Experimental setup and analysis method for the KDK ground state electron capture measurement of K-40

Abstract

Potassium-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. ANAIS-112, COSINE-100, COSINUS, DAMA/LIBRA, SABRE, etc.). The branching ratio of the electron capture directly to the ground state of Argon-40 has never been 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. This branching ratio also 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. This thesis details an experiment that 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. Here we present the characterization of the KDK detector setup in terms of energy calibration, dead time, gamma tagging efficiency and false negatives and positives. The analysis of a blinded, 44-day experimental run is also shown. Finally, a comprehensive Monte Carlo program is developed for simulating the KDK experiment and evaluating its sensitivity. We find that the KDK experiment will allow a successful ground state branching ratio measurement, with a p-value greater than 10e-7, if the true value is greater than 0.1%.