Neutron Transmutation Doped (NTD) Germanium Thermistors for CUPID

CUPID—the CUORE Upgrade with Particle Identification—is a next-generation search for the neutrinoless double-beta (0νββ) decay of ¹⁰⁰Mo utilizing a ton-scale array of scintillating Li₂MoO₄ bolometers enriched in the isotope of interest. Operated at ~10 mK temperatures, these detectors are able to measure absorbed particles' energy with very high resolution via their thermal signatures. Secondary bolometers with semiconductor wafer absorbers detect scintillation light from the Li₂MoO₄ crystals, allowing for active background discrimination through particle identification. Each primary and secondary bolometer (~4000 readout channels) is instrumented with a neutron transmutation doped (NTD) Ge temperature sensor, whose electrical resistance depends on temperature as R(T) = R₀e^{√(T₀ /T)} at sub-Kelvin temperatures, with target values of R₀ ≈ 1 – 2 Ω and T₀ ≈ 4K giving a sensitivity of ~ 0.3 – 1 MΩ/μK. These parameter values and ultra-low operating temperatures necessitate doping the high purity Ge (HPGe) material to a net dopant concentration of ~10¹⁷ cm^–3 with a very high level of uniformity, which we achieve via neutron transmutation doping: HPGe wafers are irradiated in the MITR-II nuclear reactor at nominal doses of ~10¹⁸ n/cm³. In this talk, I present the ongoing research and development of novel NTD geometries for the CUPID light detectors, particularly 1×1×1 mm³ die NTDs, and their characterization. These chips' size and dimensions reduce their heat capacity and make them suitable for large-volume production and innovative mounting arrangements. I also discuss the performance of NTDs in the Baseline Design Prototype Tower (BDPT) and the ramp-up of the CUPID NTD mass production program at the Semiconductor Detector Laboratory (SDL) at LBNL.