Actinium (Ac)

Actinium is one of very few elements where the identity of the discoverer is still genuinely argued over by historians of chemistry, and the argument is unusual. It is not a nationalist squabble over who published first. It is a technical question about whether the man who is almost universally credited had the element in his hands at all.

The conventional account: André-Louis Debierne, working alongside the Curies on pitchblende residues, announced a new radioactive element in 1899. The name is his, from the Greek aktis, a ray. The data card above departs from that convention and credits Friedrich Oskar Giesel instead, and the reason is worth setting out.

What Debierne actually reported

In his 1899 paper, Debierne described the new substance as accompanying titanium through his separations and resembling it chemically. In 1900 he revised that and said it followed thorium.

Neither is what actinium does. Actinium sits directly below lanthanum, its only common oxidation state is 3+, and in a separation it tracks the rare earths. It does not behave like titanium and it does not behave like thorium. What does follow thorium through those procedures is protactinium — element 91, undiscovered until 1917 — which is the substance several later analysts think Debierne actually had.

In 1902 Giesel, a chemist at a quinine works in Braunschweig who did radiochemistry with unusual skill on the side, isolated a strongly radioactive fraction that accompanied lanthanum. He named it emanium, for the radioactive emanation it gave off. That description matches actinium exactly, and Giesel's material was pure enough that he could report properties for it.

Why the name stuck anyway

The two substances were recognised as the same element within a few years, and the priority question could have become a fight. It did not, largely because Giesel declined to have one. He was an admirer of the Curies and had no appetite for disputing a claim that had come out of their laboratory, so he let Debierne's name stand and stopped using his own.

The revisionist case was assembled much later. H. W. Kirby reviewed the primary literature in 1971 and concluded that Giesel was the discoverer; Jean-Pierre Adloff returned to it in 2000 in a paper explicitly framed around the centenary of a controversial discovery, and reached the same conclusion. Neither has displaced the textbook attribution, which is why most sources still say Debierne, 1899. It is a good example of how a scientific credit, once fixed in the reference literature, becomes almost impossible to move even when the evidence turns.

It glows

Actinium-227 is intensely radioactive — around 150 times more active per gram than radium — and a sample of the metal or its compounds emits a pale blue light in a dark room, produced by the excitation of nitrogen molecules in the surrounding air. It is one of the very few elements that visibly announces itself.

That activity also means almost nobody works with actinium metal. Its physical constants come from tiny samples, and its behaviour in air — rapid formation of a white oxide coating — is described from microgram quantities rather than from bulk.

Chemically actinium is dull. It has one oxidation state, forms no interesting complexes that lanthanum does not form more conveniently, and gave its name to the actinide series while having no 5f electrons of its own. Whether actinium belongs in that series at all, or should be grouped with scandium, yttrium and lanthanum, is part of the wider unresolved question about the composition of group 3.

Four alphas in ten days

Actinium's importance today rests on an isotope with no bearing on the natural element at all. Actinium-225 has a half-life of 9.9 days, and its decay chain runs down through francium-221, astatine-217, bismuth-213 and polonium-213 to stable bismuth-209 — emitting four alpha particles along the way, all of them within hours of the first.

For targeted radiotherapy this is close to an ideal package. A single delivered atom produces four separate high-energy, short-range events instead of one, and the ten-day half-life matches the time an antibody or a targeting peptide takes to find its target and clear from healthy tissue. Results reported from Heidelberg in 2016, using actinium-225 attached to a prostate-specific membrane antigen ligand, showed responses in men whose disease had already progressed through the beta-emitting lutetium-177 version of the same drug.

Milking thorium-229

The problem is supply, and it is severe enough to be the field's limiting factor rather than a detail.

Almost all actinium-225 used in medicine has come from three stockpiles of thorium-229, held at Oak Ridge in the United States, at the European Commission's laboratory in Karlsruhe, and at Obninsk in Russia. Thorium-229 decays to actinium-225, so those stockpiles are milked periodically, and their combined output has run at roughly 60 to 70 gigabecquerels a year — enough to treat a few hundred patients worldwide. The thorium-229 itself is a legacy of uranium-233 production decades ago and is not being replenished.

The alternative is to make it with an accelerator, by spallation of a thorium-232 target with high-energy protons, which national laboratories in the United States and Europe have been scaling up. That yields far more, and it brings a complication: the same bombardment produces actinium-227, which has a 22-year half-life and cannot be chemically separated from actinium-225 because it is the same element. Accelerator-produced material therefore carries a long-lived contaminant that has to be accounted for in dosimetry and in waste handling.

A tracer for the deep ocean

Actinium-227 has one job that has nothing to do with medicine. It is produced continuously in seafloor sediments by the decay of protactinium-231, and it diffuses out of the sediment into the water above. Because its 22-year half-life is comparable to the timescale on which deep water mixes vertically, the shape of its concentration profile above the seabed records how vigorously that mixing happens.

Oceanographers use it as a natural clock for abyssal circulation — one of a small set of tracers that can measure something no instrument can watch directly.

There is essentially none of it

Natural actinium exists only as a transient member of the uranium-235 decay chain, at something like 0.2 milligrams per tonne of uranium ore. Extracting it from ore has never been worth doing. Every practically useful quantity of every actinium isotope is produced either by neutron irradiation of radium-226 or from the decay of stored thorium — which makes actinium a natural element that, in practice, is entirely artificial.