An international team of researchers has discovered a new configuration of nuclear particles that decays by kicking out individual protons.
With 85 protons and just 103 neutrons, the atomic nucleus is both the heaviest known to break down this way and the lightest known isotope of the element astatine (At).
Astatine itself only occurs on Earth as a decay product of heavier elements, and never for very long. All of its isotopes are radioactive and ephemeral, with half-lives ranging from hours to nanoseconds. That helps make astatine the rarest naturally occurring element in Earth’s crust. Less than 1 gram is believed to exist globally at any given time, and only in fleeting traces.
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In the new study, researchers unveil a novel astatine isotope that decays via proton emission, a route that isn’t typical. Nuclei typically decay by emitting neutrons and protons together as alpha particles or through the emission of electrons or positrons as beta decay.
“Proton emission is a rare form of radioactive decay, in which the nucleus emits a proton to take a step toward stability,” says first author Henna Kokkonen, a nuclear physicist from the University of Jyväskylä in Finland.
It’s not easy to study this type of exotic nucleus – a term for atomic nuclei with unusual numbers of protons and neutrons that render them highly unstable and prone to speedy decay. That brief existence, among other factors, requires sophisticated methods to summon and examine them.
Kokkonen and her colleagues generated this novel nucleus in the Accelerator Laboratory of the University of Jyväskylä, using a fusion-evaporation reaction in which two nuclei collide and fuse, forming an unstable compound nucleus that then sheds particles in pursuit of stability.
“The nucleus was produced in a fusion-evaporation reaction by irradiating a natural silver target with 84Sr ion beam,” University of Jyväskylä nuclear physicist Kalle Auranen says in reference to a strontium beam emitted from the lab’s cyclotron particle accelerator.
Residues from this reaction were isolated using the lab’s gas-filled recoil separator unit and then analyzed via a spectrometer and a pair of detectors.
To help interpret this experimental data, the researchers also expanded upon a theoretical framework in nuclear physics known as the non-adiabatic quasiparticle model, which illuminates the structure and mechanics of deformed nuclei.
The model accurately reproduced the measured decay rate, suggesting the nucleus is probably a prolate spheroid – a rounded object with a distance between two of its poles exceeding its equatorial diameter.
In other words, the nucleus is watermelon-shaped.
The precise reasons for this shape remain unclear, but it hints at deeper mysteries that warrant further investigation, the researchers say.
“The properties of the nucleus suggest a trend change in the binding energy of the valence proton,” Kokkonen says. “This is possibly explained by an interaction unprecedented in heavy nuclei.”
Research like this can help shed new light on the building blocks of matter, yielding fundamental knowledge about the Universe that could prove useful in many different ways.
More observations of 188At are needed, the researchers write, to clear up lingering uncertainties about how exotic nuclei like this develop and decay.
“Equally interesting would be to study the decay of presently unknown nucleus 189At,” they write, another astatine isotope that might decay by proton emission.
The study was published in Nature Communications.
