Promethium nuclides

Nucleosynthesis is the formation of elements. Hydrogen and helium were produced in the Big Bang all other elements are descended from these two, as a result of nuclear reactions taking place either in stars or in space. Some elements—among them technetium and promethium—are found in only trace amounts on Earth. Although these elements were made in stars, their short lifetimes did not allow them to survive long enough to contribute to the formation of our planet. However, nuclides that are too unstable to be found on Earth can be made by artificial techniques, and scientists have added about 2200 different nuclides to the 300 or so that occur naturally. 

Therefore, the preliminary investigation described herein examined several aspects of the behavior of the equilibrium distribution coefficients for the sorption of rubidium, cesium, strontium, barium, silver, cadmium, cerium, promethium, europium, and gadolinium from aqueous sodium chloride solutions. These solutions initially contained one and only one of the nuclides of interest. For the nuclides selected, values of Kp were then... 

For the nuclides studied (rubidium, cesium, strontium, bariun silver, cadmium, cerium, promethium, europium, and gadolinium) the distribution coefficients generally vary from about 10 ml/gm at solution-phase concentrations on the order of 10 mg-atom/ml to 10 and greater at concentrations on the order of 10 and less. These results are encouraging with regard to the sediment being able to provide a barrier to migration of nuclides away from a waste form and also appear to be reasonably consistent with related data for similar oceanic sediments and related clay minerals found within the continental United States. 

The discovery of technetium (Z = 43) in 1937 and of promethium (Z = 61) in 1947 filled the two gaps in the Periodic Table of the elements. These gaps had been the reason for many investigations. Application of Mattauch s rule (section 2.3) leads to the conclusion that stable isotopes of element 43 cannot exist. Neighbouring stable isotopes could only be expected for mass numbers A 93, A < 91, A = 103 and A > 105. However, these nuclides are relatively far away from the line of jd stability. The report by Noddak and Tacke concerning the discovery of the elements rhenium and masurium (1925) was only correct with respect to Re (Z = 75). The concentration of element 43 (Tc) in nature due to spontaneous or neutron-induced fission of uranium is several orders of magnitude too low to be detectable by emission of characteristic X rays of element 43, as had been claimed in the report. 

Nuclides are said to be either stable (nonradioactive) or unstable (radioactive). Elements that have atomic numbers greater than 83 (bismuth) are naturally radioactive, although some of the nuclides have extremely long half-lives. Some of the naturally occurring nuclides of elements 81, 82, and 83 are radioactive, and some are stable. Only a few naturally occurring elements that have atomic numbers less than 81 are radioactive. However, no stable isotopes of element 43 (technetium) or of element 61 (promethium) are known. 

The chart of the nuclides in the region of promethium is given in O Table 14.2. It is obvious now that there is a negligible amount of promethium in natural materials, and if macroscopic amounts were present, the radioactivity would be intense. The isotope with the longest half-life is 17.7-year Pm. If it were present at 1 ppm in a 1-g sample, the activity would be 6 MBq. 

It appears that when the number of protons becomes very large, Ihe proton—proton repulsions become so great that stable nucUdes are impossible. No stable nucUdes are known with atomic numbers greater than 83. On Ihe other hand, all elements with Z equal to 83 or less have one or more stable nuclides, with the exception of technetium (Z = 43), as noted in the chapter opening, and promethium (Z = 61). 

Answer Although a potent absorber of neutrons, samarium is a minor problem compared with that of xenon poisoning. Promethium-1 9 is one of the fission products of lj235 and decays on a two-day half-life to samarium-1 9 which is a stable nuclide with a large absorption cross-section (66,000 barns) for thermal neutrons. As the samarium poison accumulates, the chance for burnout by neutron capture increases, so the pile reactivity absorbed by samarium reaches a maximum or saturated value of around 600 c-mk under steady operating conditions. 

Samarium - A stable isotope (Sm-149) or nuclide resulting from the decay of Promethium-lIf9. Its absorption cross section for thermal neutrons is, 000 barns and> therefore, it must be accounted for in reactivity calculations. 

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