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 4 n +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 accompanying illustration shows a portion of the uranium disintegration series, i.e., from radium-226 to lead-206. The series beginning with thorium-232 is the 4 n series, and that beginning with uranium-235 is the 4 n +3 series, or actinide series. The 4 n +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 4 n +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).