Enhancing Sensitivity in Ge-Based Rare-Event Physics Experiments through Underground Crystal Growth and Detector Fabrication

The cosmogenic production of long-lived isotopes such as $^{3}$H,$^{55}$Fe, $^{60}$Co, $^{65}$Zn, and $^{68}$Ge poses a significant challenge as a source of background events in Ge-based dark matter (DM) and neutrinoless double-beta decay ($0\nu\beta\beta$) experiments. In the pursuit of DM, particularly within the largely unexplored parameter space for low-mass DM, new detector technologies are being developed with extremely low-energy thresholds to detect MeV-scale DM. However, isotopes like $^{3}$H, $^{55}$Fe, $^{65}$Zn, and $^{68}$Ge, produced cosmogenically within the detector material, emerge as dominant backgrounds that severely limit sensitivity in these searches. Similarly, efforts to detect $0\nu\beta\beta$, especially under a neutrino normal mass hierarchy scenario, require a sensitivity to the effective Majorana mass of $\sim$1 meV. Achieving this level of sensitivity necessitates stringent suppression of background signals from isotopes such as $^{60}$Co and $^{68}$Ge, which impose critical detection limits. To reach the targeted sensitivity for these next-generation experiments and to unlock their full discovery potential for both low-mass DM and $0\nu\beta\beta$, relocating Ge crystal growth and detector fabrication to underground environments is crucial. This approach is the most effective strategy to significantly reduce the production of these long-lived isotopes, thereby ensuring the experimental sensitivity required for groundbreaking discoveries.