Process radium decay series

The latest U.S. refinery at Weldon Springs, Missouri, operated by the Mallinckrodt Chemical Works, came into operation in 1957 and it incorporates a number of improvements over the earlier Mallinckrodt and Fernald Plants. - It processes chemical concentrates only, since there is no provision for shielding from the y- emitting radium decay series present in pitchblende or other ores. It is based upon 30 per cent TBP extraction but n-hexane is used as the inert diluent instead of kerosene. This allows faster mass-transfer and reduced times for phase separation, owing to its lower density and viscosity, thus reducing the size of plant for a given throughput. In addition, it is free from the small proportions of impurities which are present in kerosene and tend to react with nitric acid to form compounds which stabilize emulsions and form complexes with uranium. 

Radium is extremely radioactive. It glows in the dark with a faint bluish light. Radiums radioisotopes undergo a series of four decay processes each decay process ends with a stable isotope of lead. Radium-223 decays to Pb-207 radium-224 and radium-228decay to Pb-208 radium-226 decays to Pb-206 and radium-225 decays to Pb-209. During the decay processes three types of radiation—alpha (a), beta ((5), and gamma (y)—are emitted. 

The uranium decay series provides the most important isotopes of elements radium, radon, and polonium, which can be isolated in the processing of uranium minerals. Each ton of uranium is associated with 0.340 g of Ra. Freshly isolated Ra reaches radioactive equilibrium with its decay products to Pb in about two weeks (see Fig. 1.2). Many of these products emit energetic y-rays, which resulted in the use of Ra as a y-source in medical treatment of cancer (radiation therapy). However, the medical importance of radium has diminished greatly since the introduction of other radiation sources, and presently the largest use of radium is as small neutron sources (see Table 12.2). 

All uranium isotopes are radioactive. The three namral uranium isotopes found in the environment, U-234, U-235, and U-238, undergo radioactive decay by emission of an alpha particle accompanied by weak gamma radiation. The dominant isotope, U-238, forms a long series of decay products that includes the key radionuclides radium-226, and radon-222. The decay process continues until a stable, non-radioactive decay product is formed (see uranium decay series). The release of radiation during the decay process raises health concerns. 

Two methods to secure very small samples of francium for examination use the decay processes of other radioactive elements. One is to bombard thorium with protons. The second is to start with radium in an accelerator, where, through a series of decay processes, the radium is converted to actinium, which in turn rapidly decays into thorium, and finally, thorium decays naturally into francium. Following is a schematic of the decay process used for the production of small amounts of Fr-223 which, in turn, after several more decay processes ends up as stable lead (Pb) ... 

Various radium isotopes are derived through a series of radioactive decay processes. For example, Ra-223 is derived from the decay of actinium. Ra-228 and Ra-224 are the result of the series of thorium decays, and Ra-226 is a result of the decay of the uranium series. 

When an atom of any of these five isotopes decays, it emits an alpha particle (the nucleus of a helium atom) and transforms into a radioactive isotope of another element. The process continues through a series of radionuclides until reaching a stable, non-radioactive isotope of lead. The radionuclides in these transformation series (such as radium and radon), emit alpha, beta, and gamma radiations with energies and intensities that are unique to the individual radionuclide. 

Analogous to the process releasing Ra to seawater, decay of Th in sediments releases dissolved Ra which is then mixed into the ocean interior. Radium-226 decays through a series of short-lived nuclides to Pb (half-life 22.3 yr) which, like thorium and protactinium, is insoluble and readily sorbs to particles. Radioactive decay of gaseous Rn in the atmosphere also produces Pb, which is then deposited on the sea surface with aerosols and in precipitation. Although Pb and, to a lesser extent, Pa have found many applications as tracers of particle transport, by far the greatest use has been made of thorium isotopes, which form the focus of this review. 

This analysis can, for example, be applied to multistep radioactive decay reactions and to isomerization reactions. In such multistep processes, every step is by definition a first-order process. An example of multistep radioactive decay is the Actinium series (see Lederer et ah, 1968), in which Bi alpha-decays to ° T1, which beta-decays to ° Pb with respective half-lives of 2.14 and 4.77 min. Therefore, in this two-step consecutive process, k J ki =/9 = 2.14/4.77 = 0.449, very close to the Acme point. Similarly, in the Radium series, Pb beta-decays to which beta-decays to Po, which then alpha-decays very rapidly (with a half-life of only 0.16 ms) to ° Pb. This multistep decay can be closely approximated by two steps, the first with a half-life of 27 min, the second with a half-life... 

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