Jaduguda Underground Science Laboratory

• Jaduguda Underground Science Laboratory is a deep-underground facility inside a uranium mine, designed for rare-event physics experiments with ultra-low backgrounds.
• The laboratory integrates detailed ambient radiation characterization with GEANT4 simulations to optimize shielding for dark matter and neutrino studies.
• InDEx, the initial dark matter search at JUSL, employs a superheated emulsion detector to achieve competitive sensitivity and guide future experimental upgrades.

Jaduguda Underground Science Laboratory (JUSL) is an Indian deep-underground research facility established inside an operating uranium mine with 555 m of vertical rock overburden (equivalent to approximately 1.6 km water equivalent). The laboratory is designed for rare-event physics experiments requiring ultra-low backgrounds, such as direct dark matter detection, neutrinoless double beta decay, axion searches, and supernova neutrino studies. The comprehensive characterization and simulation of ambient radiation backgrounds—including gamma rays, cosmic muons, and neutrons—frame its strategic significance in the global underground laboratory network.

1. Physical Environment and Background Characterization

JUSL leverages its location deep beneath the Earth’s surface to minimize cosmic-ray-induced backgrounds. The vertical rock overburden of 555 m drastically suppresses the cosmic muon flux and provides a controlled environment for rare-event searches. Ambient radiation backgrounds measured within the facility include:

• Gamma-ray background: Dominated by primordial radionuclides ($^{40}$K, $^{238}$U, $^{232}$Th) and their decay chain products in the host rock, with $E_\gamma \lesssim 3~\mathrm{MeV}$ being the primary component. In situ measurements using CsI(Tl) scintillator arrays reveal distinct spectral peaks corresponding to these isotopes (Ghosh et al., 2021).
• Muon flux: Experimental measurement yields a flux of $$(2.051 \pm 0.142 \pm 0.009)\times 10^{-7}\ \mathrm{cm}^{-2}\mathrm{s}^{-1}$$, with an average energy near 186 GeV and a zenith-angle distribution approximated by $\cos^{3.756}\theta$ (Ghosh et al., 2021).
• Neutron background: Measured radiogenic neutron flux is $$(1.61 \pm 0.03)\times 10^{-4}\ \mathrm{cm}^{-2}\mathrm{s}^{-1}$$ (no threshold cut). Radiogenic and cosmogenic contributions are distinguished using both detector measurements and detailed GEANT4 simulations (Banik et al., 2020, Ghosh et al., 2021).

A principal feature of the laboratory is its systematic workflow of integrating experimental measurements with Monte Carlo simulations, utilizing the GEANT4 toolkit, enabling precise modeling of particle propagation, multiple scattering, and backscattering within the rock and laboratory cavern.

2. Neutron Backgrounds: Sources, Simulation, and Mitigation

Neutron backgrounds at JUSL arise primarily from two mechanisms:

• Radiogenic neutrons: Generated via $(\alpha,n)$ reactions from trace uranium and thorium in the rock and by spontaneous fission of $^{238}$U. Simulations (employing the GEANT4 "Shielding" physics list, FTFP_BERT processes and high-precision neutron models) quantify neutron yields as $6.77\pm1.12~\mathrm{yr}^{-1}\mathrm{g}^{-1}$ for $(\alpha,n)$ (from $^{238}$U), with $3.43\pm0.55~\mathrm{yr}^{-1}\mathrm{g}^{-1}$ from spontaneous fission (Banik et al., 2020). The neutron energy spectrum for fission is modeled using the Watt function:

$$W(a,b,E') = a \sqrt{\frac{4a}{\pi b}} \exp\left(-\frac{b}{4a} - aE'\right) \sinh\left(\sqrt{bE'}\right)$$

with $a=1.54245~\mathrm{MeV}^{-1}$, $b=6.81057~\mathrm{MeV}^{-1}$.

• Cosmogenic neutrons: Produced by energetic muons ($E_\mu > 300~\mathrm{GeV}$) penetrating the overburden, driving spallation, muon capture, and hadronic showers. The muon flux is simulated via Gaisser's parameterization, tracking propagation through the rock and interaction-induced neutron yields (Banik et al., 2020, Ghosh et al., 2021).

Simulations reveal fluxes of radiogenic neutrons above 1 MeV at $5.75(\pm0.58)\times10^{-6}$ cm$^{-2}$s$^{-1}$ and cosmogenic neutrons at $7.25(\pm0.65)\times10^{-9}$ cm$^{-2}$s$^{-1}$ (Banik et al., 2020). These values are closely comparable with results from other prominent facilities (DAMA, WIPP, Boulby), reinforcing the suitability of JUSL for rare-event searches.

Mitigation strategies include the use of composite passive shielding:

• Polypropylene (hydrogen-rich moderator): 40 cm thickness stops low-energy radiogenic neutrons.
• Lead (gamma attenuation but neutron producing under muon bombardment): 30 cm intermediate layer.
• Multilayer optimization: Arrangements such as [Polypropylene (40 cm) | Lead (30 cm) | Polypropylene (20 cm)] minimize transmission and suppress secondary neutron production in shielding (Banik et al., 2020).

3. Detector Technologies and Initial Results

The first direct dark matter search at JUSL is the Indian Dark matter search Experiment (InDEx), employing a superheated emulsion detector (SED) with droplets of tetrafluoroethane ($\mathrm{C_2H_2F_4}$) in a degassed gel matrix (Kumar et al., 31 Aug 2025). The superheated liquid enables acoustic discrimination of bubble nucleation events caused by energy deposition above the set threshold.

• Operating threshold: 5.87 keV (bubble nucleation).
• Exposure: 2.47 kg-days over 48.6 days.
• Sensitivity: Minimum spin-independent (SI) WIMP-nucleon cross-section for fluorine [7.939 $\pm$ (0.375)$_\text{stat}$ ($^{+1.386}_{-0.909}$)$_\text{sys}$] $\times 10^{-39}$ cm$^2$ at WIMP mass 30.67 GeV/$c^2$.

Enhanced future exposures (planned up to 100 kg-days) with lower thresholds (~0.19 keV at 55°C) are expected to extend sensitivity to hydrogen recoils and low-mass WIMPs under near-zero background conditions (Kumar et al., 31 Aug 2025). The acoustic analysis and underground setting suppress cosmogenic backgrounds, positioning InDEx competitively within global searches.

4. Facility Requirements and Adaptability for Next-Generation Experiments

For participation in the international push toward next-generation dark matter searches, JUSL must meet rigorous infrastructural standards (Cooley et al., 2022):

• Space requirements: Large experimental halls, high ceilings, and wide floor areas are essential, particularly for noble liquid (Xe, Ar) experiments and cryogenic installations such as 10 m$^3$ LN$_2$ tanks and vertical cryogenic distillation columns (up to 5.5 m tall).
• Staging and assembly areas: Cylinder farms for storage of target gases at room temperature, radon-free underground cleanrooms (<100 mBq/m$^3$), and extensive mechanical supports.
• Environmental controls: Vibration isolation (target $10^{-7}$ g/$\sqrt{\text{Hz}}$ for cryogenic bolometers), electromagnetic noise mitigation, and heat management (cryogenic columns typically release tens of kilowatts into the hall).
• Shielding versatility: Facilities must accommodate active veto systems (e.g., gadolinium-doped liquid scintillator, water layers, argon vetoes) for tagging of muon-induced events.

A plausible implication is that JUSL’s demonstrated low backgrounds, depth, and the capacity for configurable shielding position it to host not only SED-based searches but also future noble liquid, cryogenic bolometer, and CCD/point-contact Ge-based low-mass dark matter experiments if infrastructure requirements are realized (Cooley et al., 2022).

5. Simulation Technologies and Methodological Rigor

The experimental campaigns at JUSL integrate comprehensive Monte Carlo modeling with in situ measurement. GEANT4 is employed for particle tracking, interaction physics, and generation of input spectra (e.g., muons via Gaisser's formula, neutron energy via Watt and analytical fits):

• Particle propagation: Through cavern geometry and multi-material shields.
• Multiple scattering and backscattering: Shown to nearly double neutron flux inside the cavern under zero-threshold conditions; for thresholds $\gtrsim$1 MeV, the increase is ~30% (Ghosh et al., 2021).
• Analytical fits: Cosmogenic neutron spectra fitted by

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

with data-driven coefficients (Ghosh et al., 2021).

The close correspondence between simulations and experimental data underpins reliability for background estimation, shield design, and sensitivity projections for rare-event detection (Banik et al., 2020, Ghosh et al., 2021).

6. Scientific Impact and Future Directions

JUSL contributes a well-characterized, competitive underground venue for rare-event searches, with measured and simulated muon, neutron, and gamma-ray backgrounds directly comparable to leading laboratories. Initial InDEx results at moderate WIMP masses, along with background suppression strategies and planned infrastructural upgrades, underscore the laboratory’s potential for enhanced sensitivity.

Its integration with global methodologies—extending from passive and active shielding engineering to advanced detection and background modeling—positions it as an asset for pursuing diverse dark matter candidates and other rare-event processes. Ongoing and future plans (expansion of exposure, reduction of energy thresholds, support for next-generation detector infrastructure) suggest that JUSL is poised for substantial contributions, contingent on sustaining its technical and environmental standards commensurate with the evolving demands of underground physics research.