Nucleocosmochronology is the use of the abundances of radioactive nuclear species and their radiogenic decay daughters to establish the finite age of the elements and the timescale for their formation. Studies of nuclear chronometers have played, historically, an important role. To the extent that the long lived nuclear species of interest in such studies are the products of nuclear transformations proceeding in stars and supernovae over the course of our own Galaxy's history, this serves as well to measure the duration of star formation activity and concomitant nucleosynthesis in the Galaxy, and thereby to impose a solid lower limit on the age of the Universe itself. An early paper by Rutherford (1929) outlined the essential features of this science. The defining works in nucleosynthesis theory by Burbidge et al. (1957) and Cameron (1957) established that the critical long lived chronometers 187Re, 232Th, 235U, and 238U, are formed in the r-process of neutron capture. The task, since then, has been to identify the astrophysical site for the operation of this nucleosynthesis process and to calculate the appropriate rates of production as a function of time, over the course of galactic evolution.
The early developments of the use of the uranium-thorium chronometers by Fowler & Hoyle (1960) and Cameron (1962) were necessarily based upon rather simple prescriptions for the history of galactic nucleosynthesis. As our understanding of the processes of stellar and supernova nucleosynthesis improved, it became possible to address the problem of nuclear chronology in the context of increasingly realistic models of the chemical evolution of the Galaxy (Truran & Cameron 1971; Reeves 1972; Tinsley 1975). Review discussions of nuclear cosmochronology in the context of models of galactic evolution include those by Tinsley (1980), Clayton (1988), Pagel (1989), Arnould & Takahashi (1989), and Cowan, Thielemann, & Truran (1991a, b).
In this paper, we will be concerned with the determination of realistic age constraints from nucleocosmochronology, which can serve to provide a solid lower bound on the age of the Universe. The identification of the 232Th / 238U and 235U / 238U chronometer pairs as the best candidates for dating the Galaxy follows the discussions in Section 2. Critical input to these chronological studies is identified and discussed in Section 3, in the context of the astrophysical r-process of heavy element synthesis. Abundance clues to r-process history and the identification of the astrophysical site of r-process synthesis are reviewed in Section 4. In Section 5, we obtain lower bounds on the timescale of galactic nucleosynthesis, with the assumption of an early 'single event' nucleosynthesis epoch. Observational evidence for a uniform rate of nucleosynthesis, and its implications for the age of the Galaxy are presented in Section 6. The use of the thorium abundance in an extremely metal deficient halo star for dating purposes is considered in Section 7. Discussions of complications from chemical evolution and our conclusions follow.