First results of the NEXT-100 detector using 83mKr decays

The NEXT collaboration is investigating the double beta decay of ¹³⁶ Xe 136 X e using high-pressure gas electroluminescent time projection chambers, which provide excellent energy resolution together with a robust topological signature. Operating at the Laboratorio Subterráneo de Canfranc (LSC) and building on the success of the NEXT-White detector, the NEXT-100 apparatus began commissioning in May 2024 and started operation with xenon at a pressure of 4 bar in October 2024. We report here the first results obtained with NEXT-100 using low-energy calibration data from ⁸³ᵐKr 83 m Kr decays, which allow mapping of the detector response in the active volume and monitoring of its stability over time. After homogenizing the light response, we achieve an energy resolution of 4.37% FWHM at 41.5 keV for ⁸³ᵐKr 83 m Kr point-like energy deposits contained in a radius of 425 mm. In a fiducial region representing the operating conditions of NEXT-100 at 10 bar we obtain an improved energy resolution of 4.16% FWHM. These results are in good agreement with that obtained in NEXT-White, and an E-1/2 E - 1 / 2 extrapolation to Qβ β Q β β yields an energy resolution close to 0.5% FWHM, well below the 1% FWHM design target.