How One Molecule Became a Window Into the Atomic Nucleus

Physicists have turned a radioactive molecule into a miniature particle collider, no bigger than a tabletop. The trick lets them peek inside an atom’s nucleus without building a kilometers-long accelerator, opening a door to one of cosmology’s most stubborn puzzles: why the universe contains so much more matter than antimatter.

The research team at MIT paired radium atoms with fluoride to create radium monofluoride molecules, then used lasers to track the energies of electrons zipping around inside. What they found was unexpected. The electrons carried a subtle energy signature, a shift of just one millionth the energy of the laser photon used to measure them. That tiny difference told a revealing story: the electrons had briefly slipped inside the radium nucleus itself, interacted with the protons and neutrons packed there, then emerged with evidence of what they had encountered.

“When you put this radioactive atom inside of a molecule, the internal electric field that its electrons experience is orders of magnitude larger compared to the fields we can produce and apply in a lab,” explains Silviu-Marian Udrescu, a study co-author. “In a way, the molecule acts like a giant particle collider and gives us a better chance to probe the radium’s nucleus.”

A Pear-Shaped Clue to Cosmic Imbalance

Radium nuclei are unusual. Unlike most atomic nuclei, which form tidy spheres, radium’s nucleus bulges asymmetrically, like a pear. This lopsided shape may amplify subtle violations of fundamental symmetries, the kind that could explain why antimatter has all but vanished from the observable universe. According to the Standard Model, matter and antimatter should have appeared in nearly equal amounts after the Big Bang. Yet here we are, in a universe made almost entirely of matter, with antimatter appearing only in fleeting traces.

The research team used molecules as natural amplifiers. By trapping radium atoms inside radium monofluoride, they squeezed the atom’s electrons into tighter orbits, increasing the odds that electrons would penetrate the nucleus during their high-speed travels. The molecules were cooled and sent through vacuum chambers, where lasers measured the electron energies with extreme precision.

The results, published today in Science, show that the electron energies did not quite match predictions based on interactions outside the nucleus alone. The discrepancy pointed to something happening inside, a brief moment when electrons sampled the nuclear interior and carried information back out.

“There are many experiments measuring interactions between nuclei and electrons outside the nucleus, and we know what those interactions look like. When we went to measure these electron energies very precisely, it didn’t quite add up to what we expected assuming they interacted only outside of the nucleus. That told us the difference must be due to electron interactions inside the nucleus.”

Sampling a Battery from the Inside

Ronald Fernando Garcia Ruiz, who led the study, compares the achievement to measuring the electric field inside a battery rather than around it. Traditional methods for probing nuclei rely on massive facilities that accelerate electron beams to collision speeds. This molecule-based approach offers something different: a way to investigate nuclear structure without the infrastructure.

Radium’s radioactivity adds layers of difficulty. The atoms decay quickly, and radium monofluoride molecules can only be produced in tiny quantities. The team needed exceptionally sensitive techniques just to detect the molecules, let alone measure their electron energies with the precision required.

What they measured was the nuclear magnetic distribution, a property that reflects how protons and neutrons arrange themselves inside the nucleus. Each proton and neutron behaves like a small magnet, and their alignments depend on the nucleus’s internal structure. Mapping this distribution in radium could reveal whether the nucleus violates fundamental symmetries in ways that the Standard Model does not predict.

The next step involves cooling the molecules further and controlling the orientations of the pear-shaped nuclei. With that level of control, the team could map the nuclear contents in detail and search for symmetry violations that might explain the matter-antimatter imbalance.

“Radium-containing molecules are predicted to be exceptionally sensitive systems in which to search for violations of the fundamental symmetries of nature. We now have a way to carry out that search.”

The work was performed at CERN’s Collinear Resonance Ionization Spectroscopy Experiment in Switzerland, with support from the U.S. Department of Energy. Shane Wilkins, the study’s lead author and a former MIT postdoc, notes that the technique’s sensitivity makes it possible to study atoms that are both rare and short-lived. The method could extend to other heavy, radioactive nuclei, offering new routes into subatomic physics without the need for billion-dollar accelerators.

Science: 10.1126/science.adm7717