2. INTERESTING LONG-LIVED GALACTIC CHRONOMETERS

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.

Table 1. Long-lived galactic chronometers (_1/2 10⁹ yr)

			Decay	Nucleosynthesis
Nucleus	(Years)	Daughter	Mode	Process
40K	1.3x109	40Ca	-	s- / ?
40K	1.3x109	40Ar	+	s- / ?
87Rb	4.9x1010	87Sr	-	r-/s-process
138La	1.1x1011	138Ba	EC	p-process
138La	1.1x1011	138Ce	-	p-process
147Sm	1.1x1011	143Nd		s-/r-process
176Lu	3.7x1010	176Hf	-	s-process
187Re	4.5x1010	187Os	-	r-process
232Th	1.4x1010	205Pb	Decay Chain	r-process
235U	7.0x108	207Pb	Decay Chain	r-process
238U	4.5x109	206Pb	Decay Chain
107Pd	6.5x106	107Ag	-	s-/r-process
129I	1.6x107	129Xe	-	r-process
182Hf	9.0x106	182W	-	r-process
244Pu	8.2x107	232Th	Decay Chain / SF	r-process