Development of Low-Threshold Ionization Detectors Using USD-Grown Germanium Crystals for Low-Mass Dark Matter Searches*

Our objective is to leverage in-house manufactured Germanium (Ge) crystals to develop low-threshold (~sub-eV) ionization detectors at the University of South Dakota (USD) in collaboration with the Pacific Northwest National Laboratory (PNNL). This effort aims to enhance the detection of low-mass dark matter (DM) by providing Ge crystals with ultra-low cosmogenic exposures. Key tasks include fabricating charge readout Ge detectors with an internal charge amplification factor of 10-100, which will be validated at PNNL's shallow underground laboratory (SUL) at ultra-low temperatures (7.5 to 40 millikelvin).

We are working on an innovative germanium detector design featuring a high-purity Ge detector with a net impurity level of approximately 3×10^10 cm^−3 uniformly distributed across the entire detector, internal charge amplification with a gain factor of 1000, and a depleted detector operating at 77 K and continuously cooled down to helium temperature. These detectors aim to achieve an energy threshold of around sub-eV. This process involves using large concentrated electric fields to accelerate ionization excitations to energies beyond the germanium bandgap, thereby producing additional electron-hole pairs.