China Jinping Underground Laboratory (CJPL)

• China Jinping Underground Laboratory (CJPL) is a deep underground facility designed for ultra-low-background experiments in dark matter and neutrino physics.
• The facility employs rigorous environmental background characterization and material screening techniques to ensure optimal conditions for rare-event research.
• CJPL-II’s expanded infrastructure and advanced detector technologies significantly improve the sensitivity of experiments targeting rare events.

The China Jinping Underground Laboratory (CJPL) is a deep underground research infrastructure in Sichuan, China, purpose-built for ultra-low-background physics, including rare-event searches in dark matter and neutrino physics. With a vertical rock overburden of approximately 2400 meters, CJPL provides one of the most radiopure and cosmogenically shielded environments for experiments worldwide, particularly in its expanded second phase (CJPL-II). Key technical advances at CJPL include comprehensive environmental background characterization, rigorous material selection for radiopurity, and deployment of next-generation detector technologies for rare-event physics.

1. Laboratory Structure and Environmental Backgrounds

CJPL is structured in two main phases. The initial phase (CJPL-I) provides moderate laboratory space with full shielding from 2400 m of rock. The expanded phase (CJPL-II) increases available volume dramatically, with new halls such as Hall-C specifically designed to accommodate large-scale experiments (e.g., the CDEX-100 array). The laboratory's overburden yields a cosmic-ray muon flux measured as:

$$\phi_\mu = (2.0 \pm 0.4) \times 10^{-10}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}$$

This flux is among the world's lowest, reducing cosmogenic neutron production and long-lived activation to negligible levels, which is vital for rare-event search backgrounds (Wu et al., 2013, Guo et al., 2020, Zhang et al., 6 Sep 2024).

Environmental radioactivity is controlled both through selection of low-activity construction materials and through comprehensive in-situ measurements. For example, Hall-C concrete walls have been measured to exhibit concentrations of:

$${}^{238}\mathrm{U}:~6.8 \pm 1.5~\mathrm{Bq/kg}, \quad {}^{232}\mathrm{Th}:~5.4 \pm 0.6~\mathrm{Bq/kg}, \quad {}^{40}\mathrm{K}:~81.9 \pm 14.4~\mathrm{Bq/kg}$$

These values are lower than CJPL-I and lower or comparable to other international underground laboratories, reflecting effective material procurement and screening (Ma et al., 2020).

Radon control is integral. In CJPL-I, annual mean radon concentrations in the main experiment hall, under continuous ventilation, have been maintained at (53–58) Bq/m³, with spatial variations determined by ventilation efficacy (Liu et al., 2018). In the absence of mitigation (e.g., in storage tunnels), radon can reach ~345 Bq/m³. This level is intermediate among global labs but sufficient for most rare-event physics.

2. Gamma Background Measurement and Simulation Methodology

In-situ gamma-ray background assessment in CJPL-II is carried out using a portable high-purity germanium (HPGe) spectrometer. Ambient radon, a significant contributor to airborne gamma backgrounds, is continuously monitored and subtracted from the wall radioactivity measurements using an AlphaGUARD monitor. The HPGe detector's efficiency for wall-emitted gammas is calculated via explicit numerical integration relying on the Beck formula, incorporating detector angular response and geometric factors. Detection efficiency for air-originated gammas (from radon progeny) is obtained using Monte Carlo simulation in Geant4.

Detection efficiencies, ε, at selected energies (keV, conversion factors provided in the original work) are as follows:

Energy (keV)	Wall (cts/(Bq/kg) ×10⁻²)	Radon Air (cts/(Bq/m³) ×10⁻³)
295.4	2.29	2.5
351.9	4.37	4.27
609.3	4.76	3.29
1460.8	0.65	N/A

Radioactivity concentrations are then inferred by relating peak gamma counts to activities, with explicit subtraction of the radon contribution:

$C = \frac{R - C_{Rn}V\varepsilon}{F_{loc}}$

where $R$ is the observed peak rate, $C_{Rn}$ is radon concentration, $V$ is the hall volume, and $F_{loc}$ is the numerical integration factor dependent on geometry and attenuation (Ma et al., 2020).

3. Simulated Impact on Rare-Event Experimental Backgrounds

The empirical measurement of environmental backgrounds directly informs the design of next-generation experiments, specifically the CDEX-100 detector array (∼100 kg Ge mass), to be installed in Hall-C within a 6.5 m thick liquid nitrogen shielding tank.

The expected gamma-induced background in the CDEX-100 germanium experiment from primordial radionuclides (assuming secular equilibrium in wall activity) has been simulated for both light dark matter search ROI (2–4 keV) and neutrinoless double-beta decay search window (∼2 MeV):

Isotope	2–4 keV (cpkty)	2 MeV region (cpkty)
$^{238}$U	0.25	$1.5 \times 10^{-2}$
$^{232}$Th	0.38	$3.1 \times 10^{-2}$
$^{40}$K	$5.3 \times 10^{-4}$	negligible
Total	0.63	$4.6 \times 10^{-2}$

Here, cpkty = counts per keV per ton per year.

Total background levels for these energy windows, strictly from primordial wall radioactivity, are:

• $0.63~\mathrm{cpkty}$ (2–4 keV)
• $4.6\times10^{-2}~\mathrm{cpkty}$ (2 MeV region)

These values are notably lower than for the CJPL-I era, reflecting a greater than tenfold improvement due to superior materials selection and construction control (Ma et al., 2020).

4. Significance for Experimental Sensitivity and Facility Benchmarking

CJPL-II’s Hall-C provides an ultra-low environmental gamma background, among the best in deep underground laboratories worldwide. This minimizes the need for additional passive shielding and enables sensitivity to extremely rare event rates, imperative for low-threshold direct dark matter and neutrinoless double-beta decay searches.

The background achieved is not only the result of depth, but of active engineering—explicit material screening and clean construction procedures are demonstrably effective. For example, the Hall-C wall concentrations are significantly lower than those observed in other major labs, directly improving sensitivity and statistical reach of experiments under identical exposure and detector technology constraints (Ma et al., 2020).

5. Technical Advances and Equations Governing Background Inference

Quantitative background inference depends on a precise relationship between source radioactivity and detected gamma lines. The detection efficiency is computed with:

$$\frac{N_f}{A} = f(E, \theta) = \frac{N_f}{N_0} \times \frac{N_0}{\phi} \times \frac{\phi}{A}$$

where $N_f$ is detected counts, $A$ is source activity, $f(E,\theta)$ incorporates spectral and angular dependencies, and $\phi$ is gamma flux at the detector. For distributed sources (wall), the calculation integrates over hall surfaces and includes self-attenuation through the concrete lining (Ma et al., 2020).

For contributions from airborne radon, detailed Geant4 Monte Carlo incorporates source distribution in the hall volume and detector response, accounting for attenuation and scattering. Subtraction techniques ensure that wall concentration determination is unbiased by this persistent background.

6. Implications for Experimental Design and Future Prospects

With a measured gamma integrated count rate of 46.8 cps (60–2700 keV) in Hall-C, and wall U/Th/K concentrations amongst the lowest globally, future next-generation rare-event experiments at CJPL will operate under conditions favorable for probing new parameter spaces.

Planned or deployed arrays such as CDEX-100 will benefit from both the reduced radiogenic gamma background and the thorough predictive modeling enabled by the calibrated measurement and simulation approaches established at CJPL-II. The low environmental background sets a benchmark for any forthcoming rare-event experiment planning large-scale germanium or other high-sensitivity detector deployments, ensuring feasibility of sensitivity goals otherwise limited by irreducible backgrounds in less optimized settings.

7. Summary Table: Key Background Parameters in CJPL-II Hall-C

Parameter	Measured Value	Relevance
Wall ${}^{238}$U	$6.8 \pm 1.5$ Bq/kg	Inputs gamma simulations
Wall ${}^{232}$Th	$5.4 \pm 0.6$ Bq/kg	Inputs gamma simulations
Wall ${}^{40}$K	$81.9 \pm 14.4$ Bq/kg	Dominant at 1.46 MeV, otherwise minor
Gamma integral count rate	46.8 cps (60–2700 keV)	Overall gamma field level
Simulated background (2–4 keV)	$0.63~\mathrm{cpkty}$	Dominant Dark Matter ROI
Simulated background (2 MeV)	$4.6\times10^{-2}~\mathrm{cpkty}$	$0\nu\beta\beta$ search window

CJPL-II, as characterized by these measurements, thus defines a premier background environment for the most sensitive forthcoming searches in underground rare-event physics (Ma et al., 2020).