Evidence for ground-state electron capture of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi mathvariant="normal">K</mml:mi><mml:mprescripts /><mml:none /><mml:mn>40</mml:mn></mml:mmultiscripts></mml:math>

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 neutrinoless double-beta decay. The decays this long-lived must be precisely known its use as geochronometer, to account presence in low-background experiments. There are several decay modes potassium-40, but predicted electron-capture directly the ground state argon-40 has never been observed. existence mode fields, while theoretical predictions span an order magnitude. Here we report on first, successful observation rare mode, obtained by KDK (potassium decay) Collaboration using novel combination low-threshold x-ray detector surrounded tonne-scale, high-efficiency γ-ray tagger at Oak Ridge National Laboratory. A blinded analysis reveals distinctly nonzero ratio intensities ground-state electron-captures (IEC0) over excited-state ones (IEC*) IEC0/IEC*=0.0095±stat0.0022±sys0.0010 (68% CL), with null hypothesis rejected 4σ [Stukel et al., Phys. Rev. Lett. 131, 052503 (2023)]. In terms branching ratio, unambiguous signal yields IEC0=0.098%±stat0.023%±sys0.010%, roughly half commonly used prediction. This first third-forbidden unique electron capture improves our understanding low-energy backgrounds dark-matter searches implications nuclear-structure calculations. For example, shell-model based estimate half-life calcium-48 increased factor 7−2+3. Our measurement shifts geochronological up percent; illustrated Earth solar system chronologies.Received 22 November 2022Accepted 23 May 2023DOI:https://doi.org/10.1103/PhysRevC.108.014327©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeta decayGeophysicsNeutrinoless double beta decayParticle matterProperties39 ≤ 58TechniquesNuclear & decaysRadiation detectorsShell modelNuclear Physics