Technetium and Promethium

Because of the small cross section (0.13 b) the yield of the ( . y) reaction is low. Relatively large amounts of Tc are produced by nuclear fission of (fission yield 6.2%). After a burn-up of 35 000 MW d per ton of uranium with an initial enrichment of 3% - U, the spent fuel contains about 1 kg Tc per ton. The longest-lived isotope of Tc is Tc (ti/2 — 4.2 - 10 y) in contrast to Tc, it has no practical significance. 

Although Tc is present in uranium ores in extremely small concentrations, the main importance of technetium is that of a man-made clement and technetium is counted as an artificial radioclement, due to its production in nuclear reactors and by nuclear explosions. 

Element 61 (Pm) could not be found in nature, and the gap in the Periodic Table of the elements remained until 1947 when the clement was discovered by Marinsky, 

Glendenin and Coryell in the fission products of uranium after separating the rare-earth fraction by oxalate precipitation. is produced with a fission yield of 

The element was named promethium im memory of Prometheus who, according to Greek mythology, brought fire to mankind. 

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This book presents a unified treatment of the chemistry of the elements. At present 112 elements are known, though not all occur in nature of the 92 elements from hydrogen to uranium all except technetium and promethium are found on earth and technetium has been detected in some stars. To these elements a further 20 have been added by artificial nuclear syntheses in the laboratory. Why are there only 90 elements in nature Why do they have their observed abundances and why do their individual isotopes occur with the particular relative abundances observed Indeed, we must also ask to what extent these isotopic abundances commonly vary in nature, thus causing variability in atomic weights and possibly jeopardizing the classical means of determining chemical composition and structure by chemical analysis. 

Prior to 1940 only the naturally occurring actinides (thorium, protactinium and uranium) were known the remainder have been produced artificially since then. The transactinides are still being synthesized and so far the nine elements with atomic numbers 104-112 have been reliably established. Indeed, the 20 manmade transuranium elements together with technetium and promethium now constitute one-fifth of all the known chemical elements. 

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. 

Table A. 1 comprises the stable elements from hydrogen to bismuth with the radioactive elements technetium and promethium omitted. Natural variations in isotopic composition of some elements such as carbon or lead do not allow for more accurate values, a fact also reflected in the accuracy of their relative atomic mass. However, exact masses of the isotopes are not affected by varying abundances. The isotopic masses listed may differ up to some 10 u in other publications.

For example, the fission products technetium and promethium are unique, in that they do not have any stable isotopes and do not occur in nature in measureable amounts. While promethium has a number of chemical analogues in the other rare-earth elements, this is not the case for technetium, and it is thus difficult to predict its behaviour in the geosphere. 

The problem of the existence of the heavy elements must also be considered. The short half-lives of all isotopes of technetium and promethium adequately accounts for their absence on earth. However, no element with atomic number greater than ssBi has any stable isotope. Many of these (notably 84P0, 85 At, sfiRn, 8 Fr, 89Ac and 91 Pa) can be... 

Ninety-one elements occur naturally on earth. Minute traces of plutonium-244 have been discovered in rocks mined in Southern California. This discovery supports the theory that heavy elements were produced during creation of the solar system. While technetium and promethium have not yet been found naturally on earth, they have been found to be present in stars. Technetium has been identified in the spectra of certain late type stars, and promethium lines have been identified in the spectra of a faintly visible star HR465 in Andromeda. Promethium must have been made near the star s surface for no known isotope of this element has a half-life longer than 17.7 years. 

Astatine and francium are formed from uranium only in most minute quantities, the scarcity explaining why they were not discovered earlier. Technetium and promethium are formed in even smaller quantities, and are unusual in that they are the elements of atomic number less than 84 which sess no stable isotopes at all. 

More than 90 per cent of elements occurring in nature are stable, i.e. not radioactive. They occupy boxes from 1 to 83 in the periodic table, i.e. from hydrogen to bismuth. There are two gaps in this sequence corresponding to the elements with Z = 43 (technetium) and Z = 6i (promethium). The strange properties of atomic nuclei have made all the isotopes of these elements radioactive with relatively short lifetimes therefore, technetium and promethium have not been preserved in nature but decayed and transformed into the neighbouring stable elements. 

But in the interval between the syntheses of the isotopes At and At a remarkable event occurred. The scientists from the Vienna Radium Institute B. Karlik and T. Bernert managed to find natural astatine. This was an extremely skillful study straining to the utmost the capacity of radiometry. The work was crowned with success and element 85 was born for the second time. As in the cases of technetium and promethium, we can name two dates in the history of astatine, namely, the year of its synthesis (1940) and the year of its discovery in nature (1943). 

The symbols At and Fr were permanently installed in boxes 85 and 87 of the periodic table and their properties proved to be exactly the same as predicted from the table. But in comparison with their unstable mates born by nuclear physics, technetium and promethium, their position is clearly unfavourable. 

Because of a small difference in stability of valence neutrons and protons, combined with fortuitous combinations of symmetry and Coulomb energies, technetium and promethium have no stable isotopes. 

On the earth, the most likely source of macroscopic amounts for technetium and promethium is the reprocessing of irradiated uranium and plutonium from fission reactors. This mechanism produces only a few, and not the longest-lived, isotopes. 

Radioactive isotopes of all the natural elements have been produced by artificial transmutation. In addition, production of technetium and promethium by artificial transmutation has filled gaps in the periodic table. Their positions are shown in red in Figure 2.10. 

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