Radioactivity

In 1896, the French physicist Henri Becquerel accidentally found that an ore of uranium , pitchblende, emits an invisible form of radiation, somewhat similar to light. The phenomenon was soon given the name radioactivity and materials like pitchblende were called radioactive.

The radiation Becquerel discovered actually consists of three distinct parts, called alpha, beta, and gamma rays. Alpha and beta rays are made up of rapidly moving parti cles—helium nuclei in the case of alpha rays, and electrons in the case of beta rays. Gamma rays are a form of electromagnetic radiation with very short wavelengths.

Alpha rays have relatively low energies and can be stopped by a thin sheet of paper. They are not able to penetrate the human skin and, in most circumstances, pose a relatively low health risk. Beta rays are more energetic, penetrating a short distance into human tissue, but they can be stopped by a thin sheet of aluminum . Gamma rays are by far the most penetrating form of radiation, permeating wood, paper, plastic, tissue, water, and other low-density materials in the environment . They can be stopped, however, by sheets of lead a few inches thick.

Radioactivity is a normal and ubiquitous part of the environment. The most important sources of natural radioactivity are rocks containing radioactive isotopes of uranium, thorium, potassium, and other elements. The most common radioactive isotope in air is carbon-14, formed when neutrons from cosmic ray showers react with nitrogen in the atmosphere . Humans, other animals, and plants are constantly exposed to low-level radiation emitted from these isotopes, and they do suffer to some extent from that exposure. A certain number of human health problems—cancer and genetic disorders, for example—are attributed to damage caused by natural radioactivity.

In recent years, scientists have been investigating the special health problems related to one naturally occurring radioactive isotope, radon-226. This isotope is produced when uranium decays, and since uranium occurs widely in rocks, radon-226 is also a common constituent of the environment. Radon-226 is an alpha-emitter, and though the isotope does have a long half-life (1,620 years), the alpha particles are not energetic enough to penetrate the skin. The substance, however, is a health risk because it is a gas that can be directly inhaled. The alpha particles come into contact with lung tissue, and some scientists now believe that radon-226 may be responsible for a certain number of cases of lung cancer . The isotope can be a problem when homes are constructed on land containing an unusually high concentration of uranium. Radon-226 released by the uranium can escape into the basements of homes, spreading to the rest of a house. Studies by the Environmental Protection Agency (EPA) have found that as many as eight million houses in the United States have levels of radon-226 that exceed the maximum permissible concentration recommended by experts.

Though Becquerel had discovered radiation occurring naturally in the environment, scientists immediately began asking themselves whether it was possible to convert normally stable isotopes into radioactive forms. This question became the subject of intense investigation in the 1920s and 1930s, and was finally answered in 1934 when Irène Curie and Frèdèric Joliot bombarded the stable isotope aluminum-27 with alpha particles and produced phosphorus-30, a radioactive isotope. Since the Joliot-Curie experiment, scientists have found ways to manufacture hundreds of artificially radioactive isotopes. One of the most common methods is to bombard a stable isotope with gamma rays. In many cases, the product of this reaction is a radioactive isotope of the same element.

Highly specialized techniques have recently been devised to meet specific needs. Medical workers often use radioactive isotopes with short half-lives because they can be used for diagnostic purposes without remaining in a patient's body for long periods of time. But the isotope cannot have such a short half-life that it will all but totally decay between its point of manufacture and its point of use.

One solution to this problem is the so-called "molybdenum cow." The cow is no more than a shielded container of radioactive molybdenum-99. This isotope decays with a long half-life to produce technetium-99, whose half-life is only six hours. When medical workers require technetium-99 for some diagnostic procedure, they simply "milk" the molybdenum cow to get the short-lived isotope they need.

Artificially radioactive isotopes have been widely employed in industry, research, and medicine. Their value lies in the fact that the radiation they emit allows them to be tracked through settings in which they cannot be otherwise observed. For example, a physician might want to know if a patient's thyroid is functioning normally. In such a case, the patient drinks a solution containing radioactive iodine, which concentrates in the thyroid like stable iodine. The isotope's movement through the body can be detected by a Geiger counter or some other detecting device, and the speed as well as the extent to which the isotope is taken up by the thyroid is an indication of how the organ is functioning.

Artificially radioactive isotopes can pose a hazard to the environment. The materials in which they are wrapped, the tools with which they are handled, and the clothing worn by workers may all be contaminated by the isotopes. Even after they have been used and discarded, they may continue to be radioactive. Users must find ways of disposing of these wastes without allowing the release of dangerous radiation into the environment, a relatively manageable problem. Most materials discarded by industry, medical facilities, and researchers are low-level radioactive waste . The amount of radiation released decreases quite rapidly, and after isolation for just a few years, the materials can be disposed of safely with other non-radioactive wastes.

The same cannot be said for the high-level radioactive waste produced by nuclear power plants and defense research and production. Consisting of radioactive isotopes, such wastes are produced during fission reactions and release dangerously large amounts of radiation for hundreds or thousands of years.

Nuclear fission was discovered accidentally in the 1930s by scientists who were trying to produce artificial radioactive isotopes. In a number of cases, they found that the reactions they used did not result in the formation of new radioactive isotopes, but in the splitting of atomic nuclei, a process that came to be known as nuclear fission.

By the early 1940s, nuclear fission was recognized as an important new source of energy. That energy source was first put to use for destructive purposes, in the construction of nuclear weapons . Later, scientists found ways to control the release of energy from nuclear fission in nuclear reactors.

The most serious environmental problem associated with fission reactions is that their waste products are largely long-lived radioactive isotopes. Attempts have been made to isolate these wastes by burying them underground or sinking them in the ocean. All such methods have proved so far to be unsatisfactory, however, as containers break open and their contents leak into the environment.

The United States government has been working for more than four decades to find better methods for dealing with these wastes. In 1982, Congress passed a Nuclear Waste Policy Act, providing for the development of one or more permanent burial sites for high-level wastes. Political and environmental pressures have stalled the implementation of the act and a decade after its passage, the nation still has no method for the safe disposal of its most dangerous radioactive wastes.

See also Ecotoxicology; Hazardous waste; Nuclear fusion; Nuclear winter; Radiation exposure; Radiation sickness; Radioactive fallout; Radioactive pollution; Radioactive waste management

[David E. Newton ]

RESOURCES

BOOKS

Gofman, J. W. Radiation and Human Health. San Francisco: Sierra Club Books, 1981.

Inglis, D. R. Nuclear Energy: Its Physics and Social Challenge. Reading, MA: Addison-Wesley, 1973.

Jones, R. R., and R. Southwood, eds. Radiation and Health. New York: Wiley, 1987.

Wagner, H. N., and L. E. Ketchum. Living With Radiation. Baltimore: Johns Hopkins University Press, 1989.

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

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 (1882–1945)

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 Geiger–Mueller 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.

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 (1852–1908). 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 ]

RADIOACTIVITY

If an atomic nucleus is to be radioactive, two conditions must be satisfied: 1) the nucleus must be unstable; 2) the instability must be rapid enough to be observable in a reasonable amount of time.

Some nuclei are completely stable. For example, if the ¹⁶₈O nucleus is in its ground state, it will remain in that state forever, unless it is perturbed. This is because the ¹⁶₈O ground state is the configuration of sixteen nucleons with the lowest possible energy. Any other configuration has more energy, and thus the ground state ¹⁶₈O will continue to exist unless an outside agent supplies energy to it. The ground state ¹⁶₈O nucleus is said to be energetically stable, or absolutely stable.

Other nuclei are energetically unstable. For example, the ²³⁸₉₂U nucleus in its ground state is not the lowest energy combination of 238 nucleons. If these nucleons were rearranged to form a ²³⁴₉₀Th nucleus in its ground state plus an α particle (a ⁴₂He nucleus), the total energy would be lower by 4.2 MeV (4.2 × 10⁶ electron volts). Thus an undisturbed ²³⁸₉₂U nucleus has enough energy to emit an α particle, and every ground state ²³⁸₉₂U nucleus will eventually disintegrate in this way.

The force that drives the α particle out of the ²³⁸₉₂U nucleus is the electrical repulsion between the two protons in the α particle and the ninety protons in the ²³⁴₉₀Th nucleus that is left behind. This repulsion must overcome the attractive nuclear forces that tend to keep the ²³⁸₉₂U nucleus together. The competition between these attractive and repulsive forces determines how long the ²³⁸₉₂U nucleus is likely to last before it emits the α particle. In general, nuclei that emit more energetic α particles will emit these α particles more quickly. This property is expressed in terms of the half-life of the nucleus, which is the time it takes for half of the nuclei in a sample to decay. The half-life of ²³⁸₉₂U is 4.5 × 10⁹ years. But ²²⁴₉₂U, which emits an 8.46 MeV α particle, has a half-life of only a thousandth of a second.

The ²³⁴₉₀U nucleus left behind by the α emission from ²³⁸₉₂U is itself energetically unstable. It disintegrates by a process called β -decay, with a half-life of only 24 days, leading to the ²³⁴₉₁Pa nucleus, which is also unstable. This succession of decays continues until ²⁰⁶₈₂Pb is reached. ²⁰⁶₈₂Pb is also unstable (with respect to disintegration into ²⁰²₈₀Hg plus an α particle), but the excess energy of ²⁰⁶₈₂Pb is so small (about 1 MeV), that the half-life of ²⁰⁶₈₂Pb is very long—many orders of magnitude longer than the age of the universe. Thus, for all practical purposes, ²⁰⁶₈₂Pb can be regarded as a stable nucleus, in the sense that it is highly unlikely that anyone will ever see a ²⁰⁶₈₂Pb nucleus decay.

Thus we can distinguish between three types of nuclei: those that are energetically stable, those that are energetically unstable but have half-lives that are so long compared to the age of the universe that they can be regarded as stable, and those that are unstable with half-lives short enough for us to be able to

observe their decay. Nuclei in this last class are said to be radioactive.

The sequence of nuclei ²³⁸₉₂U, ²³⁴₉₀Th, ²³⁴₉₁Pa,...., ²⁰⁶₈₂Pb, are said to form a radioactive chain. There are three other radioactive chains, beginning, respectively, with ²³²₉₀Th, ²³⁵₉₂U, and ²³⁷₉₃Np. All the nuclei in a chain either have the same nucleon number or differ in nucleon number by a multiple of four. This is a consequence of the types of decay processes that occur within a chain (see below).

All the nuclei in our universe that are heavier than iron were either formed in the very energetic processes that accompanied a supernova explosion or are the descendants of nuclei formed in this way. The nuclei formed in this explosion were probably all radioactive, with half-lives that are short compared to the age of the Earth (about 5 × 10⁹ years). Therefore these nuclei have already undergone radioactive decay and are no longer present on the Earth. However, the ²³⁸₉₂U that was formed in a supernova explosion has a half-life of 4.5 × 10⁹ years, which is long enough so that some of this ²³⁸₉₂U still remains. Similarly, each of the other radioactive chains begins with a long-lived relic from a supernova explosion. The radioactive nuclei included in these chains are said to exhibit natural radioactivity. Artificial radioactivity refers to the radioactivity of nuclei that do not occur naturally but can be made when nuclei are bombarded by neutrons produced by a nuclear reactor or by the charged particles in an accelerator beam.

Several types of processes can occur when nuclei undergo radioactive disintegration:

1. Emission of photons. If a nucleus is in an excited state, it can undergo a transition to a lower-energy excited state or to the ground state. The energy difference between the initial and final nuclear states is usually carried away by a photon. This process is precisely analogous to the emission of light by atoms or molecules. The photon energy can vary from a few keV (10³ electron volts) to several MeV. The lower energy photons (with energy ≤50 keV) are usually referred to as X-rays. Higher energy photons are called γ-rays.
2. Emission of electrons or positrons. Due to the so-called weak interaction, it is possible for a neutron to transform into a proton, while emitting an electron and an antineutrino. Alternatively, a proton in the nucleus may transform into a neutron, while emitting a positron and a neutrino. A related process is the capture by a proton in the nucleus of an atomic electron, which causes the nuclear proton to become a neutron. Early in the study of this type of radioactivity, the emitted electrons were called β-rays.
3. Emission of α particles. This process is a special case of the process of fission, the disintegration of a nucleus into two or more parts (fission fragments). As for α emission, the driving force behind fission is the electrical repulsion between the fission fragments. Fission does not occur in the natural radioactive chains.

The activity of a radioactive source is the rate at which disintegrations occur. The units of activity are becquerels (disintegrations/sec) or curies (3.7 × 10¹⁰ disintegrations/sec). The unit of 1 curie was defined to be the activity of 1 gram of ²²⁶Ra. In general, the activity of a radioactive sample is given by the formula

See also:Fermi, Enrico; Neutrino, Discovery of; Pauli, Wolfgang; Quantum Tunneling; Radioactivity, Discovery of; Rutherford, Ernest

Bibliography

Krane, K. S. Introductory Nuclear Physics (Wiley, New York, 1988).

Benjamin Bayman