  Potassium-40 ($^{40}$K) 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. $^{40}$K also
plays a role in geochronological dating techniques. The branching ratio of the
electron capture directly to the ground state of argon-40 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 $^{40}$K 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 KSr$_2$I$_5$:Eu
scintillator as source and detector.
