Observation of Time-Dependent Internal Charge Amplification in a Planar Germanium Detector at Cryogenic Temperature

TL;DR

This study reports the first observation of time-dependent internal charge amplification in a cryogenic planar germanium detector, revealing impact ionization effects influenced by impurity levels and electric fields, with implications for dark matter detection.

Contribution

It demonstrates the occurrence of impact ionization-induced charge amplification in germanium detectors at cryogenic temperatures, highlighting the role of impurity levels and electric fields in this phenomenon.

Findings

Charge amplification occurs temporarily after bias application.

Amplification depends on applied bias voltage and impurity levels.

Potential for ultra-low energy detection in dark matter searches.

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 minutes 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.