The identification of 232Th, 235U, and 238U as the most useful, and currently most reliable, nuclear chronometers follows from several considerations. For our present purposes, we wish to consider long lived chronometers, with lifetimes comparable to or greater than the age of the Galaxy we seek to measure. Table 1 presents a compilation of such potentially interesting galactic chronometers and identifies the nucleosynthesis mechanisms by which they are believed to be formed. Note particularly the potential additional long lived (1/2 > 109yr) galactic chronometers 187Re, 40K, 87Rb, 138La, 147Sm, 176Lu, that we have chosen, for a variety of reasons, not to emphasize in our discussion. 40K cannot yet be utilized as a useful chronometer, since neither the nuclear process by which it is formed nor its level of production is known to a reasonable degree of certainty. The effective use of 87Rb is limited, among other things, by the fact that both the s-process and the r-process can contribute significantly to its production; the same is true of 147Sm. 138La is a p-process nucleus, for which no reliable determination of its production history is yet possible. 176Lu is an s-process nucleus for which, again, an accurate quantitative measure of its production history is lacking. Finally, the case of 187Re deserves special mention. The long lifetime of 187Re makes the 187Re-187Os chronometer pair a particularly attractive choice for dating galactic nucleosynthesis (Clayton 1964). A major difficulty here is the fact that 187Os is also produced in the s-process. The uncertainties introduced by the subtraction of the s-process contribution to isolate the cosmo-radiogenic component (Bao & Kappeler 1987) are significant. Further complications are associated with the fact that the beta decay rate of 187Re in stellar environments is sensitive to temperature (Clayton 1969; Cosner & Truran 1981; Arnould et al. 1984). For the purposes of this paper, we will then restrict ourselves to the 232Th-235U and 235U-238U chronometer pairs.
|40K||1.3x109||40Ca||-||s- / ?|
|40K||1.3x109||40Ar||+||s- / ?|
|244Pu||8.2x107||232Th||Decay Chain / SF||r-process|