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12.3 RADIOACTIVE SERIES

Four decay sequences that each end in a stable daughter

Most of the radionuclides found in nature are members of four radioactive series,

with each series consisting of a succession of daughter products all ultimately derived

from a single parent nuclide.

The reason that there are exactly four series follows from the fact that alpha decay re-

duces the mass number of a nucleus by 4. Thus the nuclides whose mass numbers are

all given by A 4 n, where nis an integer, can decay into one another in descending or-

der of mass number. The other three series have mass numbers specified by A 4 n

1, 4n2, and 4n3. The members of these series, too, can decay into one another.

Table 12.3 lists the four radioactive series. The half-life of neptunium is so short

compared with the age of the solar system that members of this series are not found

on the earth today. They have, however, been produced in the laboratory by bombarding

other heavy nuclei with neutrons, as described later. The sequence of alpha and beta

decays that lead from parent to stable end product is shown in Fig. 12.7 for the uranium

series. The decay chain branches at^214 Bi, which may decay either by alpha or beta

emission. The alpha decay is followed by a beta decay and the beta decay is followed

by an alpha decay, so both branches lead to^210 Pb.

430 Chapter Twelve

Table 12.3Four Radioactive Series

Half-Life, Stable End

Mass Numbers Series Parent Years Product

4 n Thorium^23290 Th 1.39 1010 20882 Pb

4 n  1 Neptunium^23793 Np 2.25 10620983 Bi

4 n  2 Uranium^23892 U 4.47 10920682 Pb

4 n  3 Actinium^23592 U 7.07 10820782 Pb

Astronaut Charles M. Duke, Jr., collecting rocks from the surface of the moon

during the Apollo 16 expedition in 1972. The rocks were dated radiometrically.

The youngest was found to be 3 billion years old, so igneous activity such as

volcanic eruptions must have stopped at that time.

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