Protactinium (Pa)

Almost every element has a price per kilogram. Protactinium has a price per gram, and the number comes from a single industrial campaign that nobody has ever repeated. Between 1959 and 1961 the United Kingdom Atomic Energy Authority put sixty tonnes of uranium-processing residue through a twelve-stage separation and recovered 125 grams of the metal at 99.9% purity. The cost is usually quoted at around half a million dollars of the day. That batch became the world's working supply, parcelled out to laboratories at roughly 2,800 dollars a gram, and it is still the reason most published protactinium measurements exist at all.

Why there is so little of it to find

Protactinium is not synthetic. It occurs in every uranium deposit on Earth, which makes the scarcity harder to explain than it looks. The answer is a decay chain and a ratio.

Protactinium-231 is a daughter of uranium-235, and it sits in secular equilibrium with it: as fast as it decays away with a 32,760-year half-life, the far slower uranium above it makes more. Equilibrium fixes the abundance at the ratio of the two half-lives, and uranium-235 is itself only 0.72% of natural uranium. Multiply the two and you get something close to a third of a gram of protactinium per tonne of uranium. There is no ore of it, no mineral in which it is a major component, and no deposit anywhere that concentrates it. Every gram that has ever been isolated came out of somebody else's tailings.

That is the first half of the difficulty. The second is chemical. Protactinium in solution hydrolyses readily and adsorbs onto more or less any surface it touches — glassware, tubing, precipitates it was never meant to join. Radiochemists have complained about this since the 1920s, and it is the reason a twelve-stage process was needed rather than a three-stage one.

Four discoveries and a name that lost a syllable

The single credit rendered above compresses a genuinely contested sequence, and it is worth separating the claims.

• 1900, William Crookes. He pulled from uranium a fiercely radioactive fraction he called UX, established that it was not uranium, and got no further. He had protactinium in hand without knowing what he was holding.
• 1913, Kasimir Fajans and Oswald Göhring. Working on the uranium decay series, they found a short-lived activity and named it brevium — "brief" — after its 1.17-minute half-life. What they had was the metastable isotope protactinium-234.
• 1917–18, Otto Hahn and Lise Meitner in Berlin. They isolated the long-lived protactinium-231, the isotope that makes the element a substance rather than a flicker. Much of the bench work was Meitner's; Hahn spent long stretches of the war seconded to the German army.
• 1918, Frederick Soddy and John Cranston in Glasgow. They found the same isotope independently, and were beaten into print.

Hahn and Meitner proposed protoactinium, from Greek protos — the element that stands before actinium, because it decays into it. Soddy's papers used the same clumsy five-syllable form. The contraction to protactinium came later, and the shortened spelling was not formally settled until the IUPAC conference at Amsterdam in 1949, three decades after the discovery. The metal itself was not produced until 1934, when Aristid von Grosse reduced the oxide and obtained a tenth of a milligram.

The ratio that reads the Atlantic

Protactinium's one genuinely important application is not as a material at all. It is as a signal in mud.

Two isotopes are constantly produced in seawater by uranium decay: protactinium-231 and thorium-230. Both stick to sinking particles and end up in seafloor sediment, but they do not stick equally well. Thorium is scavenged out of the water quickly and falls close to where it formed; protactinium lingers about ten times longer, long enough to be carried horizontally by deep currents before it settles. In the Atlantic, that current carries it south and out.

So the protactinium-to-thorium ratio buried in a core of North Atlantic sediment records how vigorously the ocean was overturning when that layer was laid down. A slow circulation leaves protactinium behind; a fast one exports it. In 2004 Jerry McManus and colleagues used exactly this ratio in Nature to show that Atlantic overturning had nearly shut down during the cold snap that ended the last glacial period, and had weakened sharply again during the Younger Dryas. Palaeoceanographers have used the measurement ever since. The same isotope is also used to date corals and cave carbonates back around a quarter of a million years, over a window where radiocarbon has long since run out.

An element with almost no use as a thing

Set the sediment work aside and protactinium is close to useless. It alloys with nothing anyone wants, catalyses nothing commercially, and its radioactivity offers no advantage over cheaper sources. It does have one quiet physical curiosity: the metal becomes a superconductor below about 1.4 K, which is unusually high for an actinide and has been studied for what it says about 5f-electron bonding rather than for any prospect of use.

The 1961 batch, then, was not bought for an application. It was bought so that the element could be characterised at all — so that somebody could measure its crystal structure and its chemistry on a sample large enough to weigh. Sixty tonnes in, 125 grams out, and the return was knowledge.