학술논문
Evidence for ground-state electron capture of $^{40}$K
Document Type
Working Paper
Author
Hariasz, L.; Stukel, M.; Di Stefano, P. C. F.; Rasco, B. C.; Rykaczewski, K. P.; Brewer, N. T.; Stracener, D. W.; Liu, Y.; Gai, Z.; Rouleau, C.; Carter, J.; Kostensalo, J.; Suhonen, J.; Davis, H.; Lukosi, E. D.; Goetz, K. C.; Grzywacz, R. K.; Mancuso, M.; Petricca, F.; Fijałkowska, A.; Wolińska-Cichocka, M.; Ninkovic, J.; Lechner, P.; Ickert, R. B.; Morgan, L. E.; Renne, P. R.; Yavin, I.
Source
Physical Review C, 108-014327 (2023)
Subject
Language
Abstract
Potassium-40 is a widespread 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 $^{40}$K, but a predicted electron-capture decay directly to the ground state of argon-40 has never been observed, while theoretical predictions span an order of magnitude. The KDK Collaboration reports on the first observation of this rare decay, obtained 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 ($I_{\text{EC}^0}$) over excited-state ones ($I_{\text{EC}^*}$) of $I_{\text{EC}^0} / I_{\text{EC}^*}=0.0095\stackrel{\text{stat}}{\pm}0.0022\stackrel{\text{sys}}{\pm}0.0010$ (68% CL), with the null hypothesis rejected at 4$\sigma$ [Stukel et al., DOI:10.1103/PhysRevLett.131.052503]. This unambiguous signal yields a branching ratio of $I_{\text{EC}^0}=0.098\%\stackrel{\text{stat}}{\pm}0.023\%\stackrel{\text{sys}}{\pm}0.010$, roughly half of the commonly used prediction. This first observation of a third-forbidden unique electron capture improves understanding of low-energy backgrounds in dark-matter searches and has implications for nuclear-structure calculations. A shell-model based theoretical estimate for the $0\nu\beta\beta$ decay half-life of calcium-48 is increased by a factor of $7^{+3}_{-2}$. Our nonzero measurement shifts geochronological ages by up to a percent; implications are illustrated for Earth and solar system chronologies.
Comment: This is a companion submission to Stukel et al (KDK collaboration) "Rare $^{40}$K decay with implications for fundamental physics and geochronology" [arXiv:2211.10319; DOI: 10.1103/PhysRevLett.131.052503]. As such, both texts share some figures and portions of text. This version updates the text following its review and production process
Comment: This is a companion submission to Stukel et al (KDK collaboration) "Rare $^{40}$K decay with implications for fundamental physics and geochronology" [arXiv:2211.10319; DOI: 10.1103/PhysRevLett.131.052503]. As such, both texts share some figures and portions of text. This version updates the text following its review and production process