GE-Act: Advances in Germanium 0νββ Searches

• GE-Act is an initiative encompassing advanced low-background detection techniques for germanium-based neutrinoless double beta decay, notably using bare HPGe detectors immersed in liquid argon.
• It leverages multi-layer shielding, pulse shape analysis, and anti-coincidence methods to significantly reduce background noise and enhance sensitivity to rare 0νββ events.
• The methodology integrates rigorous material screening, innovative detector design, and dedicated infrastructure like the HEROICA facility to target challenges in probing the neutrino mass hierarchy.

GE-Act refers to the ensemble of technical and procedural advances made in the context of low-background, germanium-based searches for neutrinoless double beta decay (0νββ), exemplified by the GERDA (GERmanium Detector Array) experiment. GERDA is specifically designed to operate bare, high-purity germanium detectors enriched in $^{76}$Ge in a liquid argon (LAr) environment with comprehensive background suppression methodologies. The overarching aim of this activity is the direct measurement or further constraint of the 0νββ process, a rare nuclear transition whose observation would establish the Majorana nature of neutrinos and inform the absolute neutrino mass scale.

1. Scientific Motivation and Physics of 0νββ Decay

The search for 0νββ decay of $^{76}$Ge is motivated by its unique potential to demonstrate the Majorana character of neutrinos, thereby violating lepton number conservation and providing direct sensitivity to the effective Majorana neutrino mass $⟨m_{\beta\beta}⟩$. The 0νββ decay process can be generically written as: $^{76}\text{Ge} \rightarrow\; ^{76}\text{Se} + 2e^{-}$ with no accompanying neutrinos. The inverse half-life is related to physical parameters via: $$(T_{1/2}^{0\nu})^{-1} = G^{0\nu} \cdot |M^{0\nu}|^2 \cdot \left(\frac{⟨m_{\beta\beta}⟩^2}{m_e^2}\right)$$ where $G^{0\nu}$ is the phase space factor, $M^{0\nu}$ the nuclear matrix element, and $m_e$ the electron mass (0812.4194, Collaboration et al., 2012). A distinctive monoenergetic peak at the $Q$-value of 2039 keV in the summed electron energy spectrum is the experimental signature. Sensitivity to longer half-lives directly constrains $⟨m_{\beta\beta}⟩$ to lower values, probing into the region relevant for the inverted neutrino mass hierarchy.

2. Detector Concept: Bare HPGe Diodes in Cryogenic Liquids

GERDA employs high-purity germanium (HPGe) semiconductor diodes, isotopically enriched up to $\sim$90% in $^{76}$Ge. These detectors act simultaneously as both the decay source and detector, maximizing detection efficiency and energy resolution. The detectors are operated “naked,” i.e., without conventional vacuum cryostats but directly immersed in a liquid argon bath. The use of LAr provides thermal management and significantly attenuates environmental $\gamma$ and neutron backgrounds (0812.4194, Collaboration et al., 2012).

A major technical advance, highly relevant for “GE-Act,” was the solution to instabilities (increased leakage current under gamma irradiation) in the absence of passivated surfaces. R&D demonstrated that omitting the passivation layer, which in vacuum setups prevents surface leakage, yields stable operation in LAr, as charges no longer accrue at the interface (Collaboration et al., 2012).

3. Multi-Layer Shielding and Background Control

The background rate at Q$_{\beta\beta}$ is dominated by natural radioactivity and cosmogenic isotopes. GERDA utilizes a multi-layer shielding concept:

• The HPGe detectors are immersed in liquid argon for both cooling and $\gamma$-ray attenuation.
• The LAr cryostat is surrounded by a copper lining and further encased in a large water tank ($\sim$10 m diameter), providing shielding against external $\gamma$-rays and neutrons.
• The water tank doubles as an active Cherenkov muon veto, featuring a photomultiplier array for muon event rejection.
• A “mini-shroud” of thin copper foil enveloping the detectors prevents the drift of $^{42}$K ions from $^{42}$Ar in LAr onto the detector surfaces, thereby suppressing an unexpected background identified during early operation (Collaboration et al., 2012).

Material selection, radiopurity screening (gamma spectroscopy, radon emanation, ICP-MS), and environmental radon control are enforced at all levels (0812.4194, Collaboration et al., 2012).

4. Advanced Signal Discrimination and Event Analysis

Background discrimination capitalizes on several detector and event properties:

• Segmentation: Phase II introduces segmented n-type coaxial HPGe detectors (up to 18 segments/unit), enabling spatial topology analysis. Energy deposits consistent with multi-site interactions (characteristic of Compton-scattered backgrounds) are rejected, exploiting the point-like deposition expected for 0νββ decay events (0812.4194).
• Pulse shape analysis (PSA): Refinement of PSA methods inherited from HDM and IGEX experiments aids the identification of single-site events, critical for further suppressing multi-site (background) contamination.
• LAr scintillation anti-coincidence: Events producing concurrent signals in LAr scintillation (detected by PMTs with wavelength shifters) and Ge detectors indicate interactions spanning both materials (often background). Anti-coincidence vetoing based on this signal has been demonstrated to offer suppression factors >100 for key $\gamma$ backgrounds (e.g., $^{208}$Tl, $^{214}$Bi) (0812.4194).
• Time-correlation vetoes: Decay chains from cosmogenic isotopes (such as $^{68}$Ge/$^{68}$Ga) are further suppressed using timing cuts.

These techniques collectively reduce the background index (BI) in the Q$_{\beta\beta}$ region to $2.0 \times 10^{-2}$ counts/(keV·kg·yr), an order of magnitude improvement over earlier experiments. Energy resolution of $\sim$4.5 keV FWHM at $Q_{\beta\beta}$ ensures the narrow signal window separates potential 0νββ events from the continuum background (Collaboration et al., 2012).

5. Infrastructure for Detector Production, Screening, and Deployment

Large-scale, low-background $^{76}$Ge-based experiments require extensive detector prototyping, testing, and quality control chains. Facilities such as HEROICA (HADES Underground Research Laboratory, Mol, Belgium) provide:

• Automated scanning tables with collimated sources (e.g., $^{241}$Am) for detailed topological charge collection mapping.
• Environmental suppression of cosmic muons (flux reduction by four orders of magnitude) to minimize cosmogenic activation (notably $^{68}$Ge and $^{60}$Co production).
• On-site energy resolution and depletion voltage characterization, pulse shape discrimination assessment, active volume and dead layer measurement, rapid QA cycles (Andreotti et al., 2013).

This infrastructure was central to the deployment of advanced Broad Energy Germanium (BEGe) detectors in GERDA’s upgrades. BEGe diodes offer enhanced pulse shape discrimination and energy resolution ($<2.3$~keV at $1332$~keV), directly translating to lower backgrounds and higher sensitivity (Andreotti et al., 2013). The proximity of BEGe manufacturing and HEROICA (both in Belgium) ensures prompt characterization and rapid deployment, reducing cosmogenic exposure during logistics.

6. Phased Approach, Results, and Future Prospects

GERDA’s operational program was designed in phases:

Phase	Mass (kg)	Detector Type	Background Index (counts/(keV·kg·yr))	Half-life Sensitivity $T_{1/2}^{0\nu}$ (yr)
Phase I	~$18$	Enriched HPGe (HdM/IGEX)	$≤ 10^{-2}$	$\sim 1.2\times 10^{25}$
Phase II	+$22$	Segmented HPGe, BEGe	$≤ 10^{-3}$	$> 1.5\times 10^{26}$ ($90\%$ C.L.), $0.09$–$0.29$ eV
Phase III (planned)	$\gtrsim 1000$	Large-scale, advanced design	$≤ 10^{-4}$	Probes inverted ordering

Phase I scrutinized previous 0νββ claims; subsequent Phase II and potential Phase III upgrades target the inverted hierarchy region, requiring further background reduction and increased detector mass (0812.4194). Improvements in BI, energy resolution, and mass scale are anticipated to either detect 0νββ or set stringent constraints on $⟨m_{\beta\beta}⟩$.

This suggests that the GE-Act advances exhibited in GERDA—direct immersion of bare detectors, multilevel shielding, anti-coincidence with LAr scintillation, segmentation, and quality-assured BEGe production—constitute a paradigm for future $^{76}$Ge-based neutrinoless double beta decay searches.

7. Experimental Challenges and Solutions

Notable challenges addressed in the GERDA/GE-Act context include:

• Leakage current under LAr: Omitting the passivation layer eliminated gamma-induced leakage current increases by preventing charge accumulation (critical for long-term detector operation in LAr) (Collaboration et al., 2012).
• $^{42}$K background from LAr: Rapid event rates traced to $^{42}$K ions were mitigated by copper mini-shrouds and alternative biasing schemes (AC coupling with grounded n$^+$ contact).
• Cosmogenic activation: Underground storage at HEROICA during production and testing suppressed the activation of $^{76}$Ge, preserving low backgrounds.
• Electronics and calibration: Custom front-end electronics designed to operate at LAr temperature while preserving energy resolution were extensively prototyped and tested.

Continued developments in materials screening, event topology analysis, and environmental control are regarded as central to future large-scale (ton-class) endeavors and for meeting the statistical requirements necessary for probing the neutrino mass hierarchy.

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In summary, "GE-Act" encapsulates the technical regime of state-of-the-art, high-purity, low-background germanium-based 0νββ decay searches, operationalized by GERDA and its associated detector production infrastructure. Its methodology integrates advanced cryogenic operation, comprehensive shielding, topological event discrimination, and stringent QA protocols, establishing benchmarks for future rare-event searches in neutrino physics (0812.4194, Collaboration et al., 2012, Andreotti et al., 2013).