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New UMass Lowell-led study could change what scientists know about nuclear theory

This equipment, known as a Segmented Germanium Array (SeGA), allowed researchers led by UMass Lowell’s Andrew Rogers, assistant professor of physics, to see inside the nucleus of atoms in order to study them. The machine is located inside the National Superconducting Cyclotron Laboratory in East Lansing, Michigan.
This equipment, known as a Segmented Germanium Array (SeGA), allowed researchers led by UMass Lowell’s Andrew Rogers, assistant professor of physics, to see inside the nucleus of atoms in order to study them. The machine is located inside the National Superconducting Cyclotron Laboratory in East Lansing, Michigan.Andrew Rogers for UMass Lowell (custom credit)/Andrew Rogers for UMass Lowell

A new study from a team led by University of Massachusetts Lowell nuclear physicists could change the way scientists understand the universe and nuclear theory.

The study was published in the journal Nature on April 1 and showed that the minuscule cores of atoms, known as atomic nuclei, are not as simple or symmetrical as previously thought.

The research was conducted at the Michigan State University National Superconducting Cyclotron Laboratory over the course of eight days in 2017, researchers said. It was funded through a $1.2 million grant from the U.S. Department of Energy.

“Our findings are another piece in the puzzle of how the atomic nucleus works,” said lead researcher Andrew Rogers, an assistant professor of physics at UMass Lowell. “Understanding the behavior of atomic nuclei allows us to investigate everything from the evolution of stars, where many of the chemical elements are born, to nuclear reactors.”

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An atomic nucleus holds almost all of an atom’s energy and mass. The nucleus is made up of uncharged neutrons and charged protons, with the number of protons determining which element the atom is.

Researchers said they were trying to find out how X-ray bursts, which are explosions on the surface of a massive star at the end of its life, create atomic nuclei in the universe. Rogers said they hoped this would help them answer one of the biggest questions in science: How are chemical elements created?

To try to gain insight into this, researchers created the rare isotope strontium-73 by smashing apart nuclei at nearly the speed of light. Strontium-73 only lives for a fraction of a second and isn’t found on Earth naturally, but it can be found during X-ray bursts.

Isotopes are different versions of an element that have the same number of protons but a different number of neutrons. Researchers compared strontium-73 to bromine-73 because they were thought to have mirror symmetry, or an inverse number of protons and neutrons. According to current nuclear theory, these mirror isotopes should appear identical in structure.

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Researchers found this wasn’t the case. When they observed strontium-73 break down and lose its energy, it behaved extremely differently than bromine-73.

“While the breakdown of mirror symmetry was not unexpected, our results were surprising and cannot be predicted by current nuclear theory,” said UMass Lowell Research Associate Daniel Hoff, who was the lead author of the study. “Now that we have seen this phenomenon for the first time, we hope to find it again and gather more pieces of the nuclear puzzle.”

The team plans to conduct more experiments to confirm its findings and further study these isotopes.

“Much still remains to be done,” said Witold Nazarewicz, Michigan State University’s John A. Hannah Distinguished Professor of Physics and chief scientist at the Facility for Rare Isotope Beams. “I am glad that the exotic beams delivered by our facility, unique instrumentation and theoretical calculations could contribute to this magnificent work.”

In a “News and Views” item in the same issue of Nature, Bertram Blank of the Centre d’Etudes Nucléaires de Bordeaux-Gradignan, wrote, “Is this breaking of mirror symmetry a disaster for our understanding of the structure of the atomic nucleus? Not at all. Deviations from expectations challenge our knowledge of nuclear structure, and allow nuclear scientists to fine-tune their models to describe atomic nuclei.”

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“The race is on to find more cases of broken mirror symmetry in nuclear ground states,” Blank wrote.

Caroline Enos can be reached at caroline.enos@globe.com. Follow her on Twitter @CarolineEnos.