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Technetium: The Element That Was Discovered Twice

Published

Author(s)

J T. Armstrong

Abstract

If you read about technetium in the Handbook of Chemistry and Physics (see Technetium at a Glance) you might think you knew all you needed to about its discovery and existence. That s how I felt the first time I had to deal with this rare element. But simple stories can be deceptive I first encountered technetium shortly after I got my Ph.D. While a researcher in an industrial microanalysis laboratory, I was asked to determine the distribution of technetium on bone surfaces using electron microscopy and x-ray analysis. It was the first time I measured x-ray spectra of technetium, and I figured it might well be the last. The technetium-99m in the radiopharmaceutical was made by neutron irradiation of molybdenum, similar to the technetium first analyzed by Perrier and Segre in 1937. Since the longest-lived isotope of technetium has a half-life of 200,000 years, the conventional wisdom was that no detectable natural technetium could be found on earth.Certainly, I didn t find any during the next twenty years. I moved on from industry and spent fifteen years in the Geological and Planetary Sciences Division at Caltech, using electron and ion microprobe analysis to study the oldest phases found in meteorites. Technetium is found in the spectra of stars and has interesting implications for nucleosynthesis. If it had stable isotopes, I would likely have studied it. But, since it didn t, I doubt that I spent as much as an hour thinking about its occurrence.It wasn t until 1998 that I took a real look at element 43. I was now at NIST in the Surface and Microanalysis Science Division. One day, an exuberant Belgian physicist, Pieter Van Assche, came into my office to ask my interpretation of an x-ray emission spectrum. The spectrum was from a 1925 article by Ida Noddack-Tacke, Walter Noddack and Otto Berg who claimed to have discovered element 43 (which they named masurium ) in samples from uranium-rich ores. Pieter speculated that, although they didn t realize it, they had actually isolated terrestrial technetium-99 formed from the spontaneous fission of uranium. I was skeptical, but after studying their paper, I realized that they were clearly not crackpots or, as Ernest Lawrence called them, apparently deluded . In the same article, the authors claimed discovery of element 75, naming it rhenium . Both claims were widely disputed at the time, but three years later, the Noddacks isolated weighable amounts of rhenium and were accepted as its discoverers. They weren t able to so concentrate masurium and the IUPAC eventually rejected that discovery. The controversy clearly affected their reputations. Little attention was paid to Ida Noddack s article in 1935 questioning Enrico Fermi s claim that he discovered the transuranium element 93 (for which he received the Nobel prize) and suggesting that his neutron bombardment of uranium may have resulted in the atoms disintegrating into fragments. It was not until Meitner et al. s discovery of nuclear fission in 1939 that she was proved right. After this time the Noddacks led lives of relative scientific obscurity.Using first principles x-ray emission spectral generation algorithms developed at NIST, I simulated the x-ray spectra that would be expected for Van Assche s initial estimates of Noddack s residue compositions. The first results were surprisingly close to their published spectrum! Over the next couple of years we refined our reconstruction of their analytical methods and performed more sophisticated simulations. The agreement between simulated and reported spectra improved further. Our calculation of the amount of element 43 required to produce their spectrum is very similar to the direct measurements of natural technetium abundance in uranium ore published in 1999 by Dave Curtis et al. at Los Alamos. We can find no other plausible explanation for their data, than that they did indeed detect fission masurium.The Noddacks were clearly among the fi
Citation
Chemical and Engineering News

Keywords

electron microprobe, Emilio Segre, Enrico Ferm, geochemistry, Ida Noddack, masurium, nuclear fission, technetium, Walter Noddack

Citation

Armstrong, J. (2008), Technetium: The Element That Was Discovered Twice, Chemical and Engineering News (Accessed April 24, 2024)
Created October 16, 2008