Primordial isotopes

The actinoid elements (or actinides An) constitute a series of 14 elements which are formed by the progressive filling of the 5/ electron shell and follow actinium in the periodic table (atomic numbers 90-103). All of the isotopes of the actinide elements are radioactive and only four of the primordial isotopes, Th, and " " Pu, have a sufficient long half-life for there to be any of these left in nature. 

Natural lead, a metallic element, is a mixture of the following four isotopes lead-204, lead-206, lead-207, and lead-208. Only lead-204 is a primordial isotope of nonradiogenic origin all the others are radiogenic, each isotope being the end product of one of the radioactive decay series of isotopes of thorium or uranium, namely, uranium-238, uranium-235, and thorium-232 the decay series of the uranium isotopes are listed in Figure 12 ... 

The primordial Li abundance was sought primarily because of its ability to constrain the baryon to photon ratio in the Universe, or equivalently the baryon contribution to the critical density. In this way, Li was able to complement estimates from 4He, the primordial abundance of which varied only slightly with baryon density. Li also made up for the fact that the other primordial isotopes, 2H (i.e. D) and 3He, were at that time difficult to observe and/or interpret. During the late 1990 s, however, measurements of D in damped Lyman alpha systems (high column-density gas believed to be related to galaxy discs) provided more reliable constraints on the baryon density than Li could do (e.g. [19]). Even more recently, the baryon density has been inferred from the angular power spectrum of the cosmic microwave background radiation, for example from the WMAP measurements [26]. We consider the role of Li plateau observations post WMAP. 

Extraterrestrial materials consist of samples from the Moon, Mars, and a variety of smaller bodies such as asteroids and comets. These planetary samples have been used to deduce the evolution of our solar system. A major difference between extraterrestrial and terrestrial materials is the existence of primordial isotopic heterogeneities in the early solar system. These heterogeneities are not observed on the Earth or on the Moon, because they have become obliterated during high-temperature processes over geologic time. In primitive meteorites, however, components that acquired their isotopic compositions through interaction with constituents of the solar nebula have remained unchanged since that time. 

Whereas the abundance of Sr in rubidium rich rocks changes over time due to the radioactive 3 decay of Rb as a function of the primordial rubidium concentration and the age of the mineral, the abundance of the stable Sr isotope and consequently the Sr/ Sr is constant in nature. The constant Sr/ Sr isotope ratio is often used for internal standardization (mass bias correction) during strontium isotope ratio measurements of Sr/ Sr. In the rubidium-strontium age dating method, the isotope ratios Sr/ Sr and Rb/ Sr are measured mass spectrometrically (mainly by TIMS or nowadays by ICP-MS) and the primordial strontium ratio ( Sr/ Sr)o at t = 0 and the age t of the rock can be derived from the isochrone (graph of measured Sr/ Sr isotope ratios (represented on the ordinate) as a function of the Rb/ Sr ratio (on the abscissa) in several minerals with different primordial Rb concentrations). The age of the minerals will be determined from the slope of the isochrone (e — 1), and the primordial isotope ratio ( Sr/ Sr)o from the point of intersection with the ordinate (see Figure 8.9). Rb-Sr age dating is today an... 

Primordial isotopes are those trapped during Earth formation and are not nucleogenic. Only globally significant other sources are included. See Ballentine and Bumard (2002) for production rates and other nuclear production mechanisms. Various mechanisms supply He isotopes from space see Torgersen (1989) for compilation. The source of variations is within the solid Earth, but has not contributed significantly to the atmosphere. 

Ultra-pure materials are needed for the constmction of the next generation of ultra-low level radiation detectors. These detectors are used for environmental research as well as rare nuclear decay experiments, e.g. probing the effective mass and character of the neutrino. Unfortunately, radioactive isotopes are found in most construction materials, either primordial isotopes, activation/spallation products from cosmic-ray exposure, or surface deposition of dust or radon progeny. 

Uranium is a radioactive element that is ubiquitous in the environment, with typical crustal and seawater concentrations of 2.7 mg kg and 3.0 pgl, respectively. Uranium-238 and U are primordial isotopes and their decay leads to the formation of series of daughter radioisotopes, amongst which is " U (Figure 1). The natural isotopic ratio of 238u 235u 234u 99.2745 0.7200 0.0055, and... 

Except for helium, the abundances of the primordial isotopes change by orders of magnitude when varying the baryon density or 77, respectively. On the other hand, He is very abundant and thus it can be observed with high accuracy. Therefore, it is possible to determine 77 from... 

Example of the extrapolation to primordial values. The primordial value of He (Tp) for zero metaUicity (zero oxygen) is inferred from a series of observations filled circles) (Olive 2001). For other primordial isotopes the slope of the interpolation would be steeper, for Li there would be a steep rise at higher metaUicities and a flat line with nearly constant abundance at low metaUicity (Reprinted from Olive 2001 with permission by Springer)... 

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