Process Development and First Cryogenic Operation of Compact Germanium Ring-Contact HPGe Prototypes

Abstract

Rare-event experiments such as LEGEND-1000 require high-purity germanium (HPGe) detectors with excellent energy resolution, low electronic noise, and scalable low-background packaging. The germanium ring-contact (GeRC) concept addresses this need through a recessed ring-and-groove electrode geometry intended to preserve point-contact-like low-capacitance signal formation in larger crystals. However, reliable GeRC fabrication has remained unproven because the non-planar groove geometry complicates machining, surface recovery, conformal passivation, and especially the eventual formation of a robust lithium-diffused outer contact. We report the fabrication and first cryogenic operation of two compact n-type GeRC process-validation prototypes produced from in-house HPGe crystals at the University of South Dakota. An optimized workflow was developed for core drilling, groove cutting, non-planar polishing, conformal amorphous-germanium (a-Ge) encapsulation, Al patterning, and GeRC-specific cryogenic mounting. Two independent sputtering systems were used to test whether the thin-film sequence remains operable across substantially different deposition environments. At 77~K, both devices biased stably, showed an inferred depletion onset near 340~V from a pulser-based capacitance proxy consistent with electrostatic modeling, and produced identifiable full-energy peaks from $^{241}\mathrm{Am}$ and $^{137}\mathrm{Cs}$. These results establish a proof-of-principle process and readout baseline for geometry-specific GeRC development. They do not yet constitute a deployment-ready large-mass GeRC technology, but they define the foundation for the next step: integrating conformal lithium-paint deposition and controlled diffusion on the ring-and-groove topology.

Figures (12)

• Figure 1: Schematic cross-sectional view of a germanium ring-contact detector geometry. The green region represents the germanium bulk. The purple boundary denotes the outer detector surface, which serves as the large-area opposing electrode in a p-type implementation. The recessed features visible on the left and right sides correspond to the two intersections of a single annular groove with the cross-sectional plane. The red segments indicate the corresponding cross-sectional intersections of the continuous ring-shaped p$^{+}$ readout contact formed on the recessed groove surface. This geometry preserves a spatially localized readout electrode while increasing the detector volume, thereby supporting low-capacitance operation in larger-mass germanium detectors Leone2022_Thesis_RingContactHull2022_PHDS_RCDHull2024_PHDS_RCDRadford2024_RingContactTalkRadford2018_PIRE_RingContact.
• Figure 2: Axial cross-sectional view of the compact GeRC geometry and electrode assignment. The green region represents the HPGe bulk. The white regions correspond to the central bore and the machined annular groove. The red segments denote the two intersections of the continuous annular ring electrode with the axial cutting plane; this ring serves as the small-area signal readout contact. The purple boundary marks the remaining surfaces assigned to the large-area HV electrode. The shallow recessed features near the top and bottom edges are wing-and-groove handling structures introduced to improve mechanical handling during fabrication.
• Figure 3: Three-dimensional rendering of the compact GeRC geometry generated using SSD.jl. This view makes the azimuthal continuity of the annular ring electrode explicit and visualizes the full electrode topology used in the detector model. The rendering shows how the localized ring readout contact is embedded within the surrounding large-area HV electrode structure, which includes the bore surface, the top and bottom faces, and the connected outer-sidewall regions above and below the groove.
• Figure 4: Electrostatic simulation results for the compact GeRC geometry in the $\varphi=0^\circ$ plane, generated using SSD.jl. (a) Electric-field magnitude map with superimposed field lines. The field is concentrated near the recessed ring-contact region and at sharp geometric edges, reflecting the strong local field shaping produced by the small readout electrode and the nonplanar electrode boundaries. (b) Electric-potential map with equipotential contours for the same cross section. The potential distribution shows a smooth bulk gradient from the large-area HV electrode toward the recessed ring contact, while the contour crowding near the groove indicates regions of enhanced field strength. Together, these results illustrate the characteristic GeRC field topology used to guide depletion-voltage estimation and prototype optimization.
• Figure 5: Representative photographs of the early mechanical fabrication sequence for compact GeRC prototypes. (a) Four cylindrical HPGe blanks after cold core drilling and through-bore formation, still supported in the aluminum drilling bath used for coolant circulation and mechanical stabilization. (b) Lathe-based machining of the outer recessed region using a diamond abrasive tool, with the crystal mounted concentrically on a mandrel to maintain alignment with the bore axis. (c) Continued groove and wing machining under direct liquid cooling at the tool--crystal interface. These steps illustrate the low-speed, low-force, coolant-assisted machining strategy adopted to minimize chipping, suppress subsurface crack formation, and preserve the structural integrity of the brittle HPGe crystals during fabrication Wei2019_JINST_aGeDetectorsDong2026_arXiv_HybridProcessPanamaldeniya2026_arXiv_ICPCaGe.