Jaduguda Underground Science Lab (JUSL)

• JUSL is an underground science laboratory providing a controlled, ultra-low-background environment ideal for rare-event searches such as dark matter and neutrinoless double beta decay.
• It employs advanced GEANT4 simulations and multilayer shielding to accurately characterize and suppress cosmic ray, gamma, and neutron backgrounds.
• The facility supports dark matter experiments like InDEx, achieving competitive sensitivity limits through meticulous background measurement and optimized detector configurations.

The Jaduguda Underground Science Laboratory (JUSL) is a deep underground research facility constructed within an existing mine in Jaduguda, India, with the primary objective of providing an ultra-low-background environment for rare-event search experiments. Central to its design is the need for rigorous suppression and accurate characterization of ambient radioactivity—including cosmic-ray muons, neutron backgrounds (radiogenic and cosmogenic), and gamma rays—fundamental for probing phenomena such as dark matter direct detection and neutrinoless double beta decay. The laboratory is at 555 m depth (1600 m water equivalent), enabling substantial reduction of cosmic-ray–induced backgrounds, a prerequisite for next-generation experiments targeting Weakly Interacting Massive Particles (WIMPs) and other elusive particles.

1. Laboratory Structure and Radiation Shielding Environment

JUSL is situated at a vertical rock overburden of 555 m, converting to approximately 1600 m water equivalent, and features a cavern modeled as a cubic hollow structure (outer dimension 8 m, inner cavity 4 m) enveloped by a 1 m thick rock shell for simulation purposes (Banik et al., 2020). This deep setting is engineered for maximal suppression of incoming cosmic particles, permitting only the most energetic muons (Eμ ≳ 300–400 GeV) to penetrate. The surrounding rock serves dual functions: it both generates and attenuates neutron backgrounds via (α,n) reactions and spontaneous fission from uranium/thorium impurities, as well as moderating cosmic-induced secondary fluxes.

Neutron backgrounds are of two principal classes:

• Radiogenic neutrons: Arising from (α,n) interactions and spontaneous fission associated with uranium/thorium decay in the rock (e.g., 8 ppm U and 16 ppm Th, with neutron yields 6.77 ± 1.12 n/yr/g for ²³⁸U and 5.33 ± 0.90 n/yr/g for ²³²Th; spontaneous fission modeled by the Watt spectrum).
• Cosmogenic neutrons: Generated by high-energy muon interactions within the geological overburden, with a measured muon flux inside the cavern of 4.49(±0.25)×10⁻⁷ cm⁻² s⁻¹, and cosmogenic neutron flux above 1 MeV of 7.25(±0.65)×10⁻⁹ cm⁻² s⁻¹ (Banik et al., 2020).

Comprehensive characterization indicates the laboratory's background levels to be on par with canonical rare-event search locations such as DAMA, WIPP, and Boulby mine, validating its suitability for high-sensitivity research.

2. Experimental Characterization of Ambient Backgrounds

Dedicated measurements and simulations have provided quantitative profiles of gamma ray, muon, and neutron backgrounds at JUSL (Ghosh et al., 2021):

• Gamma ray flux: Dominated by primordial radionuclides and their decay products in the rock for $E_\gamma \lesssim 3$ MeV. Gamma-ray spectra, measured with a CsI(Tl) scintillator, revealed discrete contributions from $^{40}$K, $^{238}$U, and $^{232}$Th chains. GEANT4 simulations established that a passive lead shield (∼30 cm) can yield suppression factors up to $10^{-6}$, a crucial design criteria for backgrounds near the $Q$-value relevant to double beta decay and nuclear recoil studies.
• Muon flux: Directly measured using large-area plastic scintillator telescopes, the integrated muon flux underground was $$(2.051\pm0.142\pm0.009)\times10^{-7}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$. Simulations employing the Gaisser parameterization confirmed agreement with these values and elucidated the zenith-angle dependence ($\cos^n\theta$ with $n\approx3.76$).
• Neutron flux: Fast neutron flux measured as $$(9.93\pm0.22\pm0.10)\times10^{-5}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$ for ∼0.1–1 MeV threshold, thermal neutron flux $$(6.15\pm0.18)\times10^{-5}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$, and total radiogenic neutron flux $$(1.61\pm0.03)\times10^{-4}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$ for no threshold cut.

Multiple scattering and backscattering effects within the rock and cavern walls were included via GEANT4 modeling, demonstrating that these mechanisms can nearly double the neutron flux observed in the detector region (∼30% contribution at 1 MeV threshold).

3. Simulation Frameworks and Methodologies

Radiogenic and cosmogenic neutron backgrounds were modeled using GEANT4 (version 10.02), leveraging the "Shielding" physics list for high-precision neutron transport (Banik et al., 2020). Key simulation techniques include:

• Generation of neutron energy spectra based on rock composition and radioactivity.
• Modeling muon energy and angular distributions via Gaisser parameterization, including lateral displacement and energy loss mechanisms through rock overburden.
• Event generation at cavern boundaries and tracking into the detector region.
• Validation of simulation outputs against experimental data for muon and neutron fluxes, including the use of fitted analytical spectral models:

$$\frac{d\Phi_n}{dE_n} = \sum_{j=1}^2 c_j \exp[-\beta_j (\ln E_n)^2 + \gamma_j \ln E_n ]$$

These methodologies facilitate optimization of shielding designs, inform experimental configurations, and ensure comparability with international standards in underground physics.

4. Shielding Strategies and Detector Sensitivity

Shielding optimization is critical for neutron and gamma suppression, as elaborated in both simulation and experimental studies. Passive shields using multilayer combinations (polypropylene for neutron moderation, lead for gamma attenuation, with additional polypropylene to capture neutrons produced within lead) were investigated (Banik et al., 2020). The optimal configuration—40 cm polypropylene, 30 cm lead, and another 20 cm polypropylene—demonstrated significant reduction in effective cosmogenic neutron transmission. For gamma rays, simulations indicated that a hollow lead box of ∼30 cm thickness can reduce background by six orders of magnitude (Ghosh et al., 2021).

Sensitivity studies using CsI and superheated droplet detectors (see below) incorporate these shielding results to estimate nuclear recoil event rates and project limits on WIMP-nucleon interactions, as exemplified by the Lewin and Smith formalism.

5. Dark Matter Search Experiments: The InDEx Campaign

JUSL hosts the Indian Dark matter search Experiment (InDEx), utilizing superheated droplet detectors (SDDs) with tetrafluoroethane ($\mathrm{C_2H_2F_4}$) as the active liquid. This technology is predicated on bubble nucleation induced by sufficient nuclear recoil energy after WIMP or neutron interactions, offering insensitivity to electromagnetic backgrounds (Kumar et al., 6 Jan 2025, Kumar et al., 31 Aug 2025, Das et al., 8 Oct 2025).

InDEx Run 1 (Kumar et al., 6 Jan 2025, Kumar et al., 31 Aug 2025)

• 500 ml detector with 2.47 kg-days exposure at a threshold of 5.87 keV.
• Acoustic signals from bubble nucleation captured via piezoelectric sensor and FPGA-based DAQ.
• Sensitivity limits:
  • Spin-independent cross-section for C: $\sigma_{SI}^{(C)} = [7.834 \pm 0.370_{stat} (^{+2.005}_{-1.241}_{sys})] \times 10^{-38}\ \mathrm{cm}^2$ at WIMP mass 22.81 GeV/$c^2$.
  • Spin-dependent (proton) for F: $\sigma_{SD}^{(F)} = [3.782 \pm 0.179_{stat} (^{+0.655}_{-0.432}_{sys})] \times 10^{-36}\ \mathrm{cm}^2$ at 30.67 GeV/$c^2$.
  • Minimum WIMP mass sensitivity: 4.44 GeV/$c^2$ (C), 5.16 GeV/$c^2$ (F).

InDEx Run 2 (Das et al., 8 Oct 2025)

• Expanded active mass (70.4 g, two detectors), operated at 35.0°C, threshold of 1.95 keV, total exposure 7.2 kg-days over 102.48 days.
• Constraints (90% CL):
  • Spin-independent: $(1.55^{+0.62_{stat}}_{-0.32}^{+0.03_{sys}}_{-0.02}) \times 10^{-40}\ \mathrm{cm}^2$ at 20.4 GeV/$c^2$.
  • Spin-dependent (proton): $(7.97^{+3.44_{stat}}_{-1.78}^{+0.15_{sys}}_{-0.18}) \times 10^{-38}\ \mathrm{cm}^2$ at 21.0 GeV/$c^2$.

Lowering the threshold produced improved exclusion limits and shifted the most sensitive WIMP mass toward lower values. Event analysis employed statistical inference via a profile likelihood ratio, as detailed in the paper.

Detector Principle and Performance

SDDs with C₂H₂F₄ provide sensitivity to multiple target nuclei (C, F, H). Operating temperature directly determines the nucleation threshold, with future plans to reach ∼0.19 keV at 55°C—opening sensitivity below the GeV mass region. Data acquisition integrates real-time acoustic signal collection, event discrimination using summed amplitude (Pvar), and calibration against neutron sources ($^{124}$AmBe).

6. Implications for Rare Event Searches and Future Prospects

JUSL’s comprehensive background assessment and shielding optimization support not only direct dark matter searches but broader rare-event physics, including neutrinoless double beta decay and axion experiments. The laboratory's metrics:

• Gamma suppression by lead up to $10^{-6}$,
• Muon flux at $$(2.051\pm0.142\pm0.009)\times10^{-7}\,\mathrm{cm}^{-2}\,\mathrm{s}^{-1}$$,
• Neutron backgrounds well-characterized via experiment and simulation, demonstrate its capability for ultra-low-background operation.

Recent improvements—from lowering detector thresholds to expanding exposures—are shifting the sensitivity toward low-mass dark matter candidates and the potential to approach nearly "zero background" conditions by increasing shielding, purifying materials, and controlling radon. Future runs are projected to scale up detector mass and exposure (towards 1000 kg-days), probe the MeV mass regime, and establish competitive benchmarks for WIMP searches using superheated liquids.

7. Summary Table: Key Metrics for JUSL and InDEx

Parameter	Measured Value	Context/Reference
Rock Overburden	555 m (1600 m w.e.)	Lab geometry (Banik et al., 2020)
Muon Flux (Underground)	~2.05×10⁻⁷ cm⁻² s⁻¹	Exp./sim (Ghosh et al., 2021)
Radiogenic Neutron Flux (>1 MeV)	5.75×10⁻⁶ cm⁻² s⁻¹	Sim (Banik et al., 2020)
Cosmogenic Neutron Flux (>1 MeV)	7.25×10⁻⁹ cm⁻² s⁻¹	Sim (Banik et al., 2020)
Pb Shielding Suppression	~10⁻⁶ (30 cm)	Gamma rays (Ghosh et al., 2021)
InDEx Run2 Threshold	1.95 keV	SDDs (Das et al., 8 Oct 2025)
InDEx Run2 Exposure	7.2 kg-days	SDDs (Das et al., 8 Oct 2025)
SI Limit (Fluorine, Run2)	1.55×10⁻⁴⁰ cm² at 20.4 GeV/$c^2$	(Das et al., 8 Oct 2025)
SD Limit (Proton, Run2)	7.97×10⁻³⁸ cm² at 21.0 GeV/$c^2$	(Das et al., 8 Oct 2025)

JUSL exemplifies a well-quantified, technically advanced underground research facility for rare-event searches. Detailed studies—spanning measurements, simulations, and active experimental campaigns—support its status as a competitive site within the global landscape of astroparticle physics.