Thorium natural decay series

Figure 32.3 The three still-existing natural decay series. A. Uranium-238 B. Uranium-235 and C. Thorium-232. (Modified from Holtzman 1969 LWV 1985 UNSCEAR 1988 Kiefer 1990 Rose etal. 1990). Principal decay products occur within the heavy borders outlined.

FIGURE 3-1. Uranium and Thorium Isotope Decay Series Showing the Sources and Decay Products of the Two Naturally-Occurring Isotopes of Thorium... 

The laws of radioactive decay are the basis of chronology by nuclear methods. From the variation of the number of atoms with time due to radioactive decay, time differences can be calculated rather exactly. This possibility was realized quite soon after the elucidation of the natural decay series of uranium and thorium. Rutherford was the first to stress the possibility of determining the age of uranium minerals from the amount of helium formed by radioactive decay. Dating by nuclear methods is applied with great success in many fields of science, but mainly in archaeology, geology and mineralogy, and various kinds of chronometers are available. 

In the early stages of dating by nuclear methods, the measurement of He formed by a decay in the natural decay series (9, 6 and 7 He atoms in the uranium series, the thorium series and the actinium series, respectively) has been applied. The preferred method was the U/He method which allows dating of samples with very low concentrations of U of the order of 1 mg/kg. Helium produced by a decay is driven out by heating and measured by sensitive methods, e.g. by MS. However, it is difficult to ensure the prerequisites of dating by the U/He method neither " He nor a-emitting members of the decay series must be lost and no " He atoms must be produced by other processes such as decay of Th and spallation processes in meteorites. 

Several useful particle-reactive tracers are found within the natural uranium and thorium ( Th) decay series ... 

Other natural decay series originate with thorium and actinium. Radon is also an intermediate link in the former, the radioactive thorium decay series ... 

The long arrows pointing down and to the left correspond to a-particle emissions. The short horizontal arrows represent emissions. Other natural decay series originate with loTh (thorium series) and (actinium series). 

Polonium, completing the elements of Group 16, is radioactive and one of the rarest naturally occurring elements (about 3 x 10 " % of the Earth s crust). The main natural source of polonium is uranium ores, which contain about lO g of Po per ton. The isotope 210-Po, occurring in uranium (and also thorium) minerals as an intermediate in the radioactive decay series, was discovered by M. S. Curie in 1898. 

O Hara MJ (1968) The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks. Earth Sci Rev 4 69-133 O Nions RK, McKenzie D (1993) Estimates of mantle thorium/uranium ratios from Th, U and Pb isotope abundances in basaltic melts. Phil Trans Royal Soc 342 65-77 Oversby V, Gast PW (1968) Lead isotope compositions and uranium decay series disequilibrium in reeent volcanic rocks. Earth Planet Sci Lett 5 199-206... 

Moore WS (1992) Radionuclides of the uranium and thorium decay series in the estuarine enviromnent. In Uranium-series Disequilibrium Applications to Earth, Marine and Enviromnental Sciences. Ivanovich M, Harmon RS (eds) Clarendon Press, Oxford, p 396-422 Moore WS (1996) Large groundwater inputs to coastal waters revealed by Tla emichments. Nature 380 612-614... 

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 ... 

Rn-220 is another isotope of radon and belongs to the thorium decay series. Due to its short half life of 55.6 s, reports on its concentrations in those gases and in natural water are still scant. They are also important for a better estimate of our exposure to natural radioactivity and also for the geochemical study of the forma tion of those radon isotopes and their underground movement. 

Several organizations (e.g., NIST, NRC-Canada, and IAEA) provide sediment reference materials containing radionuclides, many of which are only certified for artificial radionuclides ( Cs, Sr, Am, and Pu). Certain specific radionuclides have no certified natural matrix materials, including ocean, lake, and river sediments. Although these sediments are certified for a few naturally occurring and artificial radionuclides, the extent of radioactive equilibrium of the uranium and thorium decay series in these environmental materials is not provided. NIST currently offers an ocean sediment Standard Reference Material (SRM 4357) in... 

Photopeaks observed in the background include the more prominent x-rays and gamma-rays of the natural uranium and thorium decay series and the natural 40K. As reported by Cooper et al. (5), background peak heights are relatively unaffected by suppression, but the reduction in the background continuum is significant. For our 8-cm.3 Ge(Li) detector, the background at 150 k.e.v. was reduced from 0.1 to 0.02 counts/min./ k.e.v., and that at 1.5 M.e.v. was reduced from 0.0013 to 0.0005 counts/ min./k.e.v. 

Table 3-2 lists important physical properties of radium and selected radium compounds. Radioactive properties of the four naturally-occurring radium isotopes are listed in Table 3-3. In addition to the naturally occurring isotopes, there are 12 other known isotopes of radium. The principal decay schemes of the uranium and thorium decay series that produce the naturally-occurring radium isotopes are presented in Figure 3-1.

---