Filling of the JUNO Detector Has Begun

The JUNO underground neutrino observatory, an international project in which CNRS Nuclear & Particle Physics is involved, has entered its final stage before commissioning its massive detector. On December 18, 2024, the filling process began in Jiangmen, southern China, at a rate of approximately 2,000 tons of ultra-pure water per day.

This operation concerns the spherical detector, the central element of the JUNO infrastructure, as well as the 44-meter-deep, 43.5-meter-diameter tank that houses it. The filling process is divided into two phases.

During the first phase, which will last two months, both the sphere and the tank will be filled simultaneously with ultra-pure water. This approach ensures a balanced pressure between the two components. It will also allow for an initial data-taking phase to calibrate the detector.

The second phase, set to begin in early February, will involve gradually replacing the ultra-pure water inside the sphere with a scintillating liquid. Once the detector is operational, this liquid will provide the instrument with its full sensitivity.

It is the interaction between neutrinos and this scintillating liquid that will produce light, which will then be captured, amplified, and converted into an electrical signal by the photomultiplier tubes lining the inner wall of the spherical detector. Meanwhile, the ultra-pure water in the tank surrounding the detector will serve both as a shield and as a detector of background radiation. It will work in conjunction with a cosmic muon detector positioned above the setup to identify and eliminate background noise from the atmosphere and surrounding rock.

As of January 27, 95% of the central detector had been filled with water. The filling of the scintillating liquid, expected to be completed by July 2025, will pave the way for the start of experimental data collection in August.