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Is the general public at risk of radon exposure? Uranium is ubiquitous in the rocks of Earth's crust, and so therefore is radon production. Potassium feldspar‐bearing rocks such as granites and gneisses, black shales, and phosphates contain higher uranium concentrations (>100 ppm) than average crustal rocks (<5 ppm). They therefore pose a greater threat. Radon gas occurs in air spaces and is quite soluble in water; think of the dissolved oxygen that aqueous organisms use to respire or the carbon dioxide dissolved in carbonate beverages. Groundwater circulating through uranium‐rich rocks can dissolve substantial amounts of radon gas and concentrate radium, another carcinogenic isotope. Ordinarily, this is not a problem. The gas rises and is released to the atmosphere, where it is dispersed and diluted to very low levels. But if radon gas is released into a confined space such as a home, especially one that is well insulated and not well ventilated, radon gas concentrations can reach hazardous levels. Most radon gas enters the home through cracks in the walls and foundations, either as gas or in water from which the gas is released. Most of the remainder is released when water from radon‐contaminated wells is used, again releasing radon into the home atmosphere. The problem is especially bad in winter and spring months when homes are heated, basements flooded, and ventilation poor. As warm air in the home rises, air is drawn from the soil into the home, increasing radon concentrations. The insulation that increases heat efficiency also increases radon concentrations. What can be done to reduce the risk? Making sure that basements and foundation walls are well sealed and improving ventilation can reduce radon concentrations to acceptable levels, even in homes built on soils with high concentrations of uranium. Radon test kits can be purchased from hardware stores. If indoor radon levels exceed 4 pCi/l, remediation is recommended by the installation of indoor air pumps and ventilation pipes to remove gases from beneath basement floors. Radon remediation typically costs $1500 and is highly recommended as a health measure.
In the following sections we have chosen a few examples, among the many that exist, to illustrate the importance of radioactive isotopes and decay series in the study of Earth materials.
Age determinations using radioactive decay series
decay constant (λ) λ
ssss1 Systematics of radioactive isotopes important in age determinations in Earth materials.
Decay series Decay process Decay constant (λ) Half‐life Applicable dating range 14C → 14N Beta decay 1.29 × 10−4/year 5.37 Ka <60 Ka 40K → 40Ar Electron capture 4.69 × 10−10/year 1.25 Ga 25 Ka to >4.5 Ga 87Rb → 87Sr Beta decay 1.42 × 10−11/year 48.8 Ga 10 Ma to >4.5 Ga 147Sm → 143Nd Alpha decay 6.54 × 10−12/year 106 Ga 200 Ma to >4.15 Ga 232Th → 208Pb Beta and alpha decays 4.95 × 10−11/year 14.0 Ga 10 Ma to >4.5 Ga 235U → 207Pb Beta and alpha decays 9.85 × 10−10/year 704 Ma 10 Ma to >4.5 Ga 238U → 206Pb Beta and alpha decays 1.55 × 10−10/year 4.47 Ga 10 Ma to >4.5 Ga
ssss1 Progressive change in the proportions of radioactive parent (N) and daughter (D) isotopes over time, in terms of number of half‐lives.
More generally, the age of any sample may be calculated from the following equation:
where t = age, λ = the decay constant, d = number of stable daughter atoms and p = number of radioactive parent atoms. Where stable daughter atoms were present in the original sample, a correction must be made to account for them, as explained below.