Speaker
Description
Potassium-40 ($^{40}$K) is a naturally occurring, radioactive isotope impacting understanding of nuclear structure, geological ages spanning timescales as old as the Earth, and rare-event searches including those for dark matter and neutrinoless double-beta decay. The long-lived $^{40}$K radionuclide undergoes electron capture decays to either the excited or ground state of its Ar daughter, of which the latter has previously not been measured, and estimates of its branching ratio are highly variable ($I_{\text{EC}^0}\sim (0-0.8)\%$). In many dark matter searches, $^{40}$K contamination produces a challenging 3 keV background from these electron capture decays in the expected direct-detection signal region, and the poorly understood ground state contribution may affect interpretation of the DAMA/LIBRA dark-matter claim. In geochronology, the common omission of this decay branch impacts ages obtained with K/Ar and $^{40}$Ar/$^{39}$Ar techniques. This rare third-forbidden unique decay additionally provides an estimate for the effective value of the weak axial-vector coupling constant in this regime, with applications to neutrinoless double-beta decay half-life calculations. The KDK ("potassium decay") collaboration has obtained the first measurement of this elusive $^{40}$K branch using a coincidence technique between a high-resolution silicon drift detector to observe X-rays, and a high-efficiency ($\sim 98\%$) Modular Total Absorption Spectrometer (Oak Ridge National Labs) to tag gamma rays, ultimately differentiating ground and excited state electron capture decays of $^{40}$K. With our measurement, the re-evaluated $^{40}$K decay scheme yields $I_{\text{EC}^0} = 0.098\% \stackrel{\mathrm{stat}}{\pm} 0.023\% \stackrel{\mathrm{syst}}{\pm} 0.010\%$. We report on our novel methodology and successful first measurement of this rare $^{40}$K decay, highlighting the multidisciplinary implications of our result.
| Keyword-1 | Potassium-40 |
|---|---|
| Keyword-2 | Rare-event searches |
| Keyword-3 | Geochronology |
