Potassium-40 (K-40) is a long-lived, naturally occurring radioactive isotope. The decay products are prominent backgrounds for many rare event searches, especially those involving NaI-based scintillators (ex. DAMA, ANAIS-112, COSINE-100, SABRE, COSINUS etc.). The branching ratio of the electron capture directly to the ground state of Argon-40 has never been experimentally measured and presents an unknown background directly in the 2-6 keV energy region. This is the same region where the DAMA/LIBRA experiment observes their unique annual modulation. Knowledge of this branching ratio can place constraints on the allowed modulation fraction observed by the DAMA experiment. In addition, this branching ratio has important implications for nuclear physics and geochronology. KDK (Potassium (K) Decay (DK)) is an international collaboration dedicated to the measurement of this branching ratio. The experiment is performed using a silicon drift detector with a thermally deposited, enriched K-40 source inside the Modular Total Absorption Spectrometer (MTAS, Oak Ridge National Laboratory). MTAS is a large NaI detector whose high gamma-ray efficiency enables the proper discrimination between ground and excited state electron capture events. This setup has been characterized in terms of energy calibration and dead time, and a tagging efficiency of 97.89(6) % has been demonstrated (arXiv:2012.15232 submitted to NIM). We report on the analysis method and sensitivity for a 44-day K-40 physics run.
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