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A detector installed in one of the world’s deepest underground laboratories shows how particles produced in the atmosphere can help scientists investigate large volumes of rock and plan sensitive experiments inside a mountain.
Chinese researchers used a one-ton detector installed in the Jinping Underground Laboratory, about 2,400 meters beneath a mountain in southwest China, to map the rock structure above the facility.
The equipment recorded muons generated by the interaction of cosmic rays with the atmosphere and allowed the reconstruction of differences in the amount of matter traversed by these particles over a lateral range of approximately three kilometers.
The results were presented in a preprint made available on arXiv on June 4, 2026.
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The work had not yet undergone, until its release, all stages of peer review.
Instead of showing a direct image of the mountain’s interior, the method produces an opacity map, similar to the principle of an X-ray, from the reduction of the muon flux that manages to reach the laboratory.
The study also clarifies that the detector was not specifically installed in June 2026.
The prototype, developed for the Jinping Neutrino Experiment, had already been collecting data since 2017.
The analysis gathered 1,338.6 effective days of observation to determine the direction of arrival of the particles and estimate how much material they traversed before reaching the underground environment.
How muons traverse a mountain
Muons are elementary particles produced mainly when high-energy cosmic rays hit nuclei present in the Earth’s atmosphere.
Although they have a very short existence, they travel at speeds close to the speed of light and can reach the surface before decaying.
Because they are heavier than electrons and arrive on Earth with high energy, some muons can traverse large volumes of soil, concrete, or rock.
During the journey, they lose energy through different interactions with matter.
The greater the thickness or density of the material encountered, the fewer particles tend to reach the other side.
Muography precisely utilizes this variation.
Detectors count the muons coming from different directions and compare the observed flux with the expected number on the surface.
A more intense reduction may indicate that the particles crossed a longer path or a region with a greater amount of matter.
This principle is already applied in studies of volcanoes, archaeological structures, and large geological formations.
In Jinping, however, the depth represented an unusual condition: researchers attempted to perform the technique under a kilometer-scale rock cover, where only a small fraction of the higher-energy muons can reach.
One-ton detector records the direction of particles
The center of the equipment is a spherical acrylic container with a radius of 645 millimeters, equivalent to about 1.3 meters in diameter.
Inside it, there is a ton of scintillator liquid, a material that emits light when a charged particle passes through its interior.
As it passes through the detector, the muon deposits energy in the liquid and produces photons.
Thirty photomultiplier tubes distributed around the sphere record these light signals and convert them into electrical pulses.
The difference in the timing and intensity of the light captured by each sensor helps the system reconstruct the particle’s trajectory.
According to the preprint, the spherical shape offers almost uniform acceptance in different directions.
The average angular resolution obtained by the researchers was approximately 4.5 degrees, a measure that represents the precision used to estimate where each muon arrived from.
The set is installed inside a stainless steel tank filled with purified water.
A five-centimeter-thick lead wall adds protection against natural environmental radiation.
The apparatus was originally developed to validate technologies and measure radioactivity levels related to the future Jinping Neutrino Experiment.
Muography compares particle data and satellite terrain
To transform the records into information about the mountain, the team combined experimental data, models of muon flux on the surface, and simulations of particle behavior inside the rock.
The calculations were performed with Geant4, a platform used to simulate the passage of particles through matter.
The Jinping terrain was represented with data from SRTM3, a topographic survey obtained by satellite.
The model included an area with a radius of ten kilometers around the laboratory and considered the mountain predominantly composed of marble, with density and composition parameters defined by the researchers.
From this scenario, the system calculated how many muons should survive in each direction and compared the prediction with the flow effectively measured underground.
This relationship allowed estimating the so-called oblique depth, that is, the amount of matter traversed along each trajectory, which can be greater than the vertical depth of the laboratory.
The reconstruction covered the mountain structure around the first phase of the complex in a lateral range of up to three kilometers.
According to the authors, the results showed agreement with the satellite-derived terrain and did not reveal significant density variations within the statistical sensitivity achieved by the experiment.
This does not mean that all the rock is perfectly uniform, but that the analysis did not identify relevant anomalies within the limits of the method used.
Jinping Underground Laboratory occupies excavated area in China
The Jinping Underground Laboratory is located under the mountain of the same name, in the Liangshan Yi Autonomous Prefecture, Sichuan province.
The facility was built between two road tunnels approximately 17.5 kilometers long that cross the region.
The first phase began operation in 2010, with about 4,000 cubic meters.
The second stage, known as CJPL-II or Deep Underground and Ultra-Low Radiation Background Facility, expanded the complex.
The preprint describes the laboratory as a structure of approximately 300 thousand cubic meters, while institutional information released at the inauguration of the expansion, in December 2023, indicates a total capacity of 330 thousand cubic meters for the second phase.
The expansion began construction in December 2020 through a partnership between Tsinghua University and Yalong River Hydropower Development company.
The site houses or was prepared for research on dark matter, neutrinos, nuclear astrophysics, and other phenomena that require environments with low levels of interference.
The rock coverage reduces much of the cosmic radiation that reaches the equipment.
This characteristic favors the search for rare events, but it also makes it necessary to accurately calculate the few muons capable of passing through the mountain, because these particles can produce background signals and affect sensitive measurements.
Muon data helps plan eight experimental areas
After validating the model with observations made in the first phase of the laboratory, researchers estimated the total muon flux in the eight experimental areas of CJPL-II.
The forecasts were calculated for sectors A1, A2, B1, B2, C1, C2, D1, and D2.
Among these spaces are areas associated with the PandaX experiment, aimed at investigating dark matter; CDEX, which also seeks signals related to this type of matter; JUNA, intended for nuclear astrophysics studies; and the future Jinping Neutrino Experiment.
Knowing the expected flux allows for the sizing of protections, estimating noise, and separating possible physical events from signals caused by cosmic particles.
The work also calculated the average energy of the muons for each area, a parameter related to the production of neutrons when these particles interact with the rock and the components of the detectors.
The experience does not turn the laboratory into a scanner capable of observing any point on the planet.
In practice, the set functions as a muography instrument directed at the rocky coverage around Jinping.
Even so, the research tests the use of natural particles to study large geological structures without drilling or emitting artificial radiation.
With larger detectors and longer collection periods, the technique could increase sensitivity to density differences, provided the results are confirmed by new studies.
