Radionuclide Dating

TABLE 3.1 Events Leading to Large Injections of Radionuclides into Atmosphere (From Choppin, Rydberg and Liljenzin) 

Source Country Time Radioactivity (Bq) Important Nuclides 

An important application of the basic radioactive decay law is that of radionuclide dating. From Equation (3.6), we have 

by measuring the current ratio of daughter to parent atoms (D t)/P(t)) one can deduce the age of the sample. (This assumes, of course, that there are no daughter atoms present at t = 0, that they are all due to the parent decay, and that none have been lost.) 

Example Problem In a rock, one finds a nuclidic ratio of 206Pb to 238U of 0.60. 

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The extent to which molecular diffusion affects dating of fractured rock has yet to be evaluated thoroughly with proper models. Although diffusion is a slow process in dense crystalline rocks, it could still have an important influence on dates of very old groundwater. With atmospherically derived radionuclides, dates of water affected by this slow diffusion should appear too old. On the other hand, dating of water based on the accumulation of helium which diffuses out of solid rock into... 

FIGURE 14.21 Radionuclide dating can be successfully used in low-permeability soils and sediments (high in clay or organic matter) when there is little migration. 

Since the radionuclide-dating literature is extensive and our readers may have only a minor interest in the subject, we will describe the concepts briefly. 

Bischoff JL, Fitzpatrick JA (1991) U-series dating of impure carbonates An isochron technique using total-sample dissolution. Geochim Cosmochim Acta 55 543-554 Bonotto DM (1998) Implication of groundwater weathered profile interactions to the mobilization of radionuclides. J South Am Earth Sci 11 389-405... 

The results obtained to date are considerable and show that the chemistry of the TcN group may well be the most varied and interesting of the transition metal nitrido complexes [1,9,10]. The aim of this chapter is to provide a fairly comprehensive review of the literature up to the latter part of 1994. Additional data may be found in two conference volumes [11,12] and a recent review of Tc coordination chemistry [4]. For macroscopic studies with the long-lived "Tc (ti/2 = 2.11 x 10s years) the "Tc radionuclide is denoted simply as Tc. No carrier added studies and radiopharmaceutical applications utilizing the shortlived "mTc radionuclide (ti/2 = 6.01 hours) are denoted as "mTc. 

Table 1. Radionuclides of Atmospheric Origin and of Potential Use in Dating Ground Water.

A number of general reviews of the use of atmospherically produced radionuclides for dating groundwater have been written [12-16]. Most of these reviews center on the use of 3H and 14C. 

A fundamental assumption made for most dating with atmospheric radionuclides is that the cosmic radiation flux and hence, the natural production of the radionuclides has been constant with time. Various studies of this problem using 14C and tree-ring calibration have been made. Isotopic studies of meteorites have also been useful [17]. Considering the probable lack of basic accuracy of dating water, the problem of changes in cosmic ray flux is not serious. 

Three primary problem areas exist in dating groundwater. These are (1) Formulation of realistic geochemical-hydrodynamic models needed to interpret data which are generated by field and laboratory measurements, (2) development of sensitive and accurate analytical methods needed to measure trace amounts of various stable and unstable nuclides, and (3) theoretical and field oriented studies to determine with greater accuracy the extent and distribution of the subsurface production of radionuclides which are commonly assumed to originate only in the atmosphere. 

Theoretical studies [25,42] have shown that significant amounts of a number of radionuclides usually assumed to be derived only from the atmosphere may actually be produced in the subsurface, largely through interactions with secondary neutrons produced by alpha capture reactions. The alpha particles are derived mostly from normal decay of natural U and Th. Whether or not subsurface production of radionuclides can indeed influence dating has yet to be demonstrated by field and laboratory tests. The matter needs further study, particularly in relation to 14C dating of water which is more than 40,000 years old. 

All models available to date rest on two basic assumptions, viz. (i) the flux of the radionuclide to the sediment water interface has remained constant with time (or if the flux has varied, then its time variations are known) and (ii) the tracers and sediment particles are not independently mobile in the sediment pile. If these conditions are met then the temporal variations in the radionuclide concentration at depth, z, below the moving sediment-water interface would be given by [63,64,65,66] ... 

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