Radon decay chain

In addition to the assumptions of initial conditions, the validity of U-Pb methodology relies on closed system behavior of U, Pb and intermediate nuclides in the decay chain. Concordance between the two U-series decay chains is most likely to be compromised by Rn loss because Rn is the only gas in the decay chains and has a high diffusivity. Radon-222 in the decay chain has a half life of 3.8 days. This is much longer than the half-life of Rn (3.96 s) in the decay chain. Therefore, partial loss of Rn will give rise to an apparent age younger than the true age, whereas the 207p /235p ... 

Radon daughter ions and the ionization caused by the decay chains of radon and thoron in indoor air play important roles both from the contribution made by the daughter product positive ions to internal dose and from the effects of ion-pair production on the indoor atmospheric electrical parameters. 

Figure 2. Decay chain for the formation and decay of radon-222.

There is a radon isotope in each of the three major namral-decay chains ( Rn, half-... 

Figure 1 Abbreviated and decay chains illustrating the relative positions of radon...

Radon ( Rn) measurements made by the ARCAS provide a simple, reliable, real-time indicator of the relative maritime or continental nature of the air over coastal or oceanic areas (8). With a half-life of 3.8 days, Rn originates from the decay of Ra, a member of the decay chain. At least 98% of Rn originates from land masses (9). The radon flux at the surface depends on the radium content of the soils and rocks, the permeability of the source materials, atmospheric pressure, soil moisture, and vegetative cover (10). Relatively abrupt changes in the radon concentration over the ocean usually indicate changes in air masses and the passage of frontal systems. 

Figure 1. The decay chain of U leading to production of Rn and the important radon daughters (boxes highlighted). The half-lives of the chain members are listed in Table 1 below. The relatively high abundance of U in rocks and soils means that radon gas is, and always has been, present to some degree in the atmosphere.

Table 1. Elements in the uranium-238 decay chain and their half-lives. Note the very long half-life of (approximately equal to the age of the earth) which provides the continuing source of all other chain members. The half-lives of the radon daughters (highlighted in Figure 1) are all less than 30 minutes and so may reach varying degrees of equilibrium with radon in air depending on the factors which act on them.

As can be seen in Figure 1, radon itself and its polonium daughter products are alpha emitting nuclides, while the isotopes of lead and bismuth produced are beta/ gamma emitters. The short half-lives of the daughter products prior to Pb (Table 2) result in the rapid production of a mixture of airborne radioactive materials which may attain equilibrium concentrations within a relatively short time. The half-life of °Pb is 22 years and at this point in the decay chain any activity inhaled is largely removed from airways in which it is deposited before any appreciable decay occurs. 

Radon-222 is a direct decay product of radium-226, which is part of the decay series that begins with uranium-238 (see Chapter 3, Figure 3-1). Thorium-230 and thorium-234 are also part of this decay series. Uranium, thorium, and radium are the subjectof other ATSDR Toxicological Profiles. Other isotopes of radon, such as radon-219 and radon-220, are formed in other radioactive decay series. Flowever, radon-219 usually is not considered in the evaluation of radon-induced health effects because it is not abundant in the environment (Radon-219 is part of the decay chain of uranium-235, a relatively rare isotope) and has an extremely short half-life (4 seconds). Radon-220 is also usually not considered when evaluating radon-related health effects. While the average rate of production of radon-220 is about the same as radon-222, the amount of radon-220 entering the environment is much less than that of radon-222 because of the short half-life of radon-220 (56 seconds). All discussions of radon in the text refer to radon-222. 

Of the three naturally occurring radon isotopes, only Rn has a sufficiently long half-life (3.825 days) to allow for release from the soil and rocks, where it was generated. This half-life is short enough to restrict transport by pmre diffusion to short distances only, but once the radon has left the solid material and has become mixed with air, convection transport over longer distances (several meters) from the soil into both outdoor air and indoor environments is possible. In contrast, substantially less °Rn reaches the atmosphere because its short half-life (55.6 s) limits the distance it can travel before decay. As for Rn (formed in the decay chain of U), it... 

Radon has many unstable isotopes. The bad actor for humans is 222Rn, a dense, odorless, colorless gas that is part of the 238 j decay chain (see Table 22-4). Radon is a major contributor to the background radiation dose experienced by the general population and the greatest natural source of radiation. (Other significant sources are cosmic radiation, particularly at high altitudes, and medical procedures.)... 

Radioactive decay is probably not high on your list of household worries, unless you happen to live next door to a missile silo or Three Mile Island. However, private homes are the locations of greatest radon health concern simply because people spend so much time in them. Uranium decay-chain elements occur naturally in geologic formations and thus is continually formed underground. It migrates upward through the... 

The seventh item in the decay chain is Rn, with a half-hfe of 3.825 d. After loss of Rn, there is ample time for the decay of the daughter nuclides preceding before re-growth of the Rn. If, as is often the case, post-radon nuclides were measured to estimate activity, loss of radon would affect the whole activity measurement process. The solution is simple - encapsulate the sample and wait for about 10 half-lives of the Rn to allow equilibrium to be re-established - say one month. Having said that, experience shows that it is, in fact, possible to grind some geological materials without apparent loss of radon. However, that cannot be relied upon. Different materials have different radon-emanating powers, which will depend upon the moisture content and other factors. 

Radon is formed by decay of Ra in the U decay chain (see Fig. 27.3), and poses a serious health hazard in uranium mines, being linked to cases of lung cancer. ... 

Indoor air pollutants, including radon gas, can pose significant health risks. Radon is a radioactive gas found in the soils and rocks of Earth s crust. It finds its way into homes through cracks in the foundation or basement, and sometimes through the water supply. Radon is invisible and odorless, and becomes a health hazard when it is allowed to build up inside the home. The isotope of radon commonly found in homes is radon-222, a product of the uranium-238 decay chain, formed by the a decay of radium-226. Radon-222, in turn, decays by a emission. 

The most significant source of radioactivity in the indoor microatmosphere is radon, a noble gas product of radium decay that is produced below ground and that may leak into the basements above. Radon may enter the atmosphere as either of two isotopes, Rn (half-life 3.8 days) and °Rn (half-life 54.5 seconds). Both are alpha emitters in decay chains that terminate with stable isotopes of lead. The initial decay products, Po and Po, are nongaseous and adhere readily to atmospheric particulate matter. In some areas where radon is produced, homes have had to be fitted with ventilation systems to prevent radon infiltration. 

Some special types of intermediate continuous daughter loss could cause either normal or reverse discordance but no studies were found dealing with this. Starik et al (1960) do report, however, that radium and radon in the uranium decay chains may be leached from the crystal... 

Seaborgium (Z = 106) cannot be produced directly in " Ca-induced reactions, as it would require a radon target. The 1.9-min isotope Sg occurs in the decay chains arising from Cn and Fl, best produced in the reaction " Pu(" Ca,3n) [316]. The a-decay branch of the intermediate nuclide Ds is only 10%, so the effective production cross section is reduced from 4 pb to 0.4 pb. The 0.4-s isotope Sg is the decay daughter of Hs produced in the Ra( Ca,4n) reaction [133]. A single atom of Sg has been reported in the Fl decay chain, with a decay time of 2 min [353]. The nuclides produced in the " Cm( Ne,5n)... 

Both decay chains involve a gaseous isotope. Rn-220 in the Th-232 decay chain is short-lived (<1 min) and is unlikely to diffuse from solids Rn-222 in the U-238 decay chain is longer-lived (3.8 d) and diffusion before daughters grow in is more credible. Short-lived daughters after the radon gap will exist for analytical purposes if their radon parents are present. It is noted that Pb-210 at the end of the U-238 chain is long-lived on the time scale of sample analysis (22 y), and will remain after sample collection even if Rn-222 is lost subsequently. 

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