Spectral measurement of the 214 Bi β decay to the 214 Po ground state with XENONnT

We report the measurement of the 214Bi β -decay spectrum to the ground state of 214Po using the XENONnT detector. This decay is classified as first-forbidden nonunique, for which theoretical predictions require detailed nuclear structure modeling. A dedicated identification algorithm isolates a high-purity sample of ground-state β decays, explicitly excluding events with associated γ -ray emission. By comparing the measured spectrum, which covers energies up to 3.27 MeV, with several nuclear models, we find that the prediction based on the conserved vector current hypothesis provides the best description of the data. Using this dataset, we additionally derive charge and light yield curves for electronic recoils, extending detector response modeling up to the megaelectronvolt scale.