Evidence for ground-state electron capture of K 40

Journal Article · Mon Jul 31 00:00:00 EDT 2023 · Physical Review. C

DOI:https://doi.org/10.1103/physrevc.108.014327· OSTI ID:1997753

Hariasz, L. ^[1]; Stukel, M. ^[1]; Di Stefano, P. F. ^[1]; Rasco, B. C. ^[2]; Rykaczewski, K. P. ^[2]; Brewer, N. T. ^[3]; Stracener, D. W. ^[2]; Liu, Y. ^[2]; Gai, Z. ^[4]; Rouleau, C. ^[4]; Carter, J. ^[5]; Kostensalo, J. ^[6]; Suhonen, J. ^[7]; Davis, H. ^[8]; Lukosi, E. D. ^[8]; Goetz, K. C. ^[2]; Grzywacz, R. K. ^[9]; Mancuso, M. ^[10]; Petricca, F. ^[10]; Fijałkowska, A. ^[11] ; Wolińska-Cichocka, M. ^[12]; Ninkovic, J. ^[13]; Lechner, P. ^[13]; Ickert, R. B. ^[14]; Morgan, L. E. ^[15]; Renne, P. R. ^[16]; Yavin, I. ^[17]

---

1. Queen's University, Kingston, ON (Canada)
2. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
3. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS)
4. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States). Center for Nanophase Materials Sciences (CNMS)
5. Berkeley Geochronology Center, Berkeley, CA (United States)
6. Natural Resources Institute Finland, Joensuu (Finland)
7. University of Jyväskylä (Finland)
8. Univ. of Tennessee, Knoxville, TN (United States)
9. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Univ. of Tennessee, Knoxville, TN (United States)
10. Max Planck Inst. fuer Physik, Munich (Germany)
11. Univ. of Warsaw (Poland)
12. Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States); Univ. of Warsaw (Poland)
13. MPG Semiconductor Laboratory, Munich (Germany)
14. Purdue Univ., West Lafayette, IN (United States)
15. US Geological Survey, Denver, CO (United States)
16. Berkeley Geochronology Center, Berkeley, CA (United States); Univ. of California, Berkeley, CA (United States)
17. KDK Collaboration, et al.

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. The existence of this decay mode impacts several fields, while theoretical predictions span an order of magnitude. Here we report on the first, successful observation of this rare decay mode, obtained by the KDK (potassium decay) Collaboration using a novel combination of a low-threshold x-ray detector surrounded by a tonne-scale, high-efficiency $\gamma$ -ray tagger at Oak Ridge National Laboratory. A blinded analysis reveals a distinctly nonzero ratio of intensities of ground-state electron-captures ( $I_{{\mathrm{EC}}^0}$ ) over excited-state ones ( $I_{{\text{EC}}^{*}}$ ) of $I_{{\mathrm{EC}}^0}/I_{{\text{EC}}^{*}}=0.0095\stackrel{\text{stat}}{\pm }0.0022\stackrel{\text{sys}}{\pm }0.0010$ (68% CL), with the null hypothesis rejected at $4\sigma$ [Stukel et al., Phys. Rev. Lett. 131, 052503 (2023)]. In terms of branching ratio, this unambiguous signal yields $I_{{\mathrm{EC}}^0}=0.098\%\stackrel{\text{stat}}{\pm }0.023\%\stackrel{\text{sys}}{\pm }0.010\%$ , roughly half of the commonly used prediction. This first observation of a third-forbidden unique electron capture improves our understanding of low-energy backgrounds in dark-matter searches and has implications for nuclear-structure calculations. For example, a shell-model based theoretical estimate for the neutrinoless double-beta decay half-life of calcium-48 is increased by a factor of $7_{-2}^{+3}$ . Our nonzero measurement shifts geochronological ages by up to a percent; implications are illustrated for Earth and solar system chronologies.

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: National Science Foundation (NSF); US Department of Homeland Security; USDOE; USDOE Office of Science (SC)
• Contributing Organization: KDK Collaboration
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 1997753
• Alternate ID(s): OSTI ID: 1993097
• Journal Information: Physical Review. C, Journal Name: Physical Review. C Journal Issue: 1 Vol. 108; ISSN 2469-9985
• Publisher: American Physical Society (APS)
• Country of Publication: United States
• Language: English

Related Subjects

73 NUCLEAR PHYSICS AND RADIATION PHYSICS
beta decay
geophysics
neutrinoless double beta decay
nuclear structure & decays
particle dark matter
radiation detectors
shell model