The Research, Development, and Production of Neutron Transmutation Doped Germanium Thermistors for the CUPID Experiment.

CUPID—the CUORE Upgrade with Particle Identification—is an upcoming experiment at Laboratori Nazionali del Gran Sasso, searching for the neutrinoless double-beta (0νββ) decay of ¹⁰⁰Mo with a large array of scintillating Li₂MoO₄ bolometers enriched in the isotope of interest. Operating at ~10 mK temperatures, these detectors measure absorbed particles' energies with high resolution via their thermal signatures. Secondary bolometers detecting scintillation light from the Li₂MoO₄ crystals provide active background discrimination through particle identification. Each of the primary and secondary bolometers (~4000 readout channels) is instrumented with a neutron transmutation doped (NTD) Ge thermistor—a temperature sensor, whose electrical resistance depends on temperature as R(T) = R₀e ^{√(T₀ /T)} at sub-Kelvin temperatures, with target R₀ ≈ 1–2 Ω and T₀ ≈ 4K giving a sensitivity of ~0.3–1 MΩ/μK. These parameters require high (~10¹⁷ cm^–3 net p-type), yet very uniform, doping of the high purity Ge. This is achieved by the unique neutron transmutation doping process, with wafers irradiated in a nuclear reactor at nominal doses of ~10¹⁸ n/cm³. In this talk, we present the research and development of novel NTD geometries and readout schemes, as well as the CUPID NTD mass production program.