A long-held belief about one of the universe’s heaviest stable isotopes has been upended by new findings that challenge past beliefs that its atomic nucleus is perfectly spherical.
The discovery, made by an international collaboration led by researchers with the University of Surrey’s Nuclear Physics Group, is now causing scientists to rethink some of their longstanding assumptions about the heaviest elements in our universe and how they are formed.
Lead is the element with the heaviest, most highly stable isotope, Lead-208 (²⁰⁸Pb). This isotope possesses a doubly magic nucleus—essentially an atomic nucleus that has a “magic” number of both protons and neutrons—meaning it is extremely stable.
In their research, the international team reexamined Lead-208’s shape with a new experimental probe, which revealed something unexpected: rather than the perfectly spherical shape the team expected to see, the isotope’s nucleus is more akin to an elongated oval shape, not unlike a football.
“We were able to combine four separate measurements using the world’s most sensitive experimental equipment for this type of study, which is what allowed us to make this challenging observation,” said Dr. Jack Henderson, the study’s principal investigator and a researcher from the University of Surrey’s School of Mathematics and Physics.
“What we saw surprised us,” Henderson said, “demonstrating conclusively that lead-208 is not spherical, as one might naively assume.”
“The findings directly challenge results from our colleagues in nuclear theory, presenting an exciting avenue for future research,” Henderson said.
The team’s observations were made using a state-of-the-art new device, the GRETINA gamma-ray spectrometer, at Argonne National Laboratory in Illinois. The device, specially designed to study the properties and structural appearance of atomic nuclei, allowed Henderson and his colleagues to bombard lead atoms with particle beams moving at exceptional speeds, powerful enough to move at 10% of the speed of light.
Such speeds, which would allow a particle to make a full revolution around the planet in a single second, allowed the researchers an unprecedented opportunity to create gamma-ray “fingerprints” revealing the properties of energized lead-208 nuclei.
The atomic nuclei’s actual shape was more easily revealed while the isotope was in this excited state, although it didn’t match what researchers expected to find: due to the unexpected ellipsoid shape of the nucleus that was revealed, the experiments suggest that nuclear structure is more complex and mysterious than previously known, leaving physicists to reconsider existing models they use to describe atomic nuclei.
“These highly sensitive experiments have shed new light on something we thought we understood very well, presenting us with the new challenge of understanding the reasons why,” said Professor Paul Stevenson, lead theorist on the study from the University of Surrey.
Stevenson says one possibility is that the vibration of the lead isotope’s nucleus may be less regular under such conditions when it becomes highly excited, as the team’s research revealed.
“We are now refining our theories further to determine whether these ideas are right,” Stevenson says.
Fundamentally, the team’s surprise discovery presents a wrinkle in current thinking about nuclear physics, potentially one significant enough to lead to additional discoveries involving nuclear stability, which may have applications in fields that include quantum mechanics and astrophysics.
The new paper detailing the team’s findings, “Deformation and Collectivity in Doubly Magic 208Pb,” appeared in Physical Review Letters on February 14, 2025.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.