Rules for nuclear couples

Atoms are governed by nuclei and nuclei have rules of their own. Physicists like University of Tennessee, Knoxville Assistant Professor Dien Nguyen study those rules. In a just-published Nature paper, she and colleagues report on a new quantum selection process that could impact how scientists understand nuclear structure, which plays a major role in fields like medicine and energy.

How Particles Pick Partners

A nucleus is a quantum system made up of protons and neutrons (nucleons), held together by the strong nuclear force. At very short distances, those nucleons can double up momentarily into short-range correlated (SRC) pairs.

“SRC pairs play an important role in helping us understand how protons and neutrons interact at very short distances inside the nucleus,” Nguyen explained. “This nucleon-nucleon interaction is what binds protons and neutrons together and determines many properties of the nucleus.”

To find out how these partnerships form, she and the research team scattered electrons from calcium (Ca-40 and Ca-48) and iron (Fe-54) targets at the Thomas Jefferson National Accelerator Facility, detecting both scattered electrons and knocked-out protons. Nguyen said the three nuclei chosen were a special collection based on how their protons and neutrons are arranged in discrete energy levels, or “shells.”

“Calcium-40 and calcium-48 have the same number of protons, but calcium-48 has eight additional neutrons in an outer shell,” she said. “Calcium-48 and iron-54 have the same number of neutrons, but iron-54 has six additional protons in the same outer shell. This comparison allowed us to separate the effect of adding neutrons from the effect of adding protons in a specific shell.”

The six additional protons in iron translated into some surprising results.

Nguyen explained that adding eight neutrons (40 percent) in calcium-48 resulted in only about 10 percent more SRC proton pairs over calcium-40. Adding six protons (30 percent) in iron-54, however, led to about 50 percent more SRC proton pairing compared to calcium-48. She said the results show that pair formation isn’t controlled simply by the total number of protons or neutrons.

“Instead, it depends strongly on the quantum orbitals that the protons and neutrons occupy,” she said. “In other words, nucleons are much more likely to form SRC pairs when their quantum states are favorable.”

She added that the results point to a new kind of quantum selection rule governing how nucleons can pair at short distances.

“It was not known before and could have an important impact on how we understand nuclear structure, especially how protons and neutrons interact and organize themselves inside the nucleus,” Nguyen explained.

Meaning in Every Achievement

Nguyen is the first author on the Nature paper and worked with the CaFe team (as they call themselves) to lead the project. They took the first data in late August 2022, when she was a Nathan Isgur Fellow at Jefferson Lab and a week away from delivering her daughter. A good friend and fellow CaFe member had a two-month-old baby at the time, and Nguyen said the two young moms made a pact.

“We said to each other ‘Let’s get the first paper out before our kids turn four,’” she said. “And we did it: we kept our promise with our babies.”

(For Nguyen there was an additional bonus: her husband promised her an upscale “CaFe” machine for morning coffee if they published in Nature, which he’s made good on.)

She is quick to acknowledge the research success is shared among the entire CaFe team, Jefferson Lab’s Hall C collaboration, including physics graduate and paper co-author Casey Morean (PhD, ’23).

“I am truly grateful to everyone who helped bring the project to this point,” Nguyen said.

Publishing in Nature is the latest in a string of successes she’s earned since joining the physics faculty in 2024. In 2025 she won a U.S. Department of Energy Early Career Award to support her research on how spin shapes the fundamental structure of matter. In 2026 she was recognized with the UT College of Arts and Sciences Excellence in Research and Creative Achievement Award (Early Career). She’s also deeply invested in helping her students succeed and has been voted the department’s Research Advisor of the Year by both the graduate students (2025) and the undergraduate physics majors (2026).

“For me, professional life is my passion and it always goes along with my personal life and with my loved ones,” she said. “So (the) personal part is meaningful to me (in) every achievement.”