The Jiangmen Underground Neutrino Observatory (JUNO) is a medium baseline reactor neutrino experiment currently operating at Kaiping, Jiangmen in Guangdong province in Southern China. It aims to determine the neutrino mass hierarchy and perform precision measurements of the PontecorvoâÂÂMakiâÂÂNakagawaâÂÂSakata matrix elements. It will build on the mixing parameter results of many previous experiments.
The collaboration was formed in July 2014 and construction began January 10, 2015. Funding is provided by a collaboration of international institutions. Originally scheduled to begin taking data in 2023, , the US$376 million JUNO facility was completed and the experiment started on 26 August 2025. JUNO is the world's largest transparent spherical detector.
Planned as a follow-on to the Daya Bay Reactor Neutrino Experiment, it was originally to be sited in the same area, but the construction of a third nuclear reactor (the Lufeng Nuclear Power Plant) in that region would disrupt the experiment, which depends on maintaining a fixed distance to nearby nuclear reactors. Instead it was moved west to a site (Jingji town, Kaiping, Jiangmen) located 52.5 km from both of the Yangjiang and Taishan nuclear power plants.
The main detector consists of a diameter transparent acrylic glass sphere containing 20,000 tonnes of linear alkylbenzene liquid scintillator, surrounded by a stainless steel truss supporting approximately 43,200 photomultiplier tubes (17,612 large diameter tubes, and 25,600 tubes filling in the gaps between them), immersed in a water pool instrumented with 2,400 additional photomultiplier tubes as a muon veto. Construction was finished and operation began in 2025. The detector is deployed underground, helping reduce background and enabling it to detect neutrinos with excellent energy resolution. The overburden includes 270 m of granite mountain, which reduces the cosmic muon background.
The much larger distance to the reactors (compared to less than 2 km for the Daya Bay far detector) makes the experiment better able to distinguish neutrino oscillations, but requires a much larger, and better-shielded, detector to detect a sufficient number of reactor neutrinos.
The main approach of the JUNO Detector in measuring neutrino oscillations is the observation of electron antineutrinos () coming from two nuclear power plants at approximately 53 km distance. Since the expected rate of neutrinos reaching the detector is known from processes in the power plants, the absence of a certain neutrino flavor can give an indication of transition processes.
The quantitative part of the experiment requires measuring neutrino flavour oscillations as a function of distance. This seems impossible, as both the reactors and detector are completely immovable, but the speed of oscillation varies with energy (details at ). As the reactors emit neutrinos with a range of energies, a range of effective distances can be observed, limited by the accuracy with which each neutrino's energy can be measured.
Although not the primary goal, the detector is sensitive to atmospheric neutrinos, geoneutrinos and neutrinos from supernovae as well.
Daya Bay and RENO measured ø<sub>13</sub> and determined it has a large non-zero value. Daya Bay will be able to measure the value to âÂÂ4% precision and RENO âÂÂ7% after several years. JUNO is designed to improve uncertainty in several neutrino parameters to less than 1%.