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.

Figures (13)

• Figure 1: Schematic of the p-type segBEGe detector: (a) top view and (b) bottom view. In (a), the cylindrical coordinate system to describe positions in the segBEGe detector is shown. The segments had Lithium-drifted contacts which were grounded for the data taken for this paper.
• Figure 2: Top row: electric potential in the $r$-$z$-plane at $\varphi = 30^\circ$ resulting from (left) applying just the potentials to the contacts, $\phi_V$, (center) just the charge density from ionized impurities, $\phi_\rho$ and (right) total electric potential, $\phi$. The white lines depict equipotential lines at integer multiples of $-250$ V. Bottom row: corresponding electrical fields. The white lines represent field lines.
• Figure 3: (a) Drawing of the Compton Scanner, (b) photo of the setup. A second camera provided an increased acceptance compared to the original setup Abt2022.
• Figure 4: Simulated undepleted volume (yellow) of the p-type segBEGe detector at $V_B=-300$ V, using the default manufacturer values for the impurity density scaled to 89 % and a thickness of the Lithium-drifted contacts of 0.75 mm.
• Figure 5: Top view of the scan points used to image the undepleted volume of the p-type segBEGe detector. The solid and dashed black lines indicate the locations of the $\langle100\rangle$ and $\langle110\rangle$ axes, respectively.