Direct Detection of MeV-Scale Dark Matter Utilizing Germanium Internal Amplification for the Charge Created by the Ionization of Impurities

Light, MeV-scale dark matter (DM) is an exciting DM candidate that is undetectable by current experiments. A germanium (Ge) detector utilizing internal charge amplification for the charge carriers created by the ionization of impurities is a promising new technology with experimental sensitivity for detecting MeV-scale DM. We analyze the physics mechanisms of the signal formation, charge creation, charge internal amplification, and the projected sensitivity for directly detecting MeV-scale DM particles. We present a design for a novel Ge detector at helium temperature ($\sim$4 K) enabling ionization of impurities from DM impacts. With large localized E-fields, the ionized excitations can be accelerated to kinetic energies larger than the Ge bandgap at which point they can create additional electron-hole pairs, producing intrinsic amplification to achieve an ultra-low energy threshold of $\sim$0.1 eV for detecting low-mass DM particles in the MeV scale. Correspondingly, such a Ge detector with 1 kg-year exposure will have high sensitivity to a DM-nucleon cross section of $\sim$5$\times$10$^{-45}$ cm$^{2}$ at a DM mass of $\sim$10 MeV/c$^{2}$ and a DM-electron cross section of $\sim$5$\times$10$^{-46}$cm$^{2}$ at a DM mass of $\sim$1 MeV/c$^2$.