I first encountered technetium shortly after I got my Ph.D. While doing a stint as 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-99 in the radiopharmaceutical was made by neutron irradiation of molybdenum, similar to the technetium first analyzed by Carlo Perrier and Emilio Segrè in 1937. Since the longest lived isotope of technetium has a half-life of about 4 million years, the conventional wisdom was that no detectable natural technetium could be found on Earth.

Certainly, I didn't find any during the next 20 years. I moved on from industry and spent 15 years in the Geological & Planetary Sciences Division at California Institute of Technology, using electron and ion microprobe analysis to study the oldest phases 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.

	Ida Noddack-Tacke AMERICAN INSTITUTE OF PHYSICS/SCIENCE PHOTO LIBRARY

Van Assche speculated that, although the researchers didn't realize it, they had 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 International Union of Pure & Applied Chemistry eventually rejected that discovery. The controversy clearly affected their reputations.

Little attention was paid to Ida Noddack-Tacke'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 Lise Meitner and colleagues' "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 the Noddacks' 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 and colleagues at Los Alamos. We can find no other plausible explanation for the Noddacks' data than that they did indeed detect fission "masurium."

The Noddacks were clearly among the finest analytical geochemists of their time. Their search for the "missing" elements below manganese in the periodic table was part of a larger effort to accurately determine the abundance of the chemical elements in the earth and meteorites--data that provided a foundation for the science of geochemistry. Their work complemented rather than detracted from that of Perrier and Segrè.

I am pleased to see that Ida Noddack-Tacke's contributions to science are being rediscovered--on the Web and in recent books about the periodic table. As a participant in this scientific detective adventure, I'll always have a fondness for the "element that was discovered twice"--first as masurium, the first natural element discovered composed entirely of a spontaneous fission product; second as technetium, the first man-made chemical element.

John T. Armstrong is a research chemist at NIST, Gaithersburg, Md. He does fundamental and applied research on electron and X-ray spectrometry and is a past president of the Microbeam Analysis Society.

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