15  Extinct Radionuclides

 

15.1     Production and decay

 

An ‘extinct radionuclide’ is understood to be one that was formed by a process of stellar nucleosynthesis prior to the coalescence of the solar-system, and which has subsequently decayed away to zero. Most extinct nuclides have very short half-lives, but a few have long half-lives in the millions of years range. These may have persisted in solar-system materials at high enough concentrations to generate observable variations in the isotopic composition of daughter products. These parent)daughter pairs are of interest to cosmochemists because they can provide information about the origin of the solar-system and its early history.

 

            The production rate of an arbitrary solar-system nuclide as a function of time is shown schematically in Fig. 15.1. After the ‘big bang’ around 13.7 Byr ago, nucleosynthetic production in stars proceeds at a rate p which may be steady or very variable, depending on the process. However, prior to condensation in the new solar nebula, it is anticipated that much or all solar-system matter was out of nucleosynthetic ‘circulation’ for a time period in some form of interstellar cloud. The time between last nucleosynthesis (‘star death’) and major condensation (‘glob formation’) is termed ) (Fig. 15.1). Determination of ) for different extinct nuclides may reveal information about the process which led to solar-system coalescence. Therefore, it is one of the major goals of isotope cosmochemistry.

Fig. 15.1. Schematic illustration of the variation in production rate (p) of a given nuclide between the ‘big bang’ and termination of nucleosynthesis, followed by a period ‘)’ prior to solar-system coalescence. After Wasserburg (1985).

 

            In order to derive useful information from the daughter products of extinct radionuclides, it is necessary to study material which has not been significantly re-worked during the life of the solar-system. Hence, chondritic meteorites, which appear to represent most nearly the original accretion components of the solar-system, are the main objects of study. However, for some nuclide pairs, iron and stony meteorites, which were subject to early planetary differentiation processes, may also be useful. Cosmic-ray bombardment is one process to which meteorites are particularly susceptible. This can cause nuclear transformations, which must be excluded as a mechanism for generating daughter-product anomalies before the latter are attributed to extinct parents.

 

            Most of the scientifically important extinct radionuclides with half-lives over 105 years are shown in Table 15.1 in order of decreasing stability, and will be discussed below. Mean lives (1/8) are quoted in Table 15.1, in addition to half-lives, because they are helpful in understanding the production history of extinct nuclides. The most long-lived of these species is the extinct p-process nuclide 146Sm (t1/2 = 103 Myr). This will be discussed briefly for the constraints it can place on very early terrestrial evolution, rather than solar-system condensation. However, another p-process nuclide (92Nb, t1/2 = 36 Myr) is omitted because of the relatively weak constraints it provides (Sconbachler et al., 2002).

 

                        TABLE 15.1. Some important extinct radionuclides

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Parent  Daughter         Decay              Mean life          Half-life              8

                                    mode                Myr                  Myr                    yr!1

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146Sm   144Nd            "                     149                  103                  6.7 H 10!9       

244Pu      various           fission               119                  82                    8.5 H 10!9

129I         129Xe              $                      23                   16                    4.3 H 10!8

247Cm     235U                 3", 2$ 22.5                15.6                 4.4 H 10!8

182Hf       182W               2$                    13                   9.0                  7.7 H 10!8

107Pd      107Ag              $                       9.4                 6.5                  1.1 H 10!7

53Mn       53Cr               $                       5.3                 3.7                  1.9 H 10!7

60Fe         60Ni               2$                      2.1                 1.5                  4.7 H 10!7

26Al         26Mg             $                       1.0                 0.7                  9.8 H 10!7

41Ca        41K                 $                       0.15               0.1                  6.7 H 10!6

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