AMoRE searches for the neutrinoless double beta decay using 100 kg of enriched ^{100}Mo. Scintillating molybdate crystals coupled with a metallic magnetic calorimeter operate at milli-Kelvin temperatures to measure the energy of electrons emitted in the decay. AMoRE-I is a demonstrator for the full-scale AMoRE, operated at the Yangyang Underground Laboratory for over two years. The exposure was 8.02 kg year (or 3.89 kg_{^{100}Mo} year), and the total background rate near the Q value was 0.025±0.002 counts/keV/kg/year. We observed no indication of 0νββ decay and report a new lower limit of the half-life of ^{100}Mo 0νββ decay as T_{1/2}^{0ν}>2.9×10^{24} yr at 90% confidence level. The effective Majorana mass limit range is m_{ββ}<(210-610) meV using nuclear matrix elements estimated in the framework of different models, including the recent shell model calculations.

We present two variants of a magnetic microcalorimeter with paramagnetic temperature sensors and integrated dc-superconducting quantum interference device readout for high-resolution x-ray emission spectroscopy. Each variant employs two overhanging gold absorbers with a sensitive area of 150 × 150 μm2 and a thickness of 3 μm, thus providing a thickness related quantum efficiency of >98% for photons up to 5 keV and >50% for photons up to 10 keV. The first variant operated nominally but suffered from Joule power dissipation of the Josephson junction shunt resistors, athermal phonon loss, and slew rate limitations of the overall setup. Overall, it only achieved an energy resolution of ΔEFWHM=8.9 eV for 5.9 keV photons. In the second variant, we introduced an innovative tetrapod absorber geometry as well as a membrane technique for diverting dissipated heat away from the temperature sensors. When all mitigations are applied optimally, the second variant achieves an energy resolution of ΔEFWHM=1.25(18) eV for 5.9 keV photons and hence provides the present best energy resolving power E/ΔEFWHM among all existing energy-dispersive detectors for soft and tender x-rays.

The investigation of the absolute scale of the effective neutrino mass remains challenging due to the exclusively weak interaction of neutrinos with all known particles in the standard model of particle physics. At present, the most precise and least-model-dependent upper limit on the electron antineutrino mass is set by the Karlsruhe Tritium Neutrino Experiment (KATRIN) from the analysis of the tritium β-decay. Another promising approach is the electron capture in ¹⁶³Ho, which is under investigation using microcalorimetry by the Electron Capture in Holmium (ECHo) and HOLMES collaborations. An independently measured Q value for this process is vital for the assessment of systematic uncertainties in the neutrino mass determination. Here we report a direct, independent determination of this Q value by measuring the free-space cyclotron frequency ratio of highly charged ions of ¹⁶³Ho and ¹⁶³Dy in the Penning-trap experiment PENTATRAP. Combining this ratio with atomic physics calculations of the electronic binding energies yields a Q value of 2,863.2 ± 0.6 eV c⁻², which represents a more than 50-fold improvement over the state of the art. This will enable the determination of the electron neutrino mass on a sub-electronvolt level from the analysis of the electron capture in ¹⁶³Ho.