Low-Energy Nuclear Recoil Calibration of XENONnT with a $^{88}$YBe Photoneutron Source

Abstract: Characterizing low-energy (O(1keV)) nuclear recoils near the detector threshold is one of the major challenges for large direct dark matter detectors. To that end, we have successfully used a Yttrium-Beryllium photoneutron source that emits 152 keV neutrons for the calibration of the light and charge yields of the XENONnT experiment for the first time. After data selection, we accumulated 474 events from 183 hours of exposure with this source. The expected background was $55 \pm 12$ accidental coincidence events, estimated using a dedicated 152 hour background calibration run with a Yttrium-PVC gamma-only source and data-driven modeling. From these calibrations, we extracted the light yield and charge yield for liquid xenon at our field strength of 23 V/cm between 0.5 keV$<em>{\rm NR}$ and 5.0 keV$</em>{\rm NR}$ (nuclear recoil energy in keV). This calibration is crucial for accurately measuring the solar $<sup>8$B neutrino coherent elastic neutrino-nucleus scattering and searching for light dark matter particles with masses below 12 GeV/c$<sup>2$.