Physicists with the Potassium Decay (KDK) Collaboration have made the first direct observations of a very rare but critical decay path of potassium-40 to argon-40. Their results may improve current understanding of physics processes and increase the precision of geological dating.
Decay scheme of potassium-40. Image credit: Stukel et al., doi: 10.1103/PhysRevLett.131.052503.
Potassium-40 is a widespread, naturally occurring isotope whose radioactivity impacts estimated geological ages spanning billions of years, nuclear structure theory, and subatomic rare-event searches — including those for dark matter and neutrinoless double-beta decay.
The decays of this long-lived isotope must be precisely known for its use as a geochronometer, and to account for its presence in low-background experiments.
There are several known decay modes for potassium-40, but a predicted electron-capture decay directly to the ground state of argon-40 has never been observed, until now.
“Some nuclei of certain elements decay radioactively into nuclei of different elements. These decays can be useful or annoying depending on the context,” the KDK physicists said.
“This is especially true for potassium-40, an isotope that usually decays to calcium-40, but about 10% of the time decays to argon-40.”
“This decay path involves a process called electron capture which provides information about nuclear structure.”
“It can also determine the age of geological objects on billion-year time scales, because potassium-40 has a very long half-life.”
“The long half-life makes finding additional ways potassium-40 decays challenging.”
In their new research, the reseachers measured a rare decay branch of potassium-40 at the Holifield Radioactive Ion Beam Facility of Oak Ridge National Laboratory.
“Quantifying the decay rates of potassium-40 and its decay branches is difficult because measurements must be made of the parent nucleus and a sufficient number of rare offspring,” they said.
“We studied a subset of potassium-40 that decays to argon-40 by electron capture, which makes up about 10% of all potassium-40 decays.”
“While most potassium-40 electron-capture decays emit a characteristic gamma ray that is a background in most experiments, a small subset of these decays happens without any gamma-ray emission.”
“This happens when potassium-40 captures an electron that goes directly to the argon-40 ground state.”
“We made the first direct measurement of this decay. The achievement indicates other decay rates may also need reassessment.”
“The rare decay branch that we identified and measured provides unique experimental evidence of so-called forbidden beta decays, affects nuclear structure predictions, and removes a longstanding uncertainty for potassium-based geological and solar system age estimates.”
“The findings also improve the assessment of the background present in experiments searching for new physics beyond the Standard Model.”
The results appear in two papers (paper #1 and paper #2) in the journal Physical Review Letters and the journal Physical Review C.
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M. Stukel et al. (KDK Collaboration). 2024. Rare 40K Decay with Implications for Fundamental Physics and Geochronology. Phys. Rev. Lett 131 (5): 052503; doi: 10.1103/PhysRevLett.131.052503
L. Hariasz et al. (KDK Collaboration). 2024. Evidence for ground-state electron capture of 40K. Phys. Rev. C 108 (1): 014327; doi: 10.1103/PhysRevC.108.014327
