On an unverified nuclear decay and its role in the DAMA experiment

The rate of the direct decay of K40 to the ground state of Ar40 through electron capture has not been experimentally reported. Aside from its inherent importance for the theory of electron capture as the only such decay known of its type (unique third-forbidden), this decay presents an irreducible background in the DAMA experiment. We find that the presence of this background, as well as others, poses a challenge to any interpretation of the DAMA results in terms of a Dark Matter model with a small modulation fraction. A 10 ppb contamination of natural potassium requires a 20% modulation fraction or more. A 20 ppb contamination, which is reported as an upper limit by DAMA, disfavors any Dark Matter origin of the signal. This conclusion is based on the efficiency of detecting K40 decays as inferred from simulation. We propose measures to help clarify the situation.