Yemi Underground Laboratory (Yemilab)

• Yemilab is a state-of-the-art underground laboratory located beneath Mount Yemi, optimized for rare-event searches in neutrino physics, dark matter, and BSM research.
• The facility employs advanced geological engineering and infrastructure, including NATM tunneling and reinforced shotcrete, to ensure ultra-low background conditions.
• Yemilab hosts major experiments such as AMoRE-II, COSINE, and IsoDAR, driving breakthroughs in neutrino measurements and new physics explorations.

The Yemi Underground Laboratory (Yemilab) is Korea's premier deep underground science facility, designed to enable rare-event searches in neutrino physics, dark matter, double beta decay, and beyond-Standard-Model (BSM) physics. Located beneath Mount Yemi in Jeongseon, Gangwon Province, Yemilab provides up to 2,700 meters water-equivalent (m.w.e.) overburden, housing a large multi-room laboratory complex including a dedicated 6,300 m³ cylindrical cavern for kiloton-scale detectors. The laboratory serves as the principal site for experiments such as AMoRE-II (0νββ decay), COSINE (dark matter), the IsoDAR accelerator-driven neutrino source, and the LSC liquid scintillator detector. Since its completion in 2022, Yemilab has enabled best-in-class backgrounds for cutting-edge rare-event experiments and is positioned to serve as a major international center for underground physics.

1. Site Location, Geological Setting, and Infrastructure

Yemilab is situated on the southern flank of Mount Yemi (989 m peak) in Jeongseon-gun, Gangwon Province, Korea. The main experimental caverns are accessed via a 600 m vertical shaft and a 782 m, 5 × 5 m cross-section sloped tunnel (12% grade), leading to an experimental floor up to 1,029 m below surface (≃2,500–2,700 m.w.e.) (Park et al., 2024, Alonso et al., 2021). The host rock is predominantly Makdong limestone, with RMR grades 2–3, and was selected for its low uranium/thorium content (∼0.8 ppm U, 3.3 ppm Th (Alonso et al., 2021)) and minimal radon out-gassing relative to typical hard-rock sites.

The laboratory consists of 17 modular experimental rooms plus a 20 m diameter × 20 m deep cylindrical pit for large neutrino detectors. The exclusive scientific footprint is ~3,000 m², with overall excavation volume ~65,000 m³ (Park et al., 2024). Depth and rock composition are optimized to minimize cosmogenic backgrounds and rock-induced radiogenic neutrons and gamma rays. The ambient muon flux at depth (2,500 m.w.e.) is measured at 8.2 × 10⁻⁸ μ/cm²/s, a ×4 reduction relative to the Yangyang Underground Laboratory (Y2L) (Park et al., 2024, Seo et al., 2023).

Utilities include 1,600 kW (2,000 kVA) electrical capacity (with UPS and backup generators), forced ventilation (Q=39,000 m³/h main duct), Class 1,000 cleanroom facilities, comprehensive dust and water control, and 1 Gbps fiber-optic communications (Park et al., 2024, Alonso et al., 2021).

2. Background Environment: Neutron and Radon Mitigation

Neutron Backgrounds

Neutron measurements conducted at three locations in the main tunnel and halls show total neutron fluence rates in the (3.24 ± 0.11)–(4.01 ± 0.10) × 10⁻⁵ cm⁻²s⁻¹ range, with thermal components (E < 0.5 eV) between (1.32 ± 0.05)–(1.51 ± 0.05) × 10⁻⁵ cm⁻²s⁻¹ and fast neutrons (E = 1–10 MeV) at (0.27 ± 0.03)–(0.34 ± 0.10) × 10⁻⁵ cm⁻²s⁻¹ (Kim et al., 23 Jan 2026). The neutron spectrum reveals a substantial contribution from (α, n) processes in both host rock and shotcrete linings, with spatial variation linked to shotcrete U/Th concentration and seasonal humidity. These levels are comparable to Canfranc and CJPL but higher than LNGS/Modane, directly informing detector shielding strategies using polyethylene, borated plastic, and multi-layer passive and active vetoes (Kim et al., 23 Jan 2026, Seo et al., 2023).

Radon Backgrounds and Control

Ambient radon levels in the AMoRE hall average 17.6 ± 0.3 Bq/m³, with seasonal excursions up to ~2,000 Bq/m³ (Seo et al., 2024, Park et al., 2024). A dedicated Radon Reduction System (RRS) delivers 50 m³/h radon-suppressed air using a multi-stage (compressor-dryer-charcoal) system, reliably achieving ≥300× reduction (down to 0.058 ± 0.004 Bq/m³ in the cleanroom) and meeting the <0.29 Bq/m³ specification required for AMoRE crystal assembly (Seo et al., 2024). Verification is performed using a bespoke 70 L PIN photodiode radon detector with intrinsic background 23.8 ± 2.1 mBq/m³ and 72 keV FWHM alpha energy resolution at 6.0 MeV (Seo et al., 2024). The approach supports scaling to other experimental halls and is a core part of Yemilab’s low-background infrastructure.

3. Laboratory Architecture and Safety Systems

Yemilab employs the New Austrian Tunneling Method (NATM) with staged blasting, rock-bolt/shotcrete reinforcement, and deformation monitoring. All critical rooms use RMR-3 grade reinforcement, dual egress (for rooms >20 m), a 40-person mine refuge, and comprehensive fire and evacuation modeling (Park et al., 2024). The LSC pit (20 m × 20 m cylinder, V ≈ 6,300 m³) is engineered to support multi-kiloton detectors. Support infrastructure includes:

• Forced ventilation with negative pressure zones, dedicated exhaust, and HEPA/activated-carbon filtration.
• Automated groundwater drainage (yield ≈ 4 t/d).
• Access control, integrated personnel/equipment shafts, and staged heavy-equipment lanes.
• Distributed power (dynamic load sharing) and environmental monitoring throughout.

Shotcrete and structural supports are selected based on in situ stress and monitored for convergence and safety margins (Park et al., 2024). Regulatory compliance covers KGS, ASME BP&V, and ICRP 103 standards (Alonso et al., 2021).

4. Experimental Program and Detector Platforms

Core Experiments

Experiment	Target/Detector	Science Focus	Start (planned/actual)
AMoRE-II	160 kg Li₂MoO₄ (cryogenic bolometer, T ≈ 10 mK)	0νββ of ¹⁰⁰Mo	Q2 2024 (Park et al., 2024)
COSINE-100U/-200	NaI(Tl) crystals + LS, cooled shield	Dark matter (annual mod.), low-mass DM	2024–2025 (relocated from Y2L)
LSC (Liquid Scintillator Counter)	2.26 kt LS / WbLS cylinder (D=15 m, H=15 m)	Solar geo-, reactor-, supernova-ν, BSM	2024–onward (Seo et al., 2023)
IsoDAR@Yemilab	60 MeV/amu cyclotron, Be+⁷Li target, 17 m to LSC	High-statistics ν̄ₑ, sterile neutrinos, NSI, new physics	2024–2026 (Alonso et al., 2021, Alonso et al., 2021)

The LSC is planned as a three-layer system: an inner acrylic vessel of 2.26 kt LS or WbLS (linear alkylbenzene + PPO + bis-MSB, or ~1% organic scintillator in H₂O), a 1 kt mineral oil buffer (D=17 m, H=17 m) with 3,000–4,000 20” PMTs (49–65% optical coverage), and a 2.4 kt outer water veto with ~200 PMTs (Seo et al., 2023). Purity goals match or exceed Borexino (U/Th < 10⁻¹⁷ g/g). Fiducial mass and optical coverage are optimized for ultra-low backgrounds in solar, reactor, geoneutrino, and supernova neutrino detection, and for coincident BSM physics searches with accelerator and radioactive source deployments (Seo et al., 2023, Seo, 2019).

Technology and R&D Focus

• WbLS technology (1–10% LS in water) provides simultaneous sensitivity for Cherenkov and scintillation light; decisive for directional and low-threshold physics (Seo, 2019).
• Advanced PMTs (Hamamatsu H11780, TTS ≃ 1.2 ns, QE ≃ 34%) and LAPPDs (TTS ≈ 60 ps, <1 cm granularity) for timing, vertexing, and signal-background separation.
• Gas stripping, multi-stage distillation, water extraction, and adsorber columns used in LS/WbLS purification; staged calibration with radioactive, beam, and optical sources (Seo et al., 2023).

5. Physics Reach and BSM Capabilities

Neutrino Measurements

• Solar neutrinos: <0.5% uncertainty on pp flux, 1% on ⁷Be, 5% on ⁸B after 5 years; energy threshold ≈ 200 keV (LS) enables pp, Be7, pep, and CNO measurements (Seo et al., 2023). Directional and delayed-coincidence cuts for background suppression.
• Reactor ν̄̄: 1,950 IBD events/year, mainly from the Hanul complex (65 km). L/E oscillation spectral analysis enables precision Δm²₂₁, θ₁₂ extraction (Seo et al., 2023, Alonso et al., 2021).
• Geoneutrinos: ∼60 IBD/year and ∼820 ν-e ES events/year constrain Earth's U/Th content (Seo et al., 2023).
• Supernova and DSNB: 430–820 events for 10 kpc core-collapse, sensitivity to supernova relic ν's in 12–30 MeV (Seo et al., 2023).

BSM Sensitivities (with LSC + IsoDAR/linac/source)

• Sterile neutrino searches: oscillation-wave mapping in L/E over 9–27 m/MeV, >5σ coverage of 3+1/3+2/decay models (Alonso et al., 2021, Seo et al., 2023).
• Nonstandard interactions (NSI): ν̄ₑ–e scattering, 1.9% measurement of sin²θ_W, εee^L,R reach >5× improvement over existing bounds.
• Dark photon searches (e-linac+LSC): ε² exclusion down to ~10⁻¹⁶ (visible A′ decays) and ~10⁻¹³ (oscillation, m_{A′} < 2mₑ) (Seo et al., 2020, Seo et al., 2023).
• ALPs and new bosons: Primakoff/Compton processes in linac+LSC, g_{aγ}~10⁻⁷–10⁻⁹ GeV⁻¹ reach over MeV masses (Seo et al., 2023).
• Light dark matter: sensitivity parameter Y=ε²αD(mχ/m_A′)^4 down to 10⁻¹⁶–10⁻¹⁰ (Seo et al., 2023).

A summary of performance metrics and backgrounds for key detector units is tabulated below:

Detector	LS/WbLS Mass	Target Physics	Muon Flux (cm⁻²s⁻¹)	Ambient Radon (Bq/m³)	Neutron Flux (cm⁻²s⁻¹)
LSC (Yemilab)	2.26 kt	Solar/geo/reactor/SN/BSM	8.2×10⁻⁸	<0.06 (RRS+cleanroom)	(3.2–4.0)×10⁻⁵ (total)
AMoRE-II	—	0νββ, rare decay	8.2×10⁻⁸	<0.06 (RRS)	(appropriate for search)
WbLS prototype	~1–5 kt	R&D, directional ν, BSM	8.2×10⁻⁸	<0.06 (targeted)	(comparable)

6. Construction Timeline and Outlook

• Construction of the primary access tunnel and shaft was completed by August 2020; LSC pit excavation finalized by August 2022 (Park et al., 2024).
• Scientific fit-outs (crane rails, cleanroom, services) proceeded through 2023, with the staged relocation of facilities from Y2L to Yemilab by end of 2024.
• IsoDAR cyclotron, target caverns, and shielding are fully excavated, with system installation and integration aligned for commissioning and physics data starting in 2024–2026 (Alonso et al., 2021, Alonso et al., 2022, Alonso et al., 2021).
• Yemilab is designed for staged upgrades and programmatic expansion: modular halls and the central pit permit new detectors, expanded cryogenics, and next-generation platforms.

Yemilab combines multi-kiloton detector capacity, deep radiopurity, advanced environmental control, and accelerator access in a single facility. Its scientific output is positioned to inform neutrino and rare-event research at the international level, with infrastructure and operational data serving as a model for future global underground laboratories (Park et al., 2024, Seo et al., 2023, Alonso et al., 2022).