Atomic breakthrough reveals nucleus that loses protons to survive

For the first time in nearly 30 years, scientists have detected the heaviest atomic nucleus that undergoes proton emission. The isotope astatine‑188 (188At) contains 85 protons and 103 neutrons.

It decays by ejecting a proton, a rare type of radioactive decay where the nucleus loses a single proton to move toward stability.

This discovery was made at the Accelerator Laboratory of the University of Jyväskylä, Finland, using a fusion‑evaporation reaction: a beam of strontium‑84 ions hit a silver‑107 target. The RITU recoil separator then isolated the new isotope before detection.

“Proton emission is a rare form of radioactive decay, in which the nucleus emits a proton to take a step toward stability,” explained Henna Kokkonen, a doctoral researcher at Jyväskylä.

Heaviest proton-emitting nucleus

Prior to this, the heaviest known proton emitter was bismuth‑185, discovered back in 1996. That nucleus decayed in just 60 microseconds, though later experiments corrected that to around 2.8 μs.

By contrast, the proton‑emission half‑life of 188At was measured to be roughly 190 (−80 +350) microseconds. Detecting and timing just two decay events made it possible to confirm this extremely fleeting process.

Proton theory led to nucleus discovery

Proton emission was first observed in the 1970s, with early discoveries involving isotopes like cobalt‑53 and lutetium‑151.

These findings confirmed that atomic nuclei can decay by losing individual protons, not just clusters like alpha particles. 

The research also showed that quantum tunneling, the ability of particles to escape barriers they seemingly shouldn’t, applies to single protons in extreme nuclear environments.

Over time, advances in decay spectroscopy and detector technology allowed researchers to observe proton emission from increasingly heavier and shorter-lived isotopes.

Each detection involves tracking rare decay events with high precision, often from just a handful of atoms produced in a lab.

That’s what makes the discovery of 188At so significant – it builds on decades of steady progress in experimental nuclear physics.

Chasing rare isotopes

Producing exotic isotopes like 188At requires intense effort, advanced equipment, and luck. The fusion-evaporation process used in this experiment yields only a few atoms of the desired nucleus among billions of other particles.

Researchers must then filter, detect, and confirm these fleeting signals before they vanish. To identify 188At, the team at Jyväskylä used the GREAT spectrometer and a silicon strip detector to capture decay events with sub-millisecond precision.

The rarity of the signal, just two confirmed decays, demonstrates how precise the experimental setup must be for success.

Global teamwork made it possible

The discovery of 188At wasn’t the work of a single lab or discipline. It involved a broad collaboration between experimental physicists, nuclear theorists, and engineers across several institutions.

Each brought specific expertise, from beam production and isotope separation to quantum modeling of the nucleus.

By pooling resources and methods, the team was able to push beyond prior detection limits. These types of cross-border projects are increasingly necessary to study phenomena at the frontiers of physics, where no single institution has all the tools or data to work alone.

Exotic shapes and unexpected binding

Theoretical work helps explain why 188At decays this way. Scientists expanded a non‑adiabatic quasiparticle model to include very heavy nuclei.

The model indicates the proton emerges from a nucleus that is strongly prolate, or elongated in shape.

Such low‑angular‑momentum proton states also highlight the Thomas‑Ehrman shift, where the valence proton sits farther from the nucleus and is less repelled by the positive charge.

Until now, this shift hadn’t been seen in heavy nuclei. But 188At shows its single‑proton separation energy deviates markedly from the expected trend. This is likely the first evidence of the Thomas‑Ehrman effect at high atomic mass.

Why this matters

Finding 188At expands our understanding of nuclear stability at the edge of the proton drip line – the point where adding more protons forces immediate decay. Each new case tests nuclear models and the limits of matter.

“Isotope discoveries are rare worldwide, and this is the second time I have had the opportunity to be part of making history… it feels great to do research that improves understanding of the limits of matter and the structure of atomic nuclei,” said Kokkonen.

“The nucleus was produced in a fusion‑evaporation reaction by irradiating natural silver target with 84Sr ion beam. The new isotope was identified using the detector setup of the RITU recoil separator,” said Kalle Auranen, a research fellow at Jyväskylä.

Shaping nuclear models

Discoveries like 188At aren’t just milestones, they help reshape how scientists model the nucleus itself.

The behavior of the proton in this isotope required updates to the non-adiabatic quasiparticle model, expanding its ability to handle extreme deformation and odd-odd nuclei with mixed spin states.

These findings also offer rare insights into proton-neutron interactions near the proton drip line. The measured separation energy and deformation point to trends that theory didn’t fully anticipate, helping refine predictive tools for other rare isotopes and future discoveries.

Uses in medicine and space science

Understanding how unstable isotopes behave helps researchers refine the models used in nuclear medicine, where radioactive isotopes are engineered for targeted cancer therapies and diagnostics.

While 188At itself may not be used clinically, the insights from its structure and decay paths contribute to the general toolkit for predicting isotope behavior.

These findings also support models in astrophysics, particularly those describing how heavy elements are created in stellar explosions.

Proton-rich nuclei near the drip line can influence rapid proton capture processes (rp-processes), which help shape the periodic table in environments like supernovae and neutron star collisions.

What’s next for proton-nucleus research

This work is one part of Kokkonen’s doctoral research, building on her master’s discovery of astatine‑190 in 2023. The team will continue measuring 188At decays to refine its energy and lifetime.

The researchers also hope to produce astatine‑189, which might also emit protons. Discovering it would probe further into the heaviest proton‑emitter region.

Future findings could reshape nuclear theories, especially regarding proton‑rich extremes.

The study is published in the journal Nature Communications.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–