Radium Monofluoride Offers a New Way to Probe the Radium Nucleus

Investigating the inner structure of an atomic nucleus has conventionally meant using advanced particle colliders to fire electrons at nuclei and split them apart. Such machines commonly depend on vast installations, sometimes extending for kilometres, to propel electrons to extreme speeds while probing nuclear secrets.

A new study proposes a far simpler approach on a much smaller scale. Rather than relying on all that elaborate equipment, the researchers extracted information from within nuclei by using an atom’s own electrons as “messengers” in a diatomic molecule.

Radium monofluoride as a microscopic collider

The team combined a radium atom with a fluorine atom to make radium monofluoride. By exploiting the molecule’s internal conditions, they effectively built a microscopic collider: electrons from the radium atom could momentarily penetrate its nucleus.

This approach allowed the scientists to track the energies of electrons in the molecule with precision, revealing a slight but detectable energy change. The result indicated that electrons were briefly entering the radium nucleus and interacting with what it contains.

That behaviour may offer a new means of measuring a nucleus’s magnetic distribution - the way its protons and neutrons are arranged and how that structure shapes its magnetic characteristics.

The researchers stress that their work is an initial advance, but they intend to apply the method to examine the radium nucleus in a new way. Such knowledge could assist with major unresolved questions in physics, including why the Universe appears to contain vastly more matter than antimatter.

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

Why the radium nucleus matters

Existing models indicate that the infant Universe should have contained approximately equal quantities of matter and antimatter. Yet antimatter is strikingly scarce: the present-day Universe is composed overwhelmingly of matter, seemingly contradicting the anticipated symmetry between them.

Researchers suspect that particular atomic nuclei may hold the answer, since their interiors could reveal why antimatter counterparts are so uncommon.

The scientists identify radium as an especially promising option because its nucleus has a pear-like form. While most atomic nuclei are spherical, radium’s uneven structure could make violations of fundamental symmetries easier to detect.

“The radium nucleus is predicted to be an amplifier of this symmetry breaking, because its nucleus is asymmetric in charge and mass, which is quite unusual,” Garcia Ruiz says.

Even so, the challenge remains formidable.

“Radium is naturally radioactive, with a short lifetime and we can currently only produce radium monofluoride molecules in tiny quantities,” says lead author and physicist Shane Wilkins, a former MIT postdoc now at Michigan State University. “We therefore need incredibly sensitive techniques to be able to measure them.”

The essential step is placing a radium atom within a molecule, where the motion of its electrons can be contained and enhanced, explains co-author Silviu-Marian Udrescu, a physicist at Johns Hopkins University who worked on the study while a graduate student at MIT.

“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,” Udrescu says. “In a way, the molecule acts like a giant particle collider and gives us a better chance to probe the radium’s nucleus.”

Detecting electrons inside the nucleus

In radium monofluoride, the radium atom’s electrons were restricted in a manner that increased the likelihood of their reaching the nucleus. The team trapped and cooled the molecules before using lasers to measure the energies of their electrons.

Small yet meaningful shifts in those measurements suggested that the electrons had travelled into the nucleus.

“There are many experiments measuring interactions between nuclei and electrons outside the nucleus, and we know what those interactions look like,” Wilkins says.

“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,” he adds. “That told us the difference must be due to electron interactions inside the nucleus.”

According to the researchers, the finding could transform the study of atomic nuclei. Subatomic particles are famously difficult to investigate, however, and do not readily reveal their secrets.

“We now have proof that we can sample inside the nucleus. It's like being able to measure a battery's electric field. People can measure its field outside, but to measure inside the battery is far more challenging. And that's what we can do now,” Garcia Ruiz says.

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

The study was published in Science.