Lukosi’s Years-Long Search for Rare Isotope Pays Off with Spotlight in Physics
We may need to take a few tens of millions of candles off Earth’s next birthday cake.
Associate Head for Research and Facilities Eric Lukosi has spent the last few years helping to resolve a mystery of modern nuclear physics: the abundance of an invisible fraction of decaying potassium atoms.
“This isotope contributes significant background noise to rare search events, like the search for dark matter,” Lukosi said. “It is also part of a geologic dating method for early Earth history.”
Potassium (elemental symbol K) is the eighth most common metal in Earth’s crust. Most potassium atoms are stable, but 0.012% of them are a radioactive isotope known as potassium-40 or 40K, which has a half-life of 1.25 billion years. About 11.6% of 40K atoms decay into an argon isotope called 40Ar.
Rocks solidifying in Earth’s crust naturally trap some 40K atoms, some of which slowly decay to 40Ar. Since the 1950s, geologists have used the decay rate and the relative abundance of the two elements in samples to estimate the ages of ancient rocks.
Unfortunately, while around 99% of the 40K atoms that decay to 40Ar release a gamma ray—a useful signal that helps quantify the rate of that decay process—a small number of atoms convert to 40Ar more quietly, releasing only low-energy X-rays and electrons that are hard to quantify. Over the years, various models have assumed that this quiet conversion accounted for between 2% and—since it had never actually been observed—zero percent of 40K’s decay path to 40Ar.
The Potassium Decay Collaboration (KDK Collaboration) is an international group of eleven research institutions dedicated to resolving the rate at which this rare decay occurs. It was founded in 2016 in response to concerns over the dark matter claims from DAMA, a long-running observatory experiment.
The debunking of DAMA’s results in 2021 only underlined the importance of removing potassium decay from the background noise—rather than interpreting it as the presence of dark matter particles—and the Collective’s work continued.
“When I learned about the KDK Collaboration’s research, I realized that they could get more statistically significant data using a potassium-containing scintillator,” Lukosi said, referring to a material that absorbs energetic radiation like X-rays and converts it to visible photons. “UT’s nuclear engineering and physics departments are well versed in developing radiation detection systems and applying them in fundamental studies, so I offered them our expertise.”
Lukosi worked with Research Professor Chuck Melcher, the director of the Scintillation Materials Research Center, to create a customized scintillator for the KDK Collaboration. Lukosi and his students then teamed up with the Collaboration to conduct experiments and perform data analysis.
“This kind of international project lets us lend our expertise while giving our students a great, hands-on learning opportunity,” Lukosi said.
After years of searching, the researchers not only made the world’s first observation of that low-signal decay process—they discovered that the quiet conversion accounts for 1% of the 40K destined to become 40Ar.
“These results may affect the predicted ages of ancient Earth by up to one percent, up to tens of millions of years,” Lukosi said.
On July 31, the KDK Collaboration published their results in two journals of the American Physical Society (APS), a global nonprofit organization for physics professionals.
In recognition of the research’s impact, the editors of the APS shared the KDK Collaboration’s papers on social media, published free-to-read versions online, and highlighted them with an expert-written commentary article—an honor bestowed on only about 100 of the 20,000 papers published by APS every year.
“It is a pleasure knowing that our years of effort were recognized by the scientific community,” said Lukosi. “The commentary article emphasizes our result’s potential impact on multiple studies, including its implications in refining chronologies of the Earth and solar system.”
Contact
Izzie Gall (865-974-7203, egall4@utk.edu)