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

The rate of the direct decay of K-40 to the ground state of Ar-40 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 K-40 decays as inferred from simulation. We propose measures to help clarify the situation. (c) 2013 Elsevier B.V. All rights reserved.