# Record proton‑emitting nucleus found: 188‑astatine is ‘watermelon‑shaped’ and may reshape nuclear models

*A heavy nucleus that spits out protons—and looks like a watermelon—just bent the rules.*

A team at the University of Jyväskylä, Finland, has measured the heaviest nucleus known to decay by proton emission: 188‑astatine. Reported on 16 August 2025, it appears strongly prolate—“watermelon‑shaped”—and its behaviour hints at an interaction not seen before in heavy nuclei. The work, published in Nature Communications, could sharpen models of nuclear stability.

## What was discovered
Researchers identified 188‑astatine (85 protons, 103 neutrons) as a new isotope and a record proton emitter. It is also the lightest known isotope of astatine so far, placing it at the extreme edge of the element’s family. As Doctoral Researcher Henna Kokkonen put it, “Proton emission is a rare form of radioactive decay,” underscoring the challenge of spotting it.

## How they made and spotted it
The team produced the isotope by firing a strontium‑84 ion beam at a natural silver target in a fusion–evaporation reaction, then used the RITU recoil separator to identify the events. “The nucleus was produced in a fusion‑evaporation reaction,” said Academy Research Fellow Kalle Auranen, noting the precision needed because lifetimes are short and production rates are tiny.

## Why the shape matters
Analysis indicates a strongly prolate—‘watermelon‑shaped’—nucleus. That geometry lines up with a trend change in the binding energy of the valence proton, a sensitive clue to the forces at play inside heavy nuclei. The shape, in other words, influences whether the outermost proton is held firmly or slips away.

## What it could change in physics
The data point to an interaction unprecedented in heavy nuclei and provide a new benchmark for nuclear models. Better models refine the map of nuclides, including where the proton ‘drip line’ sits, and guide future searches for ultra‑short‑lived isotopes.

### The bigger picture
This is the first record of its kind in more than three decades; the previous comparable breakthrough dates to 1996. The study forms part of Kokkonen’s doctoral work and continues earlier research that uncovered another exotic astatine isotope.

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Strong takeaway: A record‑breaking, watermelon‑shaped nucleus is nudging theorists to rethink how protons are bound in the heaviest systems.

CTA: Want the full details? Read the original release and paper via ScienceDaily: [Scientists stunned by record‑breaking, watermelon‑shaped nucleus](https://www.sciencedaily.com/releases/2025/08/250816113512.htm).