The final stage of the construction of the Jiangmen Underground Neutrino Observatory (JUNO) began on Dec. 18 as the Project Manager WANG Yifang announced to begin the water filling at a rate of ~100 tons/hour.
JUNO’s core component is a liquid scintillator detector with an effective mass of 20,000 tons. It is located 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, 20,000 tons of liquid scintillator, 20,000 20-inch photomultiplier tubes (PMTs), 25,000 3-inch PMTs, front-end electronics, cables, anti-magnetic coils, light barriers, and many other detector components. The PMTs covering the inner wall of the central detector jointly detect the scintillation light generated when neutrinos are “captured” by the liquid scintillator, converting the light signal into electrical signals for output. Compared to current best international standards, the volume of the liquid scintillator in the central detector is 20 times larger, the photoelectron yield is three times larger, and the energy resolution isa factor of two better, reaching an unprecedented 3%.
The central detector will be immersed in a cylindrical pool, which will serve as both a water Cherenkov detector and a shielding body. At the top of the pool there will be a cosmic-ray tracking detector with an area of approximately 1,000 square meters. The water Cherenkov detector and the cosmic-ray tracking detector will jointly detect cosmic rays, thereby eliminating their impact to neutrino detection. In addition, the water in the pool will shield against natural radioactivity from rocks and the large amount of secondary particles generated by cosmic rays in nearby rocks.
The liquid filling process will be carried out in two stages. During the first two months, the inner and outer spaces of the acrylic sphere will be filled with ultrapure water. Over the following six months, the ultrapure water inside the acrylic sphere will be replaced with liquid scintillator. Data collection will begin in August 2025.
JUNO is a multipurpose neutrino experiment designed to determine neutrino mass ordering and precisely measure oscillation parameters. JUNO is built by a large international team with more than 700 members from 17 countries and regions.
The JUNO Collaborators Onsite (Credit: IHEP)
