Exploring the Potential of Residual Impurities in Germanium Detectors for Low-Mass Dark Matter Detection

The pursuit of low-mass dark matter detection hinges on achieving an unprecedentedly low energy detection threshold. Germanium (Ge) detectors, when meticulously tailored with precise impurity compositions, hold the promise of expanding their sensitivity to energy levels below the sub-electronvolt (sub-eV) range. In this study, we embark on an exploration of residual impurities inherent to Ge detectors, focusing on their behavior at helium temperatures. These impurity atoms undergo a captivating freeze-out phenomenon, transitioning into localized states known as excited dipole states. Through compelling evidence derived from the measurement of relative capacitance, we elucidate the transition of impurity atoms from free charge states to these localized dipole states, as the temperature plummets from 11 K to 6.5 K. Our investigation encompasses the intricate formation of these dipole states in both n-type and p-type impurities. Furthermore, we shed light on the electric field generated by these dipole states, unveiling their capacity to ensnare charges and foster the creation of cluster dipole states. Drawing from prior measurements, we affirm that these excited dipole states exhibit a binding energy of less than 10 meV, thus forging a path toward an exceptionally low detection threshold for low-mass dark matter. Capitalizing on this concept, we propose the development of a 1-kg Ge detector equipped with internal charge amplification, an innovative approach poised to surpass electrical noise and, in turn, empower the detection of low-mass dark matter with unprecedented sensitivity.