Friday, March 13, 2009

The Discovery of Technetium

In the 4th edition of Intermediate Physics for Medicine and Biology, Russ Hobbie and I discuss the biomedical properties of the element technetium (Tc), which plays an important role in nuclear medicine.
The most widely used isotope is 99m-Tc. As its name suggests, it does not occur naturally on earth, since it has no stable isotopes... [It decays by emitting] a nearly monoenergetic 140-keV gamma ray. Only about 10% of the energy is in the form of nonpenetrating radiation. The isotope is produced in the hospital from the decay of its parent, 99-Mo, which is a fission product of 235-U and can be separated from about 75 other fission products. The 99-Mo decays to 99m-Tc.
The Search For the Elements, by Isaac Asimov, superimposed on Intermediate Physics for Medicine and Biology.
The Search For the Elements,
by Isaac Asimov.
Technetium has an interesting history. When Dmitri Mendeleev proposed the periodic table, he predicted that holes in his table were missing elements that had not yet been discovered. In The Search for the Elements, Isaac Asimov writes
The first elements produced [by artificial transmutation] was the missing number 43 [technetium]. A claim to discovery of this element had been made in 1925 by Noddack, Tacke, and Berg, the discoverers of rhenium. They had named element number 43 “masurium” (after a district in East Prussia). But no one else was able to find masurium in the same source material, so their supposed discovery had remained a question mark. It was, in fact, just a mistake. In 1937 Emilio Gino Segre of Italy, an ardent hunter for the element, identified the real number 43.

[Ernest O.] Lawrence had bombarded a sample of molybdenum (element number 42) with protons accelerated in his cyclotron. Finally he got some radioactive stuff which he sent to Segre in Italy for analysis. Segre and an assistant, C. Perrier, traced some of the radioactivity to an element which behaved like manganese. Since the missing element 43 belonged in the vacancy in the periodic table next to manganese, they were sure this was it.

It turned out that element number 43 had several isotopes. Oddly, all of them were radioactive. There were no stable isotopes of the element!... Segre named the element number 43 technetium, from a Greek work meaning artificial, because it was the first element made by man.
Asimov tells the standard history of the discovery of technetium, but recently there has been a new twist to the story. John Armstrong of the National Institute of Standards and Technology (NIST) suggested that maybe masurium really was technetium. In an abstract to a NIST Sigma Xi colloquium in 2000 titled The Disputed Discovery of Element 43 (Technetium), Armstrong and P. H. M. Van Assche write
In 1925, Noddack, Tacke and Berg reported discovery of element Z = 43, which they named Masurium, based on line identification of x-ray emission spectra from chemically concentrated residues of various U-rich minerals. Their results were disputed and eventually the discovery of element 43 (Technetium) was generally credited to Perrier and Segre, based on their chemical separation of neutron-irradiated molybdenum in 1937. Using first principles x-ray emission spectral generation algorithms from the N.I.S.T. DTSA spectral processing program, we have simulated the x-ray spectra that would be expected using their likely analytical conditions (from their papers and contemporaneous reports) and the likely residue compositions suggested by Noddack et al. and Van Assche. The resulting spectra are in close agreement with that reported by Noddack et al., place limits on the possible residue compositions, and are supportive of the presence of detectable amounts of element 43 in their sample. Moreover, the calculated mass of element 43 shown in their spectrum is consistent with the amount that would be now expected from the spontaneous fission of U present in the ores they studied. The history of the original masurium/technetium controversy and the means used to reexamine the original record will be presented in this scientific detective story.
Was masurium really technitium? You will have to look at the evidence and decide for yourself. The story certainly is fascinating, and will interest readers of Intermediate Physics for Medicine and Biology.

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