Inside the Nucleus: The Molecular Method Changing How We Study Atoms

Dr. Ronald Fernando Garcia Ruiz, Associate Professor of Physics at the Massachusetts Institute of Technology, and his team have developed a new way to look inside an atom’s nucleus using the atom’s own electrons. Their study, demonstrates a molecule-based method that allows researchers to probe nuclear interiors on a tabletop setup; without the need for massive particle accelerators.

Wilkins, S. G., Udrescu, S. M., Athanasakis-Kaklamanakis, M., Garcia Ruiz, R. F., Au, M., Belošević, I., Berger, R., Bissell, M. L., Breier, A. A., Brinson, A. J., Chrysalidis, K., Cocolios, T. E., de Groote, R. P., Dorne, A., Flanagan, K. T., Franchoo, S., Gaul, K., Geldhof, S., Giesen, T. F., … Zülch, C. (2025). Observation of the distribution of nuclear magnetization in a molecule. Science, 390(6771), 386–389. https://doi.org/10.1126/science.adm7717

For decades, physicists have relied on high-energy facilities spanning kilometers to explore atomic nuclei. These accelerators fire beams of particles into atoms, breaking them apart and revealing how protons and neutrons are arranged inside. While such experiments have been vital in building our understanding of matter, they are costly, time-consuming, and limited in the range of isotopes they can handle; especially those that exist only briefly before decaying. The MIT team’s method offers a smaller-scale, yet highly precise, alternative.

Instead of relying on collisions, Garcia Ruiz’s group turned to molecules, specifically radium monofluoride (RaF). In this molecule, a radium atom is paired with a fluoride atom, creating an environment where the radium’s electrons are strongly confined. The intense internal electric fields within this molecular bond cause some of these electrons to briefly enter the nucleus before returning to their orbits. By measuring subtle changes in their energy, the researchers could infer details about the magnetic structure within the nucleus itself.

Dr. Ronald Fernando Garcia Ruiz, Associate Professor of Physics at the Massachusetts Institute of Technology, stated,

“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.”

The experiment begins with the creation of radium monofluoride molecules. Radium is a naturally radioactive element, so the process involves careful handling of small quantities produced at specialized facilities. Once formed, the molecules are cooled and trapped in vacuum chambers to minimize interference from outside particles. Using precisely tuned lasers, the researchers then measure how the molecules absorb and emit light; a process known as laser spectroscopy.

In traditional nuclear physics, probing the interior of a nucleus typically involves high-speed particle collisions. In contrast, Garcia Ruiz and his collaborators use the natural motion of electrons. When an electron briefly penetrates the nucleus, it interacts directly with the protons and neutrons inside, carrying away information about the forces it encountered. These interactions are extraordinarily delicate, producing energy shifts that are minuscule; roughly one millionth of the energy of the laser photons used to excite the molecules.

Despite their small size, these shifts are measurable. By comparing them to theoretical predictions, the researchers could confirm that electrons had indeed entered the nuclear region. It was a subtle but groundbreaking observation: direct evidence of electrons “sampling” the interior of the nucleus from within a molecule.

Garcia Ruiz explains that this is akin to measuring the inside of a battery rather than its external field. While previous experiments could describe how electrons behave outside the nucleus, this method makes it possible to sense what occurs deep inside the atomic core.

A central finding of the research was the observation of the Bohr–Weisskopf effect. This phenomenon arises from the way nuclear magnetism is distributed within the nucleus. Protons and neutrons act like tiny magnets, and their arrangement determines how they interact with nearby electrons. If the magnetic field were generated by a single point, the interaction would follow one pattern. But because the nucleus has a finite size and its magnetization is spread out, the resulting effect deviates slightly from this idealized case.

By carefully measuring these deviations, the team was able to map the magnetic distribution of the radium nucleus. This represents a new level of precision in understanding how nuclear magnetism manifests. In radium monofluoride, the internal molecular fields amplify these effects, making them detectable even in such small quantities.

The researchers found that the observed differences between theory and experiment could only be explained by accounting for the finite spread of nuclear magnetization. The agreement between experimental results and advanced theoretical models strengthens confidence in the use of molecular systems to study nuclear properties.

Radium is not just any atom; it is an especially intriguing one for fundamental physics. Unlike most atomic nuclei, which are roughly spherical, the radium nucleus has an asymmetric, pear-like shape. This unique geometry enhances certain effects that are sensitive to violations of fundamental symmetries in nature, such as those related to time reversal and matter-antimatter imbalance.

One of the great unsolved mysteries in physics is why the universe is composed primarily of matter rather than equal parts matter and antimatter. According to the Standard Model, the laws of physics treat both almost symmetrically, yet our observable universe tells a different story. It is believed that some hidden mechanism; possibly a violation of fundamental symmetry within nuclei; may help explain this discrepancy.

The pear-shaped nucleus of radium is a particularly sensitive probe for such violations. Because its charge and mass are unevenly distributed, the nucleus can amplify small symmetry-breaking effects that might otherwise go unnoticed. By studying the magnetic and structural properties of radium nuclei through molecules like radium monofluoride, scientists hope to uncover hints of new physics beyond the Standard Model.

The practical side of this experiment is as striking as its scientific implications. The setup occupies a laboratory bench rather than a sprawling accelerator facility. This shift opens the door for more laboratories worldwide to perform precision nuclear measurements without needing vast infrastructure.

However, the approach is not without its challenges. Producing radium monofluoride molecules is difficult because radium is scarce and radioactive. Each molecule exists for a limited time before decaying, and the quantities available for study are minuscule. Detecting the faint signals from these molecules demands extremely sensitive instruments capable of distinguishing minute energy changes amid background noise.

Despite these hurdles, the team’s success demonstrates that it is possible to gather meaningful nuclear data from molecules. The next steps involve cooling the molecules even further and aligning the pear-shaped nuclei so that their orientations are controlled. Doing so would allow for even more precise measurements of nuclear properties and may make it possible to map the internal forces within the nucleus in greater detail.

This method does more than reveal the structure of one atom’s nucleus. It establishes a framework for exploring nuclear phenomena in a wide range of elements, particularly those that are short-lived or difficult to produce in large quantities. It also offers a new way to test theories about fundamental symmetries that shape the universe.

If refined, this technique could become a cornerstone of experimental nuclear physics, bridging the gap between atomic physics, quantum chemistry, and particle physics. Researchers could use similar molecule-based systems to explore other isotopes, gaining insights that were once accessible only through large-scale accelerator experiments.

The broader scientific impact lies in its accessibility. A tabletop technique for nuclear probing could democratize this branch of research, allowing smaller institutions to contribute to the study of nuclear structure and symmetry violation. Over time, this might accelerate discoveries about the nature of matter itself.

As Garcia Ruiz notes, the team’s achievement marks a turning point: “We now have proof that we can sample inside the nucleus. It’s like finally being able to look inside a closed box that we’ve only been able to observe from the outside for decades.”