A novel experimental system for the KDK measurement of the 40K decay scheme relevant for rare event searches

Potassium-40 (40K) is a long-lived, naturally occurring radioactive isotope. The decay products are prominent backgrounds for many rare event searches, including those involving NaI-based scintillators. 40K also plays a role in geochronological dating techniques. The branching ratio of the electron capture directly to the ground state of  40Ar has never been measured, which can cause difficulty in interpreting certain results or can lead to lack of precision depending on the field and analysis technique. The KDK (Potassium (K) Decay (DK)) collaboration is measuring this decay. A composite method has a silicon drift detector with an enriched, thermally deposited 40K source inside the Modular Total Absorption Spectrometer. This setup has been characterized in terms of energy calibration, gamma tagging efficiency, live time and false negatives and positives. A complementary, homogeneous, method is also discussed; it employs a KSr2I5:Eu scintillator as source and detector.