Radium decay sequence

Evidence for the second viewpoint comes from measurements of longer-lived radionucleides within the radium decay sequence, specifically bismuth-210 and lead-210. The major routes for nuclei conversion within the radium decay scheme are shown in Fig. 7-27. The direct decay product of radium-226, an alpha-emitter, is radon-222, which escapes the Earth surface. Only the continents are a source the contribution from the oceans is negligible. Since the half-life time of radon-222 is only 3.8 days, its distribution in the troposphere is rather uneven. Over the continents the mixing ratio declines with increasing altitude (see Fig. 1-9). Over the oceans, the vertical gradient is reversed, as the oceans act as a sink and the zonal circulation keeps supplying material from the middle and upper troposphere. The immediate... 

Once it became clear that radioactive elements were decaying to new elements, which were themselves radioactive, a great deal of effort was expended in working out the decay sequences. In most cases the new elements were at first obtained in quantities too small to be weighed and were distinguished from each other only by the type of decay they exhibited and the rate at which the decay occurred. Three decay series were elucidated the uranium series proceeded through radium and terminated with the stable radium G the thorium series ended in the stable thorium D and the actinium series ended in actinium D. Between them, these series contained around 25 new radioelements. 

Marguerite Catherine Perey, an assistant to Marie Curie, is credited with the discovery of francium-223 in 1939. Perey discovered the sequence of radioactive decay of radium to actinium and then to several other unknown radioisotopes, one of which she identified as francium-223. Since half of her sample disappeared every 21 minutes, she did not have enough to continue her work, but a new element was discovered. 

Estimation of this radiation dose is sometimes accomplished by modeling the sequence of events involved in the acquisition, deposition, clearance, and decay of radium within the body. While based on the current understanding of experimental data on radium toxicokinetics, different models make different assumptions about these processes, thereby resulting in different estimates of dose and risk. These models are described in numerous reports including BEIR IV (1988), ICRP (1979), and Raabe et al. (1983). In this section, the toxicokinetics of radium are described based on the available experimental data rather than on descriptions derived from models. 

The final members of the decay series are stable nuclides ° Pb at the end of the thorium family, Pb at the end of the uranium-radium family, Pb at the end of the actinium family, and Bi at the end of the neptunium family. In all four decay series one or more branchings are observed. For instance, Bi decays with a certain probabihty by emission of an a particle into Tl, and with another probability by emission of an electron into Po. os-pj decays by emission of an electron into Pb, and Po by emission of an a particle into the same nuclide (Table 4.1), thus closing the branching. In both branches the sequence of decay alternates either a decay is followed by P decay or p decay is followed by a decay. 

An important characteristic of radioactive decay is that the momentum of the emitted particle must be balanced by the momentum of the product nucleus. In suitable cases it should be possible to detect the recoil nucleus. Thorium-228, which decays to radium-224 having an energy of 97 keV, has been used to study the oxidation of a number of metals. Its advantages are its low volatility (as thorium oxide) and its relatively low rate of diffusion in lighter metals. The maximum range of recoils in solids is of the order of 300-500 A and for thinner oxide layers, its distance from the surface can be measured. One difSculty with quantitative work is that the radium undergoes a sequence of further decay, which complicates calculation of recoil ranges, and calibration may be necessary. 

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