JUNO Observatory: the detector filled with 20,000 tons of liquid scintillator and the first data acquisition
On August 26, 2025, the JUNO observatory reached a major milestone: its 20,000-ton liquid scintillator detector was filled and began recording data.
Located 700 meters underground in southern China, JUNO's initial operations and data collection show that key performance indicators are meeting or exceeding design expectations, enabling JUNO to address one of the major open questions in particle physics this decade: the order of neutrino masses, in particular which neutrino is the lightest or heaviest, a key element in deciphering the early universe.
A reactor neutrino detected on August 24, 2025, here is the fast component of the event with an estimated energy of 5.7 MeV.
© JUNO collaboration.Prof. Yifang Wang, spokesperson for JUNO and Director of the Institute of High Energy Physics (IHEP), said: « Completing the filling of the JUNO detector and starting data taking marks a historic milestone. For the first time, we have in operation a detector of this scale and precision dedicated to neutrinos. JUNO will allow us to answer fundamental questions about the nature of matter and the universe. »
At the same time, Prof. MA Xiaoyan, chief engineer at JUNO, praised the work accomplished:
« 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. »
For up to nearly six years, the data collected will come mainly from two nearby nuclear power plants. However, JUNO will also study solar neutrinos and those produced by natural radioactivity inside the Earth (uranium and thorium), in order to better understand convection in the Earth's mantle and its role in plate tectonics. Finally, JUNO will open up new perspectives for exploring as yet unknown physics, notably with research into proton decay.
The Subatech laboratory is heavily involved in JUNO and in measuring mass order, notably with pioneering publications*, but also in the possible measurement of supernova neutrinos.
*https://www.nature.com/articles/s41598-022-09111-1.
