Scientists detect first proton-emitting heavy nucleus in nearly 30 years

A rare new map point has been added at the edge of nuclear stability: astatine-188, or 188At, the lightest known astatine isotope and now the heaviest proton-emitting nucleus ever measured. For nuclear physicists, that matters because proton emission is one of the cleanest ways to test how far a nucleus can be pushed before it falls apart.

The nucleus was created at the Accelerator Laboratory of the University of Jyväskylä in Finland by firing an 84Sr ion beam at a natural silver target. In the fusion-evaporation reaction 107Ag(84Sr, 3n)188At, the new isotope was identified with the RITU recoil separator, a setup built for finding products that exist only briefly and in tiny numbers. The research team described exotic nuclei like this as especially hard to study because of their short lifetimes and very low production cross sections.

What makes the result stand out is not just the new isotope itself, but what the decay told the team about its structure. The experimental decay rate was reproduced with an expanded non-adiabatic quasiparticle model, and the nucleus appears strongly prolate, the kind of elongated shape physicists shorthand as watermelon-shaped. The calculations also pointed to a dominant s1/2 proton component in the wave function. Even more intriguingly, the single-proton separation energy of 188At deviated from extrapolated systematics, which the authors interpret as possible first evidence of a Thomas-Ehrman-shift-like effect in heavy nuclei.

That is a careful claim, not a sweeping one, but it hints at an interaction never before seen in this mass region. Proton emission itself is rare, a radioactive decay mode in which an unstable nucleus sheds a proton as it moves toward greater stability. The new measurement is the first comparable breakthrough in nearly 30 years, since 185Bi was identified in 1996 as the previous heaviest known proton emitter.

The paper puts 188At into a very small club. Proton emission was first seen from an isomeric state of 53Co in the 1970s, then from the ground state of 151Lu in the early 1980s. Since then, roughly 50 proton-emitting cases have been observed between 108I and 185Bi. Against that backdrop, the Jyväskylä result, led by doctoral researcher Henna Kokkonen and Academy Research Fellow Kalle Auranen, lands as a precise measurement with a big cartographic payoff: one more point on the nuclear landscape, and one more test of where the shoreline of matter really lies.