($\alpha $, $\gamma )$ reaction induced background events for rare event experiments

We report an observation of ($\alpha $, $\gamma )$ reaction at the Soudan mine. With a 12-Liter scintillation neutron detector at Soudan mine for about 5 years of data taking, we have observed ($\alpha $, $\gamma )$ reaction, which can generate potential background events for dark matter and neutrinoless double-beta decay experiments. We have simulated the alpha flux from radon decay using the measured radon concentration in Soudan mine. The convolution of the alpha flux and the cross-section of ($\alpha $, $\gamma )$ allows us to determine the rate of high energy gamma from ($\alpha $, $\gamma )$ reaction. This rate is compared to the measured event rate. We demonstrate that the modulation of ($\alpha $, $\gamma )$ event rate has similar pattern as the radon modulation observed independently in Soudan mine.