Low energy calibration in DUNE far detector prototypes

The Deep Underground Neutrino Experiment (DUNE) is a next-generation long-baseline neutrino experiment. In addition to GeV-scale oscillation measurements ($\delta_{CP}$, $\theta_{23}$ octant, mass ordering), DUNE features a low-energy (MeV-scale) program targeting solar, supernova burst (SNB), and Diffuse Supernova Background (DSNB) neutrinos. Accurate reconstruction and background understanding are critical. $^{39}$Ar $\beta$-decays, naturally present in LAr, provide a uniform background and can be used for calibration. This proceeding presents an analysis of isolated MeV-scale energy deposits in the ProtoDUNE-HD (PDHD) prototype. Using cosmic + beam data (run 28086), we analyze energy spectra and spatial distributions of $^{39}$Ar, $^{232}$Th, and $^{207}$Bi. A calibration factor $c_A = (3.9 \pm 0.3)\times 10^{-2}~\text{MeV}/\text{ADC} \times \text{tick}$ and recombination factor $R = 0.60 \pm 0.05$ are extracted, consistent with expectations. The $^{207}$Bi source is spatially resolved with cm-level precision, and $^{232}$Th hot spots align with the field cage structure, offering further calibration potential. These results demonstrate PDHD capability for MeV-scale reconstruction, supporting DUNE low-energy physics goals.