3.2 illustrates the 14‐step process required to convert radioactive uranium‐238 (238U) into the stable daughter isotope lead‐206 (206Pb). All of the intervening isotopes are unstable, so that the radioactive decay process continues. The first step, the conversion of238U into thorium‐234 by alpha decay, is slow, with a half‐life of 4.47 billion years (4.47 Ga). Because many of the remaining steps are relatively rapid, the half‐life of the full decay, sequence is just over 4.47 Ga. As the rate at which238U atoms are ultimately converted into206Pb atoms is known, the ratio238U/206Pb can be used to determine the crystallization ages for minerals, especially for those formed early in Earth's history, as explained below.
Those isotopes that decay rapidly, beginning with protactinium‐234 and radon‐222, produce large amounts of decay products in short amounts of time. Radioactive decay products, especially alpha particles, can produce notable damage to crystal structures and significant tissue damage in human populations (Box 3.2). Radioactive isotopes also have significant applications in medicine, especially in cancer treatments. Radioactive decay provides a significant energy source through nuclear fission in reactors. Radioactive materials remain important global energy resources, even though the radioactive isotopes in spent fuel present long‐term hazards, expecially with respect to their disposal. As noted above, radioactive decay is also the primary heat engine within Earth and is partly responsible for driving plate tectonics and core–mantle convection. Without radioactive heat, Earth would be a very different kind of home.
Figure 3.12 Three types of radioactive decay: alpha decay, beta decay, and electron capture (gamma decay) and the changes in nuclear configuration that occur as the parent isotope decays into a daughter isotope.
Table 3.2 The 14‐step radioactive decay sequence that occurs in the conversion of the radioactive isotope238U into the stable isotope206Pb.
| Parent isotope | Daughter isotope | Decay process | Half‐life |
|---|---|---|---|
| Uranium‐238 | Thorium‐234 | Alpha | 4.5 × 109 years |
| Thorium‐234 | Protactinium‐234 | Beta | 24.5 days |
| Protactinium‐234 | Uranium‐234 | Beta | 1.1 minutes |
| Uranium‐234 | Thorium‐230 | Alpha | 2.3 × 105 years |
| Thorium‐230 | Radium‐226 | Alpha | 8.3 × 104 years |
| Radium‐226 | Radon‐222 | Alpha | 1.6 × 103 years |
| Radon‐222 | Polonium‐218 | Alpha | 3.8 days |
| Polonium‐218 | Lead‐214 | Alpha | 3.1 minutes |
| Lead‐214 | Bismuth‐214 | Beta | 26.8 minutes |
| Bismuth‐214 | Polonium‐214 | Beta | 19.7 minutes |
| Polonium‐214 | Lead‐210 | Alpha | 1.5 × 10−4 seconds |
| Lead‐210 | Bismuth‐210 | Beta | 22.0 years |
| Bismuth‐210 | Polonium‐210 | Beta | 5.0 days |
| Polonium‐210 | Lead‐206 | Alpha | 140 days |
Box 3.2 Radon and lung cancer
Inhalation of radon gas is the second largest cause of lung cancer worldwide, second only to cigarette smoking. In the 1960s, underground uranium miners began to show unusually high incidences of lung cancer. The cause was shown to be related to the duration of the miner's exposure to radioactive materials. To cause lung cancer, the radioactive material must enter the lungs as a gas. It then causes progressive damage to the bronchial epithelium or lining of the lungs. What is the gas and how does it originate? Table 3.2 shows the many radioactive isotopes that are produced by the decay of the common isotope of uranium (238U). Uranium miners would be exposed to all of these, but which one would they inhale into their lungs? Because radon possesses a stable electron configuration, it tends not to combine with other elements. Like most noble elements, under normal near surface conditions, it tends to exist as separate atoms in the form of a gas. In the confined space of poorly ventilated underground mines, radioactive decay in the uranium series produces sufficient concentrations of radon to significantly increase the incidence of lung cancer. The other property that makes radon‐222 so dangerous is its short half‐life (3.825 days). Within days, most of the radon inhaled by miners decays into polonium‐218 with the emission of alpha particles (4He nuclei). Subsequently, most of the radioactive218Po decays within hours into lead‐210 with the release of more alpha particles. Lung damage leading to lung cancer largely results from continued rapid release of alpha particles over long periods of exposure. Scientific studies on radon exposure have been complicated by the fact that many miners were also smokers. It turns out that smoking and radon exposure act synergistically to multiply the risk of developing lung cancer.
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
