Primordial nuclides

Extremely metal-poor stars have a well defined nuclide composition for elements from C to Zn, except for C, N and Na, displaying some intrinsic scatter. This composition is in fairly well agreement with theoretical yields of primordial SNe of masses in the range 15 to 35 M , but not with those of Pair-Instability SNe. 

In the next section each light nuclide is considered in turn, its post-BBN evolution briefly reviewed along with identification of a few of the potential challenges to accurately inferring the primordial abundances from the observational data. Then, having established that the current data - taken at face value - are not entirely consistent with SBBN, I investigate whether changes in the early universe expansion rate can reconcile them. 

An important application of isotope ratio mass spectrometry is geochronology, which is based on the decay of a radioactive nuclide (e.g., 87Rb,187 Re or 238U) to a stable isotope. The total number of daughter atoms Dt at time t is then equal to the sum of the daughter atoms formed due to the radioactive decay and the number of primordial daughter atoms (D0) at time t = 0 ... 

The decay of extinct 244Pu is deduced from excess abundances of the nuclides 136Xe, 134Xe, and 132Xe, produced by the spontaneous fission of 244Pu. Uncertainties arise because there is no stable isotope of Pu that can be used in the way that 127I is used in Equation (3.60) and the use of other heavy nuclides U or Th as substitutes leads to difficulties due to differences in primordial production and chemistry. 

A number of short-lived radionuchdes also existed at the time that the Sun and the rocky bits of the solar system were forming (Table 1). These nuclides are sufficiently long-lived that they could exist in appreciable quantities in the earhest solar system rocks, but their mean fives are short enough that they are now completely decayed from their primordial abundances. In this sense they are referred to as extinct nuchdes. Although less familiar than the still-extant radionuclides, these short-lived isotopes potentially play similar roles their relative abundances can, in principle, form the basis of various chronometers that constrain the timing of early chemical fractionations, and the more abundant radioisotopes can possibly provide sufficient heat to drive differentiation (i.e., melting) of early accreted planetesimals. The very rapid rate of decay of the short-lived isotopes, however, means that inferred isotopic differences translate... 

The primordial nuclear reactor is short-lived, quickly encountering an energy crisis. Because of the falling temperatures and the coulomb barriers, nuclear reactions cease rather abruptly when the temperature drops below roughly 30 keV, when the universe is about 20 minutes old. As a result there is nuclear freeze-out since no already existing nuclides are destroyed (except for those that are unstable and decay) and no new nuclides are created. In 1000 seconds BBN has run its course. 

The primordial abundances of D, 3He, and 7Li(7Be) are rate limited, depending sensitively on the competition between the nuclear reactions rates and the universal expansion rate. As a result, these nuclides are potential baryometers since their abundances are sensitive to the universal density of nucleons. As the universe expands, the nucleon density decreases so it is useful to compare the nucleon density to that of the CMB photons r) = n /n7. Since this ratio will turn out to be very small, it is convenient to introduce... 

In contrast to the other light nuclides, the primordial abundance of 4He (mass fraction Y) is relatively insensitive to the baryon density, but since virtually all neutrons available at BBN are incorporated in 4He, it does depend on the competition between the weak interaction rate (largely fixed by the accurately measured neutron lifetime) and the universal expansion rate (which depends on geff)- The higher the nucleon density, the earlier can the D-bottleneck be breached. At early times there are more neutrons and, therefore, more 4He will be synthesized. This latter effect is responsible for the very slow (logarithmic) increase in Y with rj. Given the standard model relation between time and temperature and the nuclear and weak cross sections and decay rates measured in the laboratory, the evolution of the light nuclide abundances may be calculated and the frozen-out relic abundances predicted as a function of the one free parameter, the nucleon density or rj. These are shown in Fig. 1. 

As with the other relic nuclides, the dominant uncertainties in estimating the primordial abundance of 7Li are not statistical, they are systematic. Lithium is observed in the atmospheres of cool stars (see Lambert (2001) in these lectures). It is the metal-poor, Pop II halo stars that are of direct relevance for the BBN abundance of 7Li. Uncertainties in the lithium equivalent width measurements, in the temperature scales for... 

The Big Bang. In what is generally known as the standard family of Big Bang (Friedmann) models, 7Li is the only LiBeB nuclide synthesised in observable amounts. This Li in full or in part is seen in warm very metal-poor stars, as the Spite plateau. Nonstandard Big Bang models in a wide variety of forms have been proposed. Often, the consequences for the primordial nucleosynthesis are a focus of these proposals. 

There are essentially three sources of radioactive elements. Primordial nuclides are radioactive elements whose half-lives are comparable to the age of our solar system and were present at the formation of Earth. These nuclides are generally referred to as naturally occurring radioactivity and are derived from the radioactive decay of thorium and uranium. Cosmogenic nuclides are atoms that are constantly being synthesized from the bombardment of planetary surfaces by cosmic particles (primarily protons ejected from the Sun), and are also considered natural in their origin. The third source of radioactive nuclides is termed anthropogenic and results from human activity in the production of nuclear power, nuclear weapons, or through the use of particle accelerators. 

Shielding parameters. The most widely used shielding parameter for stony meteorites is the ratio Ne/ Ne of the cosmogenic component (note that we will always talk here about the cosmic-ray-produced fraction of a nuclide, which often has been obtained from measured values after substantial correction for other contributions, such as primordial noble gases see Isotopic abundances of cosmogenic noble gases... 

In order to deduce the cosmic-ray-produced fraction of a noble gas nuclide in a meteorite, it is commonly necessary to correct for other noble gas components, mostly trapped primordial gases or atmospheric contamination. To this end, the isotopic composition—or at least some crucial isotopic ratios—of the various components should be well constrained. Trapped components are discussed in previous chapters by Ott... 

---