---
title: 'LEGEND-1000: Tonne-Scale 0νββ Search'
url: https://www.emergentmind.com/topics/legend-1000
type: topic
---

# LEGEND-1000: Tonne-Scale 0νββ Search

LEGEND-1000 is the planned ton-scale phase of the Large Enriched Germanium Experiment for Neutrinoless Double-Beta Decay, designed to search for \(0\nu\beta\beta\) in \(^{76}\)Ge with \(1000\) kg of enriched HPGe detectors, a live time of about \(10\) years, and a half-life discovery sensitivity beyond \(10^{28}\) years. In the preconceptual design report, the experiment is described as a quasi-background-free search with a 99.7%-CL discovery sensitivity of \(1.3\times 10^{28}\) years, corresponding to an effective Majorana mass range of \(9\)–\(21\) meV and intended to cover the inverted-ordering neutrino mass scale [2107.11462].

## 1. Programmatic position within the LEGEND collaboration

LEGEND is a phased \(^{76}\)Ge program built from the technical lineages of GERDA and the MAJORANA DEMONSTRATOR. A later collaboration summary states that the collaboration was formed in 2016 to combine GERDA’s operation of germanium detectors in liquid argon with an active LAr veto and MAJORANA’s ultra-clean materials, low-noise electronics, and outstanding energy resolution [2508.18573]. Early LEGEND design papers already defined the strategic structure: a \(200\) kg stage using existing GERDA infrastructure at LNGS, followed by a tonne-scale stage with discovery potential beyond \(10^{28}\) years [1810.00849, 1709.01980].

Within that architecture, LEGEND-200 is the intermediate operational phase and LEGEND-1000 is the final large-scale instrument. LEGEND-200 was framed as a \(\sim 200\) kg experiment with a target exposure of \(\sim 1\) ton·yr and a design background index of \(2\times 10^{-4}\,\text{counts}/(\text{keV kg yr})\), while LEGEND-1000 is the next-generation phase with \(1000\) kg of enriched HPGe and a target exposure of \(10\) ton·yr [2509.21166]. This phased structure is not merely administrative. It is the mechanism by which detector geometry, radiopurity protocols, veto instrumentation, electronics, and analysis workflows are validated at the \(10^2\) kg scale before deployment at the \(10^3\) kg scale [2508.18573].

A plausible implication is that LEGEND-1000 should be read less as an isolated future detector than as the culmination of an explicitly staged germanium program whose predecessor phase is already producing operational and background-model inputs for the tonne-scale design.

## 2. Physics rationale and neutrino-mass reach

The experimental target is neutrinoless double-beta decay of \(^{76}\)Ge. The process would violate total lepton number by two units, and its observation would imply that neutrinos are Majorana particles; multiple LEGEND summaries also connect this to leptogenesis and the cosmological matter-antimatter asymmetry [2508.18573, 2501.10046]. For \(^{76}\)Ge, the search is centered on the monoenergetic endpoint signature at \(Q_{\beta\beta} = 2039.061(7)\,\text{keV}\) [2509.21166].

For the standard light-Majorana exchange mechanism, the decay rate is written as
\[
\left[T^{0\nu}_{1/2}\right]^{-1} = G^{0\nu} \; |M^{0\nu}|^2 \; \left( \frac{\langle m_{\beta\beta} \rangle}{m_e} \right)^2,
\]
with
\[
\langle m_{\beta\beta} \rangle = \left| \sum_i U_{ei}^2 m_i \right|.
\]
Here \(G^{0\nu}\) is the phase-space factor, \(M^{0\nu}\) the nuclear matrix element, and \(\langle m_{\beta\beta}\rangle\) the effective Majorana mass [2508.18573, 1709.01980].

The central physics claim attached to LEGEND-1000 is that sensitivity beyond \(10^{28}\) years in \(^{76}\)Ge reaches the inverted-ordering mass scale. Later LEGEND descriptions state a target \(m_{\beta\beta}\) reach of approximately \(9\)–\(24\) meV for \(T_{1/2}^{0\nu}\) beyond \(10^{28}\) years [2509.21166, 2603.12884]. The preconceptual design report quotes \(9\)–\(21\) meV at \(1.3\times 10^{28}\) years [2107.11462]. Across the design literature, this reach is consistently described as covering the inverted neutrino mass hierarchy or inverted-ordering region [2508.18573, 2107.11462].

The sensitivity logic is likewise standard. Large exposure \(E=M\times t\), very low background index, and HPGe-class energy resolution jointly determine discovery reach. In a background-free or quasi-background-free regime, sensitivity scales approximately linearly with exposure; in a background-limited regime it scales more slowly, roughly with \(\sqrt{M t}\) [1709.01980, 2603.12884]. LEGEND-1000 is explicitly optimized to operate in the former regime [2508.18573].

## 3. Experimental architecture and performance targets

Later LEGEND-1000 descriptions give the core target parameters in a compact form: \(1000\) kg of germanium, \(10\) ton·yr exposure, a target background index of \(10^{-5}\,\text{counts}/(\text{keV kg yr})\), and a \(3\sigma\) discovery sensitivity for a \(0\nu\) half-life beyond \(10^{28}\,\text{yr}\) [2509.21166]. A 2026 LEGEND overview expresses the same background goal as
\[
B_{\text{LEGEND-1000}} = 10^{-5}\ \text{cts}/(\text{keV kg yr}),
\]
corresponding to
\[
B \simeq 0.025\ \text{cts}/(\text{FWHM t yr})
\]
for \(\text{FWHM}(Q_{\beta\beta}) \approx 2.5\) keV [2603.12884].

The detector technology baseline has also narrowed with time. Conference and summary papers state that only ICPC detectors are planned for LEGEND-1000 deployment [2509.21166]. The rationale is direct: ICPC geometry combines large detector mass with the point-contact field configuration needed for strong PSD and high energy resolution, while avoiding the poorer \(Q_{\beta\beta}\)-scale resolution associated with legacy coaxial detectors [2509.21166]. The preconceptual design report further specifies germanium detectors enriched to more than \(90\%\) in \(^{76}\)Ge, operated in a liquid argon active shield at a deep underground laboratory [2107.11462].

The cryogenic concept is built around nested shielding. The detectors are immersed in LAr, which supplies cooling, passive shielding, and scintillation-based active veto capability. The surrounding infrastructure includes low-radioactivity cryostat materials, external shielding, and muon-veto instrumentation; design studies discuss underground argon in the inner detector region and atmospheric argon in an outer region, preserving an active veto where radiopurity is most critical [2107.11462, 2508.18573].

A design-level implication follows from the numerical target itself. At
\[
B = 10^{-5}\ \text{cts}/(\text{keV kg yr}),
\]
\(\text{FWHM}\approx 2.5\) keV, and \(M t = 10^4\) kg·yr, the expected background in one FWHM around \(Q_{\beta\beta}\) is well below one count [2603.12884]. That quasi-background-free condition is the basis for the claim that LEGEND-1000 could make an unambiguous discovery with only a few counts at the decay \(Q\) value [2107.11462].

## 4. Background budget and enabling technologies

LEGEND-1000 inherits a layered background-rejection strategy from GERDA, MAJORANA, and LEGEND-200. The relevant subsystems include Ge-Ge anti-coincidence, active LAr scintillation veto, water Cherenkov muon veto, and waveform-based PSD using parameters such as \(A/E\) and late-charge observables [2508.18573]. Later ASIC studies emphasize that front-end electronics located close to the detector are simultaneously a noise problem and a radiopurity problem; for that reason, dedicated ASIC development has been pursued for LEGEND-1000. Tests of the XGLab CUBE preamplifier with a PPC detector obtained \(\sim 2.2\) keV FWHM at \(2039\) keV and \(\sim 2.5\) keV at \(2.6\) MeV, with PSD performance comparable to GERDA and MAJORANA [1911.07727].

One major background axis is the \(^{42}\)Ar/\(^{42}\)K chain in LAr. A dedicated LEGEND mitigation study states that \(^{42}\)K backgrounds are expected to comprise \(30\)–\(40\%\) of LEGEND backgrounds in both phases if left uncontrolled [2209.07598]. The same work studies encapsulation of each Ge detector with ultra-pure plastic, especially PEN, as a scalable mitigation. Simulation results show that about \(8\) mm of PEN would fully absorb a \(3.525\) MeV electron, while \(2\) mm PEN already produces substantial energy sharing between PEN and Ge, making veto-assisted rejection plausible when combined with dead layers and PSD [2209.07598]. This suggests that detector encapsulation is a contingency or complementary mitigation path if underground-sourced argon is limited.

A second major axis is cosmogenic neutrons and the production of \(^{77}\)Ge/\(^{77m}\)Ge. A dedicated comparison of Geant4 and MCNP for LEGEND-1000-like geometries finds that Geant4 generally predicts higher neutron scattering and higher capture rates on germanium than MCNP, implying that earlier Geant4-based cosmogenic estimates are conservative [2406.12882]. The same study proposes methane-doped outer liquid argon as a risk-mitigation option: \(10\%\) molar methane reduces neutron capture on \(^{76}\)Ge by about a factor of \(3\), and \(20\%\) methane by about a factor of \(5\), without significant modification to the baseline design [2406.12882].

Two additional R&D lines address event-classification systematics at the waveform level. The CAGE internal-source scanning cryostat was built to study surface events on HPGe detectors; its commissioning with \(^{241}\)Am achieved a beam spot precision of \(3.1\) mm and identified \(\Delta t_{2-20}\) as a robust early-rise metric for shallow gamma interactions near passivated surfaces [2602.06289]. Separately, a semi-supervised Affinity-Propagation-plus-SVM cleaning model for LEGEND waveforms reports a maximum sacrifice of physics events of \(0.024^{+0.004}_{-0.003}\%\), and is already being used to accelerate LEGEND-200 data-cleaning development while being positioned for LEGEND-1000-scale analysis workflows [2410.14701].

## 5. LEGEND-200 as the empirical precursor

The first year of LEGEND-200 provides the direct empirical basis for LEGEND-1000 projections. LEGEND-200 began stable physics running in 2023 at LNGS with \(142.5\) kg of detectors installed, and its first \(0\nu\beta\beta\) analysis used a total exposure of \(61\) kg·yr [2603.12884, 2509.21166]. Detector types included BEGe, PPC, ICPC, and legacy coaxial units, with enrichment between \(86\%\) and \(92\%\) in \(^{76}\)Ge [2603.12884].

Performance results are central. ICPC, BEGe, and PPC detectors met the LEGEND-200 energy-resolution goal of \(\leq 2.5\) keV FWHM at \(Q_{\beta\beta}\), while coaxial detectors retained the poorer resolution already known from GERDA [2603.12884]. After cuts, the first-year background indices were
\[
0.5^{+0.3}_{-0.2}\times 10^{-3}\,\text{counts}/(\text{keV kg yr})
\]
for the golden dataset and
\[
1.3^{+0.8}_{-0.5}\times 10^{-3}\,\text{counts}/(\text{keV kg yr})
\]
for the silver dataset [2509.21166]. The frequentist LEGEND-200-only half-life limit was
\[
T_{1/2}^{0\nu} > 0.5\times 10^{26}\ \text{yr}\quad (90\%~\text{CL}),
\]
and the combined GERDA + MAJORANA Demonstrator + LEGEND-200 result gave
\[
T_{1/2}^{0\nu} > 1.9\times 10^{26}\ \text{yr}\quad (90\%~\text{CL}),
\]
with median exclusion sensitivity
\[
> 2.8\times 10^{26}\ \text{yr},
\]
described as the best achieved among \(0\nu\beta\beta\) decay searches to date [2508.18573, 2509.21166].

For LEGEND-1000, the design consequences are explicit. Coaxial detectors are identified as a dominant source of elevated background and are not planned for future phases; only ICPC are planned for LEGEND-1000 deployment [2508.18573, 2509.21166]. Background characterization after the first year also triggered screening, cleaning, and redeployment steps in LEGEND-200, including movement toward ICPC-and-BEGe-only arrays and improved surface treatments [2603.12884]. This suggests that LEGEND-1000 will not simply scale the first deployed LEGEND-200 hardware mix, but the refined post-characterization configuration.

## 6. Materials, infrastructure, and role in the global \(0\nu\beta\beta\) program

The LEGEND-1000 program depends on industrially credible enriched-germanium production. A dedicated process-development paper reports hydrogen reduction of \(^{76}\)Ge-enriched GeO\(_2\) with an average yield of \(99.85\%\), subsequent zone refining to intrinsic purity with an overall Ge yield of \(99.05\%\), and an average cosmogenic exposure of \(156\) h for a \(23\) kg enriched batch [2009.07585]. The authors state that the line was developed to find the optimum solution for processing large quantities of germanium for LEGEND-1000 [2009.07585]. This suggests that isotope logistics and feedstock preparation, not only detector operation, are recognized as core enabling technologies for the tonne-scale phase.

Mechanical infrastructure is likewise under active validation. A recent LEGEND-1000 hardware campaign on OFHC copper cylinders for the reentrant-tube design used hydrostatic burst tests and an automated vision pipeline to reconstruct hoop stress-strain curves and extract yield strengths by the \(0.2\%\) offset, \(0.5\%\) EUL, and Johnson-Cook methods; the same work cross-validated video-derived diameter extraction against a non-agentic pipeline at the \(\pm 5\) pixel level and compared results to Ansys simulations [2606.18294]. The presence of such studies indicates that LEGEND-1000 design work extends beyond detector modules into long-duration cryogenic vessel mechanics and dual-argon-volume containment.

In the broader \(0\nu\beta\beta\) landscape, LEGEND-1000 is the flagship germanium-based next-generation experiment. Later LEGEND summaries emphasize the combination of source-equals-detector geometry, \(\sim 0.1\%\) energy resolution at \(Q_{\beta\beta}\), powerful PSD, and multi-layer active vetoes as the distinctive strengths of the \(^{76}\)Ge approach [2509.21166, 1901.02805]. The same literature places LEGEND-1000 among the primary experiments capable of decisively testing the inverted-ordering parameter space for Majorana neutrinos [2107.11462, 1810.00849].

If LEGEND-1000 observes \(0\nu\beta\beta\), the implication would be lepton-number violation and Majorana neutrinos. If it reaches its intended sensitivity and observes no signal, the standard light-Majorana interpretation of the inverted-ordering region would be strongly disfavored [2508.18573, 2107.11462]. In either outcome, LEGEND-1000 is designed as a decisive \(^{76}\)Ge experiment: not merely a scale-up of earlier germanium arrays, but a tonne-scale, quasi-background-free instrument built to turn sub-\(10\) meV neutrino-mass physics into an experimental question.

Source: https://www.emergentmind.com/topics/legend-1000