leading Bequerel to decide to repeat the experiment, using new film. However, he developed the earlier film, expecting to see little, if any, darkening. To his surprise the film was as dark as if sunlight had been striking the uranium throughout the whole cloudy period. He concluded that uranium was spontaneously emitting high-energy rays that caused the observed darkening of the photographic plate and asked Madame Marie Curie, a research assistant in his laboratory, to join him in further studies of this new phenomenon. Curie named the spontaneous, high-energy radiation "radioactivity."

By 1898 Madame Curie and her husband Pierre, in collaboration with Bequerel, had isolated two new elements from the radioactive decay of uranium in pitchblende ore. Both were more radioactive than uranium itself. They named the first element polonium (Po) after Madame Curie's native land (Poland), and the second was named radium (Ra). Isolation of these two elements required chemical separation of very small amounts of Po and Ra from tons of pitchblende. Radium was found to be over 300,000 times more radioactive than uranium.

The French experiments attracted the attention of Ernest Rutherford, a physicist at the University of Manchester in England. Using an electrical field, Rutherford demonstrated that the radiation emitted from a radioactive sample could be separated into three types of rays, which he named alpha, beta, and gamma rays. The α rays (alpha rays) were positively charged, as they were deflected strongly to the negative side, whereas the negative β rays (beta rays) were deflected to the positive side. The γ rays (gamma rays) were not deflected and are uncharged high-energy electromagnetic radiation, similar to x rays and light rays. Gamma rays are the result of rearrangements of neutrons and protons in nuclei that yield lower-energy states and usually accompany other forms of radioactive decay.

Emission of an α -particle produces a new nucleus with a reduction in atomic number by two and in mass number by four. When a nucleus emits a β -particle, the atomic number of the new nucleus increases by one (over that of the decaying nucleus), but the mass numbers are unchanged. Some radioactive nuclei do not increase in atomic number in decay, but decrease by one unit of mass number due to the emission of a positron (a positively charged β ray). For example, in β decay with electron emission, is converted to , whereas in positron β decay, is converted to . An alternative process to β decay involves the absorption of an orbital electron by the nucleus in a process known as electron capture, which results in a decrease in the atomic number of the product nucleus, for example, 195Au decaying to 195Pt. Gamma-ray decay results in no change in either mass number or atomic number.

Another type of radioactive decay that is observed in the heaviest elements is spontaneous fission . In this process, a nucleus splits into two roughly equal parts, simultaneously releasing a large amount of energy. For example, for the nuclide , of every 100 nuclei that decay, approximately 97 do so by α decay and 3 undergo spontaneous fission.

The rate of radioactive decay is directly proportional to the amount of radioactive species present. A radioactive nucleus is characterized by its half-life, which is the amount of time it takes for 50 percent of the atoms present initially to decay. The half-life of 131I is eight days: an original sample of 1 gram (0.035 ounces) of 131I after eight days has only 0.5 grams (0.018 ounces) remaining; after sixteen days, only 0.25 grams (0.0088 ounces), and so on. The half-life is unaffected by differences in temperature, pressure, or chemical state. This constancy has made study of the half-lives of radioactive nuclei very useful to scientists engaged in dating archaeological and geological materials.

HANS GEIGER (18821945)

Hans Geiger worked in Ernest Rutherford's laboratory manually and meticulously counting α -particle scintillations for the famous experiments that led to the discovery of the nucleus. Because of this work, he developed an α -particle detector. After World War I, Geiger developed the modern GeigerMueller counter and worked until his death to increase its speed and sensitivity.

Valerie Borek

Since the discovery of radioactivity, radioactive nuclei serving as "tracers" have been of immense value to science, agriculture, medicine, and industry.

In the use of radioactive tracers it is assumed that the radioactive isotopes studied are identical in chemical behavior to the nonradioactive isotopes. The first experiments that used radioactive tracers were carried out in 1913 in Germany and were designed to measure the solubility of lead salts via the use of a radioactive isotope of lead. In industry, radionuclides have been used for analytical purposes, for measurements of flow in pipes, and as part of many other applications. Another example of an important tracer study has been the investigation of photosynthesis of carbohydrates from atmospheric CO2 in the presence of light and chlorophyll . Scientists used , , and to identify the intermediate steps involved in the photo-synthesis of carbohydrates in plants that had been placed in an atmosphere composed of -labeled CO2 and had been irradiated with light. The presence of the radioactive carbon in the synthesized carbohydrate was evidence that O2 was involved in the synthesis .

The process of neutron activation analysis , in which radioactivity is induced in stable nuclei by their bombardment with neutrons, has allowed measurement of impurities on the level of less than one part per billion. Neutron activation analysis has been used in determining the authenticity of paintings, in criminology, in analyzing lunar soil, and in many other areas.

The largest single use of radionuclides has been in medical science. If a radioactive compound, such as a radioactively labeled amino acid, vitamin , or drug, is administered to a patient, the substance is incorporated in different organs to varying degrees. The substance undergoes chemical change within the body, and the movement of the radioactive atoms in the body can be followed with radiation detectors. Such information is of great diagnostic value toward identifying the presence of tumors and other diseases in different organs in the body. Radioactivity has also been used in medicine for therapeutic purposes (radiotherapy); for example, it attacks cancerous cells more efficiently than normal cells.

see also Nuclear Fission; Nuclear Medicine; Transactinides; Transmutation.

Gregory R. Choppin

Bibliography

Choppin, Gregory R.; Liljenzin, Jan-Olov; and Rydberg, Jan (2001). Radiochemistry and Nuclear Chemistry, 3rd edition. Woburn, MA: Butterworth-Heinemann.

Ehmann, William D., and Vance, Diane E. (1991). Radiochemistry and Nuclear Methods of Analysis. New York: Wiley.

Friedlander, Gerhart, et al. (1981). Nuclear and Radiochemistry, 3rd edition. New York: Wiley.

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Radioactivity

Radioactivity

Radioactivity is the emission of radiation by unstable nuclei. That radiation may exist in the form of subatomic particles (primarily alpha and beta particles) or in the form of energy (primarily gamma rays).

Radioactivity was discovered accidentally in 1896 by French physicist Henri Becquerel (18521908). In the decades that followed Becquerel's discovery, research on radioactivity produced revolutionary breakthroughs in our understanding of the nature of matter and led to a number of important practical applications. These applications include a host of new devices and techniques ranging from nuclear weapons and nuclear power plants to medical techniques that can be used for diagnosing and treating serious diseases.

Stable and unstable nuclei

The nucleus of all atoms (with the exception of hydrogen) contains one or more protons and one or more neutrons. The nucleus of most carbon atoms, for instance, contains six protons and six neutrons. In most cases, the nuclei of atoms are stable; that is, they do not undergo changes on their own. A carbon nucleus will look exactly the same a hundred years from now (or a million years from now) as it does today.

But some nuclei are unstable. An unstable nucleus is one that undergoes some internal change spontaneously. In this change, the nucleus gives off a subatomic particle, or a burst of energy, or both. As an example, an isotope of carbon, carbon-14, has a nucleus consisting of six protons and eight (rather than six) neutrons. A nucleus that gives off a particle or energy is said to undergo radioactive decay, or just decay.

Words to Know

Alpha particle: The nucleus of a helium atom, consisting of two protons and two neutrons.

Beta particle: An electron emitted by an atomic nucleus.

Gamma ray: A high-energy form of electromagnetic radiation.

Isotopes: Two or more forms of an element with the same number of protons but different numbers of neutrons in their atomic nuclei.

Nucleus (atomic): The core of an atom, usually consisting of one or more protons and neutrons.

Radioactive decay: The process by which an atomic nucleus gives off radiation and changes into a new nucleus.

Radioactive family: A group of radioactive isotopes in which the decay of one isotope leads to the formation of another radioactive isotope.

Stable nucleus: An atomic nucleus that does not undergo any changes spontaneously.

Subatomic particle: Basic unit of matter and energy (proton, neutron, electron, neutrino, and positron) smaller than an atom.

Unstable nucleus: An atomic nucleus that undergoes some internal change spontaneously.

Scientists are not entirely clear as to what makes a nucleus unstable. It seems that some nuclei contain an excess number of protons or neutrons or an excess amount of energy. These nuclei restore what must for them be a proper balance of protons, neutrons, and energy by giving off a subatomic particle or a burst of energy.

In this process, the nucleus changes its composition and may actually become a different nucleus entirely. For example, in its attempt to achieve stability, a carbon-14 nucleus gives off a beta particle. After the carbon-14 nucleus has lost the beta particle, it consists of seven protons and seven neutrons. But a nucleus consisting of seven protons and seven neutrons is no longer a carbon nucleus. It is now the nucleus of a nitrogen atom. By giving off a beta particle, the carbon-14 atom has changed into a nitrogen atom.

Types of radiation

The forms of radiation most commonly emitted by a radioactive nucleus are called alpha particles, beta particles, and gamma rays. An alpha particle is the nucleus of a helium atom. It consists of two protons and two neutrons. Consider the case of a radium-226 atom. The nucleus of a radium-226 atom consists of 88 protons and 138 neutrons. If that nucleus gives off an alpha particle, it must lose the two protons and two neutrons of which the alpha particle is made. After emission of the alpha particle, the remaining nucleus contains only 86 protons (88 2) and 136 neutrons (138 2). This nucleus is the nucleus of a radon atom, not a radium atom. By emitting an alpha particle, the radium-226 atom has changed into an atom of radon.

The emission of beta particles from nuclei was a source of confusion for scientists for many years. A beta particle is an electron. The problem is that electrons do not exist in the nuclei of atoms. They can be found outside the nucleus but not within it. How is it possible, then, for an unstable nucleus to give off a beta particle (electron)?

The answer is that the beta particle is produced when a neutron inside the atomic nucleus breaks apart to form a proton and an electron:

neutron proton + electron

Recall that a proton carries a single positive charge and the electron a single negative charge. That means that a neutron, which carries no electrical charge at all, can break apart to form two new particles (a proton and an electron) whose electrical charges add up to make zero.

Think back to the example of carbon-14, mentioned earlier. A carbon-14 nucleus decays by giving off a beta particle. That means that one neutron in the carbon-14 nucleus breaks apart to form a proton and an electron. The electron is given off as a beta ray, and the proton remains behind in the nucleus. The new nucleus contains seven protons (its original six plus one new proton) and seven neutrons (its original eight reduced by the breakdown of one).

The loss of an alpha particle or a beta particle from an unstable nucleus is often accompanied by the loss of a gamma ray. A gamma ray is a form of high-energy radiation. It is similar to an X ray but of somewhat greater energy. Some unstable nuclei can decay by the emission of gamma rays only. When they have lost the energy carried away by the gamma rays, they become stable.

Natural and synthetic radioactivity

Many radioactive elements occur in nature. In fact, all of the elements heavier than bismuth (atomic number 83) are radioactive. They have no stable isotopes.

The heaviest of the radioactive elements are involved in sequences known as radioactive families. A radioactive family is a group of elements in which the decay of one radioactive element produces another element that is also radioactive. As an example, the parent isotope of one radioactive family is uranium-238. When uranium-238 decays, it forms thorium-234. But thorium-234 is also radioactive. When it decays, it forms protactinium-234. Protactinium-234, in turn, is also radioactive and decays to form uranium-234. The process continues for another eleven steps. Finally, the isotope polonium-210 decays to form lead-206, which is stable.

Many lighter elements also have radioactive isotopes. Some examples include hydrogen-3, carbon-14, potassium-40, and tellurium-123.

Radioactive isotopes can also be made artificially. The usual process is to bombard a stable nucleus with protons, neutrons, alpha particles, or other subatomic particles. The bombardment process can be accomplished with particle accelerators (atom-smashers) or in nuclear reactors. When one of the bombarding particles (bullets) hits a stable nucleus, it may cause that nucleus to become unstable and, therefore, turn radioactive.

[See also Atom; Isotope; Nuclear fission; Nuclear fusion; Nuclear medicine; Nuclear power; Subatomic particles; X rays ]

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radioactivity

radioactivity When Henri Becquerel established the existence of ‘uranic rays’ in March 1896, there was no way of appreciating the far-reaching implications of this discovery. Only 75 elements had been discovered by this time, two of which, uranium and thorium, were radioactive, although this was not known. The periodic table has since been expanded to 81 stable and 31 radioactive elements.

Radioactivity is the process of emission of radiation as a radioactive material changes form, often to a different element. To understand this process, we need to be familiar with a number of concepts and terms. Atoms of each element contain a different and defining number of protons and an equal number of electrons. The nucleus of the atom contains neutrons as well as protons and different numbers of neutrons are present in different isotopes of the same element. Isotopes of an element may be either stable, or unstable and radioactive — radioisotopes. Isotopes of all elements are referred to collectively as nuclides; those that are radioactive, as radionuclides. Radionuclides are specified by the elemental name and the mass number — the combined number of protons and neutrons — for example, carbon-14 (14C), iodine-131 (131I), plutonium-239 (239Pu). Where an element is referred to as radioactive, as in the paragraph above, this is intended to mean that all isotopes of the element are radioactive. Radionuclides differ in their rate of decay as well as the radiation emitted. The rate of decay in a given mass of the radionuclide is measured in units called becquerels (Bq), where 1 Bq equals one transformation per second. Alpha-emitting radionuclides emit alpha particles, each consisting of 2 protons and 2 neutrons. Beta-emissions involve the loss of an electron from the nucleus as a beta particle during the conversion of a neutron into a proton. Gamma rays are high energy photons, often emitted together with beta or alpha radiations when the transformation has left the atom with excess energy. An important characteristic of a radionuclide, as well as the radiation emitted, is its half-life — the time taken for half the atoms present to decay to the daughter nuclide. Thus 131I is a beta-emitter with a half-life of 8 days, while 239Pu is an alpha-emitter with a half-life of 24 000 years.

We are exposed to radionuclides throughout our lives, mainly from natural sources. The greatest exposures are due to inhalation of radon-222 (222Rn) gas, present in the atmosphere due to the decay of uranium-238 contained in rocks and soil. Artificial sources include the medical use of radiopharmaceuticals and small amounts released by the nuclear industry. There is, of course, the potential for greater exposures from nuclear installations if accidents occur, the most noteable example being the accident at Chernobyl in the former Soviet Union in 1986.

The health risk associated with exposure to a particular radionuclide will depend on the radiation emitted and its chemical behaviour. Beta and gamma radiations can penetrate through the skin and may pose an external radiation hazard, but the main concern generally is the entry of radionuclides into the body by inhalation and ingestion. Intake will lead to dose delivery to the respiratory and alimentary tracts, and absorption into the blood followed by entry into other organs and tissues. Depending on their chemical behaviour, some radionuclides concentrate in specific organs and tissues. For example, iodine-131 concentrates in the thyroid gland because iodine is an essential constituent of the hormone, thyroxine. Consequently, the dose to the thyroid is much greater than doses to other tissues, presenting a potential risk of thyroid cancer. Plutonium-239 is deposited mainly in the skeleton and liver and presents a potential risk of liver and bone cancer and leukaemia. Doses are calculated for intakes of radionuclides, taking account of their distribution and retention in the body and the pattern of deposition of radiation energy in different tissues. These calculations are done primarily by the International Commission on Radiological Protection, and the calculated values of dose per unit intake (Sv per Bq) are used as a basis for restrictions on radionuclide exposure in legislation in Europe, the UK, and elsewhere.

John D. Harrison


See also imaging techniques; radiation, ionizing; radiology; radiotherapy.

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radioactivity

radioactivity The spontaneous disintegration of certain atomic nuclei accompanied by the emission of alpha-particles (helium nuclei), beta-particles (electrons or positrons), or gamma radiation (short-wavelength electromagnetic waves). Natural radioactivity is the result of the spontaneous disintegration of naturally occurring radioisotopes. The rate of disintegration is uninfluenced by chemical changes or any normal changes in their environment. However, radioactivity can be induced in many nuclides by bombarding them with neutrons or other particles. See also radiation units.

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radioactivity

ra·di·o·ac·tiv·i·ty / ˌrādēōakˈtivətē/ • n. the emission of ionizing radiation or particles caused by the spontaneous disintegration of atomic nuclei. ∎  radioactive substances, or the radiation emitted by these.

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radioactivity

radioactivity (ray-di-oh-ak-tiv-iti) n. disintegration of the nuclei of certain elements, with the emission of energy in the form of alpha, beta, or gamma rays. Naturally occurring radioactive elements include radium and uranium. See also radioisotope.
radioactive adj.

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radioactivity

radioactivity Spontaneous change of the atomic nuclei of atoms, accompanied by the emission of radiation. The process by which a radioactive nucleus disintegrates is known as radioactive decay.

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