Single Molecule Becomes a Microscopic Particle Collider to Probe Radium's Nucleus

Physicists use radium monofluoride to probe the heart of an atom

Physicists traditionally use large electron accelerators to blast nuclei and study their internal structure. In a new study published in Science, researchers demonstrate a radically smaller approach: using electrons bound to a radium atom inside a diatomic molecule as local probes, effectively turning one molecule into a microscopic particle collider.

The team formed radium monofluoride (RaF) molecules and exploited the molecule's internal electric fields to coax some of the radium atom's electrons into brief excursions inside the nucleus. Precision laser spectroscopy of the molecules revealed tiny but measurable shifts in electron energy levels that are consistent with such intranuclear interactions.

Why this matters

Those subtle energy deviations open a new window on a nucleus's magnetic distribution — how protons and neutrons arrange and generate magnetic effects. The technique offers a promising route to search for violations of fundamental symmetries at the nuclear level, which could help explain why the observable Universe is dominated by matter rather than antimatter.

"Our results lay the groundwork for subsequent studies aiming to measure violations of fundamental symmetries at the nuclear level," said MIT physicist Ronald Fernando Garcia Ruiz, a co-author. "This could provide answers to some of the most pressing questions in modern physics."

Radium is a particularly attractive target because many of its isotopes are predicted to have asymmetric, "pear-shaped" nuclei. That asymmetry can amplify symmetry-violating effects, making them easier to detect. But practical challenges remain: radium is radioactive, its isotopes are short-lived, and RaF molecules can currently be produced only in tiny quantities, requiring exceedingly sensitive measurement techniques.

To perform the experiment, researchers confined and cooled RaF molecules and used lasers to measure electron energy levels with high precision. The mismatch between observed energies and values predicted assuming only extranuclear interactions indicated an extra contribution from electrons interacting inside the nucleus.

"When we measured these electron energies very precisely, it didn't quite add up to what we expected assuming they interacted only outside the nucleus," explained lead author Shane Wilkins (formerly at MIT, now at Michigan State University). "That told us the difference must be due to electron interactions inside the nucleus."

Co-author Silviu-Marian Udrescu (Johns Hopkins University, contributing while a graduate student at MIT) added that embedding radium in a molecule amplifies the internal electric fields experienced by the electrons by orders of magnitude compared with laboratory-applied fields, making the molecule act like a tiny, highly focused collider.

The researchers describe this work as an early demonstration. They plan to refine the technique and apply it to further studies of radium nuclei and searches for symmetry violations that could illuminate the matter–antimatter asymmetry problem.

Study: Published in Science.