The Jiangmen Underground Neutrino Observatory (JUNO), a multipurpose neutrino experiment designed to determine neutrino mass ordering and precisely measure oscillation parameters, has entered the final construction phase as its Project Manager WANG Yifang announced to begin the water filling at a rate of approximately 100 tons per hour on Dec. 18.

JUNO’s core component is a liquid scintillator detector with an effective mass of 20,000 tons, situated at the center of a 44-meter-deep pool of the underground experimental hall.

The detector’s main supporting structure is a 41.1-meter-diameter stainless steel structure that holds a 35.4-meter-diameter acrylic sphere. This sphere contains 20,000 tons of liquid scintillator, along with 20,000 20-inch photomultiplier tubes (PMTs), 25,000 3-inch PMTs, front-end electronics, cables, anti-magnetic coils, light barriers, and numerous other detector components.

The PMTs lining the inner wall of the central detector work together to detect the scintillation light generated when neutrinos are “captured” by the liquid scintillator, converting this light signal into electrical signals for output.

Compared to current international standards, the volume of the liquid scintillator in the central detector is 20 times larger, the photoelectron yield is three times greater, and the energy resolution is twice as good, reaching an unprecedented 3% precision.

The central detector will be submerged in a cylindrical pool, which will serve as both a water Cherenkov detector and a shielding barrier. A cosmic-ray tracking detector, covering an area of approximately 1,000 square meters, will be installed at the top of the pool. Together, the water Cherenkov detector and the cosmic-ray tracking detector will work to detect cosmic rays, eliminating their impact on neutrino detection.

In addition, the water in the pool will provide shielding against natural radioactivity from surrounding rocks and the large amount of secondary particles generated by cosmic rays in nearby geological formations.

The liquid filling process will be carried out in two stages. In the first two months, ultrapure water will fill both the inner and outer spaces of the acrylic sphere. Over the following six months, the ultrapure water inside the sphere will be replaced with liquid scintillator. Data collection will begin in August 2025.

JUNO is a collaborative effort involving over 700 members from 17 countries and regions, united in the pursuit of advancing our understanding of neutrino physics.

The JUNO Collaborators onsite (Credit: IHEP)