A 350 keV Monoenergetic Neutron Source using a DD-Neutron Source and a Deuterated Scintillator Neutron Reflector

Nuclear recoil (NR) calibrations are vital for understanding detector responses to dark matter candidates and neutrino-nucleus signals in direct detection experiments. Low-mass ($<$5 GeV) dark matter candidates and $^8$B neutrinos drive the need for high-statistics/low-systematic calibrations at even lower NR energies. We report the results of measurements made at Brown University demonstrating the effectiveness of an Adelphi Technologies Inc. DD neutron generator and deuterated scintillator in a carefully shielded geometry to shift the neutron beam energy from 2.45 MeV (94 keV FWHM) to 350 keV (85 keV FWHM). This low energy, monoenergetic source is fully portable and usable in situ to measure NR events in a range of detector technologies. The lower neutron speed allows the tagging of distinct S1 signals for multiple scatters within tonne-scale liquid noble time projection chambers (TPCs) and permits direct in situ measurement of light yield (L$_y$) independent of charge yield (Q$_y$). The scintillator reflector allows per-neutron energy determination via time-of-flight (ToF) and pulse size measurements, providing a powerful calibration source with few systematic uncertainties. A ToF-based hydrogen reflector source with a tunable neutron energy from 10-100 keV is also discussed.