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

Abstract

Annual modulation of $γ$ rays from ($α$, $γ$) reactions in the Soudan Underground Lab has been 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 Lab. 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.

7 pages, 8 figures, and 4 tables