Search for: All records

For the first time, time-dependent internal charge amplification through impact ionization has been observed in a planar germanium (Ge) detector operated at cryogenic temperature. In a time period of 30 and 45 min after applying a bias voltage, the charge energy corresponding to a baseline of the 59.54 keV$$\gamma $$$\gamma$rays from a$$^{241}$$${}^{241}$Am source is amplified for a short period of time and then decreases back to the baseline. The amplification of charge energy depends strongly on the applied positive bias voltage with drifting holes across the detector. No such phenomenon is visible with drifting electrons across the detector. We find that the observed charge amplification is dictated by the impact ionization of charged states, which has a strong correlation with impurity level and applied electric field. We analyze the dominant physics mechanisms that are responsible for the creation and the impact ionization of charged states. Our analysis suggests that the appropriate level of impurity in a Ge detector can enhance charge yield through the impact ionization of charged states to achieve extremely low-energy detection threshold (< 10 meV) for MeV-scale dark matter searches if the charge amplification can be stabilized.

There is considerable evidence for widespread subsonic turbulence in galaxy clusters, most notably from Hitomi. Turbulence is often invoked to offset radiative losses in cluster cores, both by direct dissipation and by enabling turbulent heat diffusion. However, in a stratified medium, buoyancy forces oppose radial motions, making turbulence anisotropic. This can be quantified via the Froude number Fr, which decreases inward in clusters as stratification increases. We exploit analogies with MHD turbulence to show that wave–turbulence interactions increase cascade times and reduce dissipation rates ϵ ∝ Fr. Equivalently, for a given energy injection/dissipation rate ϵ, turbulent velocities u must be higher compared to Kolmogorov scalings. High-resolution hydrodynamic simulations show excellent agreement with the ϵ ∝ Fr scaling, which sets in for Fr ≲ 0.1. We also compare previously predicted scalings for the turbulent diffusion coefficient D ∝ Fr2 and find excellent agreement, for Fr ≲ 1. However, we find a different normalization, corresponding to stronger diffusive suppression by more than an order of magnitude. Our results imply that turbulent diffusion is more heavily suppressed by stratification, over a much wider radial range, than turbulent dissipation. Thus, the latter potentially dominates. Furthermore, this shift implies significantly higher turbulent velocities required to offset cooling, compared to previousmore »models. These results are potentially relevant to turbulent metal diffusion in the galaxy groups and clusters (which is likewise suppressed), and to planetary atmospheres.