Lutetium

Lutetium was discovered in 1907, independently and almost simultaneously by the French chemist Georges Urbain, the Austrian Carl Auer von Welsbach and the American Charles James, as the last stable element to be prised from the rare-earth ores. Urbain, who prevailed in the naming, called it lutetium after Lutetia, the Roman name for Paris. It is a hard, dense, silvery rare-earth metal, the last of the lanthanides, used in catalysts and in specialised detectors. The densest and hardest of the lanthanides, lutetium is scarce and costly; it serves as a catalyst in refining petroleum, in the crystals of positron-emission tomography scanners, and its isotopes are used in dating rocks and in targeted cancer therapy.

Lutetium oxyorthosilicate and its cerium-doped variant, commonly called LSO and LYSO, are the standard scintillator crystals in most modern positron-emission tomography scanners, valued for converting incoming gamma rays into detectable light flashes quickly and efficiently; because lutetium is the scarcest and most expensive of the stable rare earths to refine, PET-scanner crystal manufacturing is one of the relatively few industrial applications that can justify its cost. Naturally occurring lutetium also carries a faint trace of a long-lived radioactive isotope, lutetium-176, whose slow decay into hafnium-176 gives geologists a further radiometric clock for dating some of the oldest rocks and meteorites.

Lutetium's name honours ancient Paris, Lutetia having been the Roman settlement on the site of the modern city, a fittingly classical tribute from the French chemist who claimed priority. The discovery, however, was bitterly contested: Auer von Welsbach had reached the same element and proposed the name cassiopeium, which was used in Germany for decades, while the priority quarrel between the rival claimants was one of the sharpest in the history of the rare earths.

The international standard eventually settled on Urbain's lutetium. The element has no ancient or esoteric tradition, being a twentieth-century discovery closing the lanthanide series; its interest lies in its name, a Roman echo of Paris, and in the rivalry that attended its finding. German-language chemistry literature continued to use Auer von Welsbach's cassiopeium for decades after Urbain's claim was internationally endorsed, and the name was not formally retired in favour of lutetium until an IUPAC ruling in 1949, an unusually long-running and explicit standardisation of a disputed element name. A further twist survives in spelling: since 1949 the accepted form has been lutetium rather than Urbain's original lutecium, a minor orthographic adjustment that still occasionally trips up older reference works.