Page 109 - Radiochemistry and nuclear chemistry

98 Radiochemistry and Nuclear Chemistry

TABLE 5.2. Primordial radionuclides for Z < 82 (Pb)

Nuclide Isotopic abundance Decay mode and Half-life (years)
% particle energy MeV
4~ 0.0117 ~- EC 1.31 1.26 x 109
"~V 0.250 B- EC (0.601) > 1.4 x 1017
S7Rb 27.83 B- 0.273 4.88 x 101~
llSIn 95.72 fl- 1.0 4.4 x 1014
lZ'~re 0.905 EC (0.052) 1.3 x 1013
13SLa 0.092 3- EC 1.06 x 1011
144Nd 23.80 o~ 2.1 x 1015
147Sm 15.0 et 2.23 1.06 X l0 II
14SSm 11.3 tx 1.96 7 X 1015
1761m 2.59 /J- (1.188) 3.8 x 101~
174Hf 0.162 ct 2x1015
lSTRe 62.60 B- 0.0025 4.2x 101~
19~ 0.012 ot 6.5x10 II
Values within parenthesis refer to decay energies.
, , ,, ,, , , , . . . . . . , , , ,, ,
Because of the long half-lives of these nuclides they must have been formed at the time
of (or possibly even before) the formation of the solar system and of the earth. When the
earth's crust solidified, these radionuclides became trapped in rocks. As they decayed,
decay products accumulated in the closed rock environment. By measuring the amount of
parent and daughter nuclides, it is possible with the half-life to calculate how long this
environment (e.g. a rock formation) has existed. This is the bases for nuclear dating (also
called "radioactive clocks'), and almost all of the nuclides in Table 5.2 can be used for this
purpose. In w 5.8 we discuss dating methods for the K-Ar and Rb-Sr systems.
A careful look at these naturally occurring long-lived nuclei reveals that some of them
appea:r in short decay series, e.g. 152Gd ~ 148Sm ~ 144Nd --* 140Ce and 19~ ~ 186Os --~
182W. The heavy element series beginning with U and Th isotopes are therefore referred
to as long decay series.

5.2.2. Elements in the natural radioactive decay series

In Chapter 1 we briefly discussed the existence of four long series of genetically related
radioactive nuclides beginning with Th, U or Np and ending with Pb or Bi. In Figure 5.1
we present all known isotopes of elements 81TI to 92 U. Some of these nuclides occur
naturally in the long decay series shown in Figure 2.11. Others are produced through
nuclear reactions according to the schemes in Figure 4.8 and principles described in Ch.
12-15. The first series in Figure 1.2 is known as the thorium decay series, and consists of
a group of radionuclides related through decay in which all the mass numbers are evenly
divisible by four (the 4n series). It has its natural origin in 232Th which occurs with 100 %
isotopic abundance. Natural thorium has a specific activity S of 4.06 MBq/kg, as its

1 The student can easily trace the decay series in Fig. 5.1 with a transparent marker.