An Fe-shielded ¹²⁴Sb⁹Be 24 keV photoneutron source for calibration of low threshold dark matter detectors

Direct detection dark matter searches are extending their reach to sub-GeV dark matter masses, requiring new detector technologies with low thresholds and new low-energy calibration methods. For many detectors searching for dark matter - nucleus interactions, the ideal calibration source is a beam of monoenergetic neutrons with ~keV energy. We will discuss recent progress in making such neutron-based calibrations practical in a university lab environment via novel shielding of an ¹²⁴Sb⁹Be photoneutron source. The 24 keV neutron flux is well-matched to a narrow spectral feature of neutron transparency in Fe, meaning an Fe shield can stop the ¹²⁴Sb gamma background flux while transmitting the 24 keV neutron flux with relatively little moderation. Such an Fe-shielded ¹²⁴Sb⁹Be source has been designed and assembled, and measurements of the neutron and gamma flux and energy spectrum will be presented. Additionally, the detection of the 24 keV neutrons with liquid scintillator, which would allow for tagging neutron recoil angle, will be demonstrated.