Eur. Phys. J. C (2023) 83: 278
https://doi.org/10.1140/epjc/s10052-023-11432-y

Special Article - Tools for Experiment and Theory

Observation of time-dependent internal charge amplification in a planar germanium detector at cryogenic temperature

¹ Department of Physics, The University of South Dakota, 57069, Vermillion, SD, USA
² School of Physics and Astronomy, University of Minnesota, 55455, Minneapolis, MN, USA
³ Department of Physics and Astronomy, Texas A &M University, 77843, College Station, TX, USA

^c Dongming.Mei@usd.edu

Received: 3 February 2023
Accepted: 24 March 2023
Published online: 6 April 2023

Abstract

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 $\gamma$ rays from a ${}^{241}$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.