The GemX project started with the procurement of 23 kg of high-purity germanium enriched in the isotope Ge-76 >89% in the chemical form of GeO2. We established a detailed quality assurance protocol with elemental and isotopic analysis carried out by the GemX team throughout the production. During and after the enrichment process, the material was stored underground at the producer’s site to minimize activation through cosmic radiation (i.e. production of interfering elements such as 68Ge or 60Co). After verifying the quality of the raw material by measurements ICPMS carried out by the GemX team, the germanium material was transported from the production site in a 15 ton shielded steel container to Munich, Germany, and stored underground.
The enriched germanium material was converted from oxide to high-purity metal. For this purpose, an optimized process for hydrogen reduction and zone refinement of germanium dioxide was developed by GemX team in collaboration with crystal growth experts. The process was optimized using a kinetic model for unreacted shrinkage and initially tested with a batch of GeO2 with natural isotopic composition. The reduction process of the isotopically enriched germanium was carried out with an average yield of 99.85%. Subsequently, the germanium was purified to intrinsic purity by zone-refining, and an overall germanium yield of 99.05% was achieved. With the intermediate underground storage and the point-to-point car transportation, an average cosmogenic exposure of only 156 h was accumulated over the entire processing period.
In preparation for the detector production, we studied the performance of the novel high-purity germanium detector’s signals. To this end, we investigated the collective effects in clusters of charge carriers in germanium detectors and the impact of such effects on signal formation, with a particular focus on 0-like signals. We determined that the deformation of the signal due to collective effects is relevant for detectors with long drift paths. Using Monte Carlo and pulse shape simulations of gamma radiation from 208Tl and 0s of 76Ge, we determined that such volume dependence does not significantly impact the pulse shape discrimination performances provided an optimized detector geometry and analysis strategy. We could show that despite these significant effects, the inverted coaxial detector design, which GemX will realize, is high-performance for the search for neutrinoless decay.
