Radioactivity | Research & Encyclopedia Articles

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Radioactive decay Summary

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.