JUNO Completed Liquid Filling and Begins Data Taking

JIANGMEN, China — August 26, 2025

      The Jiangmen Underground Neutrino Observatory (JUNO) successfully completed the filling of its 20,000 tons of liquid scintillator and begun data taking. After more than a decade of preparation and construction, JUNO is the first of the new generation of large-scale neutrino experiments to reach this stage. Initial trial operation and data taking shows that key performance indicators met or exceeded design expectations, enabling JUNO to tackle one of this decade’s major questions in particle physics: the ordering of neutrino masses—whether the third mass state (ν₃) is heavier than the second (ν₂).

     “Completing the filling of the JUNO detector and beginning data collection marks a historic milestone. For the first time, we now have in operation a detector of this scale and precision dedicated to neutrino research. JUNO will enable us to tackle some of the most fundamental questions about the nature of matter and the universe,” said Prof. Yifang Wang, spokesperson for JUNO and a researcher at the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences.

      Located 700 meters underground in Jiangmen city, Guangdong Province, JUNO detects antineutrinos produced 53 kilometers away by the Taishan and Yangjiang nuclear power plants and measures their energy spectrum with record precision.Unlike other approaches, JUNO’s determination of the mass ordering is independent of matter effects in the Earth and largely free of parameter degeneracies. JUNO will also deliver order‑of‑magnitude improvements in the precision of several neutrino‑oscillation parameters and pursue cutting‑edge studies of supernova and solar neutrinos, geoneutrinos, and searches for sterile neutrinos and proton decay.

      Proposed in 2008 and approved by the Chinese Academy of Sciences and Guangdong Province in 2013, JUNO began underground construction in 2015. Detector installation started in December 2021 and was completed in December 2024, followed by a phased filling campaign. Within 45 days, the team filled 60,000 tons of ultra‑pure water, keeping the liquid‑level difference between the inner and outer acrylic spheres within centimeters and maintaining a flow‑rate uncertainty below 0.5%, safeguarding structural integrity. Over the next six months, 20,000 tons of liquid scintillator were filled into the 35.4‑meter‑diameter acrylic sphere while displacing the water. Throughout, stringent requirements on ultra‑high purity, optical transparency, and extremely low radioactivity were achieved for both media. In parallel, the collaboration conducted detector debugging, commissioning, and optimization, enabling a seamless transition to full operations at the completion of filling.

      At the heart of JUNO is a central liquid‑scintillator detector (effective mass 20,000 tons) at the center of a 44‑meter‑deep water pool. A 41.1‑meter‑diameter stainless steel truss supports the 35.4‑meter acrylic sphere, the scintillator, 20,000 20‑inch photomultiplier tubes (PMTs), 25,600 3‑inch PMTs, front‑end electronics, cabling, anti‑magnetic compensation coils, and optical panels. All PMTs operate simultaneously to capture scintillation light from neutrino interactions and convert it to electrical signals.

      Dr. Xiaoyan Ma, JUNO Chief Engineer, remarked:“Building JUNO has been a journey of extraordinary challenges. It demanded not only new ideas and technologies, but also years of careful planning, testing, and perseverance. Meeting the stringent requirements of purity, stability, and safety called for the dedication of hundreds of engineers and technicians. Their teamwork and integrity turned a bold design into a functioning detector, ready now to open a new window on the neutrino world.”

      JUNO is designed for a scientific lifetime of up to 30 years, with a credible upgrade path toward a world‑leading search for neutrinoless double‑beta decay. Such an upgrade would probe the absolute neutrino mass scale and test whether neutrinos are Majorana particles, addressing fundamental questions spanning particle physics, astrophysics, and cosmology, and profoundly shaping our understanding of the universe.

      JUNO is hosted by the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences, and involves more than 700 researchers from 74 institutions across 17 countries and regions.

      Prof. Wei Wang, a JUNO collaboration board member representing Sun Yat-sen University and also an executive board member of the collaboration, further explained:“JUNO is the product of a truly international effort, and SYSU has been one of the core institutes since the very beginning. Our JUNO group has taken responsibility for a wide range of hardware and software tasks, including—but not limited to—the PMT quality assurance and characterization system, the filling system, the acrylic detector bonding, and software tools such as the detector event display. SYSU also contributed the Top Veto Tracker (TVT), which serves as a cosmic muon veto for the JUNO TAO detector system. Over the past decade, the SYSU team has worked side by side with colleagues throughout the construction process, and it is deeply rewarding to see our combined expertise come together in a detector that will serve the global physics community for decades to come.”

Fig.1 The JUNO detector seen from outside.

Fig.2 The central acrylic sphere and PMTs.

Fig.3 Top tracker above the water pool.

Fig. 4 A neutrino event seen by JUNO.

图源：中国科学院高能物理研究所

初审：周越

审核：刘李云

审定发布：徐瑶