Observation of annual modulation induced by γ rays from (α,γ) reactions at the Soudan Underground Laboratory

Annual modulation of γ rays from (α,γ) reactions in the Soudan Underground Laboratory was observed using a 12-liter scintillation detector. This significant annual modulation, measured over 4 years, can mimic the signature for dark matter and can also generate potential background events for neutrinoless double-β decay experiments. The measured annual modulation of the event rate from (α,γ) reactions is strongly correlated with the time-varying radon concentration observed independently in the laboratory. The α flux from radon decay is simulated starting from the measured radon concentration, and the γ-ray flux is determined using the convolution of the α flux and the cross sections for (α,γ) reactions. The calculated γ-ray flux is sufficient to generate the measured event rate that exhibits an annual modulation.