Astatine-211

Astatine-211 decays with a 7.2-hour half-life entirely by alpha emission, branching between two paths: about 41.8% via direct alpha emission to long-lived 207Bi, and about 58.2% via alpha decay to 211Po (half-life 0.52 s), which itself alpha-decays to stable 207Pb. Because astatine sits in the halogen group, it can in principle use much of the same radiochemistry developed for radioiodine (e.g. astato-benzylguanidine as an analogue of iobenguane).

It is made almost exclusively by bombarding natural, monoisotopic bismuth-209 targets with alpha particles in a cyclotron via the 209Bi(α,2n)211At reaction, which requires a beam of around 28-29.5 MeV; energies above this range increasingly co-produce the long-lived, more radiotoxic contaminant 210At (which decays to 210Po), so machines are tuned to stay just below or at that threshold.

Supply is the binding constraint: as of the mid-2020s only a handful of cyclotrons worldwide routinely produce clinical-grade 211At, including Copenhagen (Denmark), Arronax in Nantes (France), Duke University, the University of Washington and Ionetix (Michigan, first US commercial supplier, shipping since September 2026) in the US, with new or restarting capacity in Poland, Czechia, the UK and South Korea. Because of the 7.2-hour half-life, 211At must be produced close to where it will be used and cannot be stockpiled or shipped over long distances.