Neutron captures and delayed decays of reaction products are common sources of backgrounds in ultrarare event searches. In this work, we studied ${}^{13}\mathrm{C}(\alpha ,n){}^{16}\mathrm{O}$ reactions induced by $\alpha$ particles emitted within the calibration sources of the Majorana Demonstrator. These sources are thorium-based calibration standards enclosed in carbon-rich materials. The reaction rate was estimated by using the 6129-keV $\gamma$ rays emitted from the excited ${}^{16}\mathrm{O}$ states that are populated when the incoming $\alpha$ particles exceed the reaction $Q$ value. Thanks to the excellent energy performance of the Demonstrator's germanium detectors, these characteristic photons can be clearly observed in the calibration data. Facilitated by Geant4 simulations, a comparison between the observed 6129-keV photon rates and predictions by a talys-based software was performed. The measurements and predictions were found to be consistent, albeit with large statistical uncertainties. This agreement provides support for background projections from $(\alpha ,n)$ reactions in future double-beta decay search efforts.

• Received 2 April 2022
• Accepted 2 June 2022

DOI:https://doi.org/10.1103/PhysRevC.105.064610