Uranium isotopes, decay chains

ThSiO "Th and Th are present in naturally occurring uranium Th and Th occur in uranium minerals as members of the decay chain. The remaining isotopes are formed upon neutron bombardment of those isotopes discussed, or by charged particle bombardment of various targets. 

Soon after this discovery the harnessing of the technique to the measurement of all the U isotopes and all the Th isotopes with great precision immediately opened up the entire field of uranium and thorium decay chain studies. This area of study was formerly the poaching ground for radioactive measurements alone but now became part of the wonderful world of mass spectrometric measurements. (The same transformation took place for radiocarbon from the various radioactive counting schemes to accelerator mass spectrometry.)... 

The paper of 1939 [1 ], On the Chain Decay of the Main Uranium Isotope, studies the effects of elastic and non-elastic neutron moderation and concludes that chain fission reactions by fast neutrons in pure metallic natural uranium are impossible. The 1940 paper, On the Chain Decay of Uranium under the Influence of Slow Neutrons [2 ], is classic in the best sense of this word its value is difficult to overestimate. The theoretical study performed showed clearly that the effect of resonance absorption of neutrons by nuclei of 238U is a governing factor in the calculation of the coefficient of neutron breeding in an unbounded medium it was concluded that a self-sustained chain reaction in a homogeneous natural uranium-light water system is impossible. 

The uranium-graphite nuclear reactor (or nuclear pile ) was important not merely because it proved the feasibility of a self-sustaining fission chain. It could be used, with minor modification, for neutron irradiation of a sample by placing the sample in the interior of the reactor. Also, the system could be used as a source for the easily fissionable Pu239. This isotope (half-life 24,100 years) is a product in the decay chain from U239, which in turn results from the (n,y) reaction on U238 ... 

Some of the neutrons released in the controlled chain reaction strike the nuclei of non-fissionable U-238 atoms. In this case, the U-238 captures the neutron and becomes a heavier uranium isotope, U-239, which eventually decays to produce plutonium-239. 

The isotope Cm is the largest contributor to the alpha activity of irradiated uranium fuel from power reactors. It is an important source of the 2n + 2 decay chain in the high4evel wastes from fuel reprocessing. The alpha activity of Cm results in an internal heat-generation rate of 120 W/g of pure Cm. Separated Cm, prepared by the neutron irradiation of Am, provides a useful alternative for a thermoelectric source and for radionuclide batteries when relatively high outputs are desired over short periods of the order of its half-Ufe of 163 days. For example, a space power generator denoted as SNAP-11 contained 7.5 g of Cm and produced 20 W of thermoelectric power. Cm is also the decay source of Pu, which is used as a longer-lived radioisotope heat source. 

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. 

Very unexpectedly, the elements 99 einsteinium, Es) and 100 fermium, Fm) were detected in 1952 in the debris from a thermonuclear explosion. The Es and Fm isotopes found and identified by a collaborative effort of American laboratories (Ghiorso et al. 1955b) were Es (Tin = 20.5 d) and Fm (20 h). They were presumably produced by the successive capture of 15 or even more neutrons in in an enormous neutron flux on such a rapid timescale that no radioactive decay occurred between neutron captures until the P -decay half-lives of the very neutron-rich uranium isotopes became sufficiently short to compete and, thus, to feed long chains of subsequent P -decays ending in the heaviest elements. Macroscopic amounts of the elements berkelium to einsteinium were produced since the late 1960s by multiple neutron capture in curium in a high flux reactor in Oak Ridge. 

A second surprise was the observation of more than one such chain. Since represents 99.3% of natural uranium, the contribution of the other isotopes of uranium should be negligible. Consequently, one was expecting one and only one decay chain starting with... 

Schematic diagram of production of heavy uranium isotopes by successive neutron captures in followed by their subsequent beta-decay to spontaneously fissioning or alpha-decaying nuclides. Mass chains detected in debris from the Mike thermonuclear test are shown...

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