Development of Low Threshold Detectors for Low-Mass Dark Matter Searches

This study investigates the charge transport behavior in a cryogenic P-type Ge detector, fabricated from a crystal grown at USD. The influence of cryogenic temperatures on charge dynamics is explored using an Am-241 source to observe time-dependent impact ionization phenomena. The results reveal the formation of cluster dipole states at low temperatures, leading to time-dependent impact ionization. Several key physical aspects are examined, including charge collection efficiency, impact ionization rate, electric field-dependent trapping cross-sections, and the binding energies of cluster dipole states. The impact ionization scattering cross-section is measured within the range of 10^-13 cm². The binding energies of dipole and cluster dipole states are investigated under two modes: a constant electric field at cryogenic Kelvin temperature, following the Maxwell-Boltzmann distribution, and a variable electric field at cryogenic millikelvin temperature, following the Fermi-Dirac distribution. The first mode reveals binding energies below 2.5 meV, while the second mode indicates binding energies below 1 meV. This observation suggests the formation of distinct charge states influenced by temperature and electric field, offering potential implications for the search for low mass dark matter.