Once having identified the specific nuclear chronometers (in this case
the r-process
nuclei ^{232}Th, ^{238}U, and ^{235}U) for
dating the epoch of galactic nucleosynthesis, further
critical input astrophysical quantities include:

- the abundance ratios characterizing the matter which condensed into
meteorites when the solar system formed.
- the production ratios of these isotopes in the relevant (r-process)
nucleosynthesis
site (note the significant advantage of a single nucleosynthesis process).
- the time dependence of the nucleosynthesis rate (read 'star
formation rate')
*over the course of galactic evolution.*Abundance determinations for the thorium and uranium isotopes of interest are provided by analyses of meteoritic material. The situation for the

^{232}Th-^{238}U pair is complicated by the chemical differences between the two elements. Anders & Ebihara (1982) and Anders & Grevesse (1989) found the present day value of Th/U to be 3.6, which translates into a primordial solar system ratio (^{232}Th /^{238}U)_{SS}= 2.32. This is the value we have adopted for this paper. We have also used the ratio (^{235}U /^{238}U)_{SS}= 0.317 provided by these authors.The determination of the production ratios (

^{232}Th /^{238}U)_{r-process}and (^{235}U /^{238}U)_{r-process}resulting from r-process nucleosynthesis is, in general, a sensitive function of both the input nuclear physics and the physical conditions under which the r-process proceeds (Kratz et al. 1993, 1994). There are considerable uncertainties associated with the nuclear properties of the unstable neutron-rich progenitors of the uranium and thorium isotopes of interest. For purposes of illustration, Table 2 (adapted from the review articles of Cowan, Thielemann, & Truran (1991a, b)) lists representative values for these production ratios as determined by the indicated references. Note specifically that the predicted production ratios span a broad range: 1.40 (^{232}Th /^{238}U)_{r-process}1.90 and 0.89 (^{235}U /^{238}U)_{r-process}1.89. In general, the final abundances of these long lived isotopes involve summations over the abundances of a number of short-lived progenitors in alpha decay chains, with the number of progenitors being constrained by the occurrence of spontaneous fission at high masses. The effective numbers of progenitors arrived at by such considerations are 5.8, 6, and 3.35, for the isotopes^{232}Th,^{235}U, and^{238}U, respectively. If one assumes that the various progenitors are produced in equal abundances in the r-process, these numbers may be used directly to provide a crude estimate of the production ratios of these isotopes:^{232}Th /^{238}U) = 1.73 and^{235}U /^{238}U = 1.79. These values are also included in Table 2 as identified by `equal abundances assumed.' In our subsequent discussions, we have adopted the values (^{232}Th /^{238}U)_{r-process}= 1.65 ± 0.20 and (^{235}U /^{238}U)_{r-process}= 1.35 ± 0.30, which represent averages of the values presented in the Table 2.One additional factor should be emphasized here. For many other possible chronometers, dating is complicated by the fact that either the parent or the daughter can be formed by more than one astrophysical process - e.g., the 'mixed' r- and s-process chronologies noted in the previous section. The fact that the three radioactive isotopes with which we are concerned -

^{232}Th,^{235}U, and^{238}U - are formed in a single astrophysical process lessens the effects of uncertainties in models of galactic evolution and allows improved and more secure estimates to be obtained of their relative levels of production.The additional input required for dating is the nucleosynthesis history. The basic equations governing the time evolution of the abundance of a radioactive nuclear species are straightforward. In the following, we follow the notation of Tinsley (1980; see, also, Cowan, Thielemann, & Truran 1991a, b). Assuming a homogeneous ISM and instantaneous return of the products of stellar nucleosynthesis into the surrounding gas, the time evolution of species N

_{i}can be written**Table 2.**Production Ratios from r-Process Calculations

^{232}Th /^{238}U*^{235}U /^{238}UReference

1.65 1.42 Fowler (1972) Truran & Cameron (1971) 1.90 1.89 Seeger & Schramm (1970) 1.70 0.89 Wene & Johanson (1976) 1.80 1.42 Fowler (1977) 1.90 1.50 Symbalisty Schramm (1981) 1.50 1.10 Krumlinde et al. (1981) 1.40 1.24 Thielemann et al. (1983a,b) 1.71 1.34 Fowler (1987) 1.60 1.16 Cowan et al. (1987) 1.53 1.26 Thielemann et al. (1989) 1.73 1.79 Equal abundances assumed * The r-process calculations of Kratz and Thielemann (1996) yield values for

^{232}Th /^{238}U ranging from 1.37 to 2.18, depending upon the choice of atomic mass formula.

where (t) represents the gain or loss of mass due to accretion and winds, (t) is the rate of conversion of mass into stars,

_{i}is the decay rate of species i, and P_{i}is the production rate of species i, per unit mass going into stars. This equation has the general solutionwhere

The astrophysical input to these equations then involves the rate of formation of the range of stellar masses within which r-process nucleosynthesis occurs. While calculations of r-process nucleosynthesis have been carried out for a variety of plausible astrophysical sites (see, e.g., the reviews by Hillebrandt 1978; Schramm 1982; Mathews & Ward 1985; and Cowan, Thielemann, & Truran 1991a), a firm identification of the appropriate environment has become possible only recently. Observations of heavy element abundance patterns in metal deficient halo stars, described in the next section, point strongly to the identification of r-process nucleosynthesis with the environments provided by the evolution of massive stars and supernovae of Type II. In this context, the most promising mechanism of r-process synthesis would appear to be that associated with the neutrino heated "hot-bubble" supernova ejecta (Woosley & Hoffman 1992; Meyer et al. 1992; Takahashi, Witti, & Janka 1994), although an r-process associated with the decompression of cold neutron matter from neutron star mergers (Lattimer et al. 1977) provides a viable alternative. An important consequence of the identification of the r-process with such massive stars (M 10 M

_{}) of short lifetimes ( 10^{8}years), as discussed in the next section, is that we can reasonably expect that the age we determine from r-process chronometer studies is indeed representative of the age of the Galaxy itself.