Speakers
Description
Potassium-40 ($^{40}$K) is a naturally-occurring, radioactive isotope of interest to rare-event searches as a challenging background. In particular, NaI scintillators contain $^{40}$K contamination which produces an irreducible $\sim $3 keV signal originating from this isotope's electron capture (EC) decays. In geochronology, the $\mathcal{O}(\text{Gy})$ lifetime of $^{40}$K is utilized in dating techniques. The direct-to-ground-state EC intensity ($I_\text{EC}$) of this radionuclide has never been measured, and theoretical predictions are highly variable ($I_\text{EC}\sim (0.064(19)–0.22(4))\%$). The poorly understood intensity of this branch may affect the interpretation or precision of experimental results, including those probing dark matter signals in the (2-6) keV region. The KDK (``potassium decay") experiment is carrying out the first measurement of this $I_\text{EC}$ branch, using a coincidence technique between a high-resolution silicon drift detector for $\mathcal{O}(\text{keV})$ X-rays and Augers, and a high-efficiency ($\sim 98\%$) Modular Total Absorption Spectrometer (Oak Ridge National Labs) for $\mathcal{O}(\text{MeV})$ gammas, to differentiate ground and excited state EC decays of $^{40}$K. We report on the analysis of the main $^{40}$K result, and on a measurement of $^{65}$Zn decays used to test methods.
