Compton imaging of undepleted volumes of germanium detectors

Abstract

The shape of the undepleted volume of a p-type high-purity Broad Energy Germanium detector, dependent on the bias voltage, has been imaged by measuring spatially-resolved Compton-scattering efficiency. The bias voltage was raised stepwise from $-50\,\text{V}$ to the full-depletion voltage. The geometric acceptance was determined at full depletion. Below full depletion, the relative acceptance observed for $2\times2\times2\,\text{mm}^3$ voxels was used to create the image of the undepleted volume for each bias voltage. The images were used to extract the impurity density profile of the detector by fitting predictions of the open-source software package SolidStateDetectors$.$jl to the images. The result is shown and compared to the impurity density profile deduced from capacitance measurements. This is the first time that three-dimensional images of the undepleted volumes of a germanium detector have become available and have been used to deduce an impurity density profile.