radioactivity: Radioactive Disintegration Series

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 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 4n series, and that beginning with uranium-235 is the 4n+3 series, or actinide series. 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.

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