The collaboration was formed in July 2014[5] and construction began January 10, 2015.[6] Funding is provided by a collaboration of international institutions. Originally scheduled to begin taking data in 2023,[7] , the US$376 million JUNO facility[8][9][10] was completed and the experiment started on 26 August 2025.[11][12][13]:8 JUNO is the world's largest transparent spherical detector.[12]
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.[14]:9 Instead it was moved west to a site (Jingji town, Kaiping, Jiangmen)[6] located 52.5km from both of the Yangjiang and Taishan nuclear power plants.[14]:4[13]:8
Detector
JUNO detector
The main detector consists of a 35.4m (116ft) diameter transparent acrylic glass sphere containing 20,000tonnes of linear alkylbenzene liquid scintillator, surrounded by a stainless steel truss supporting approximately 43,200 photomultiplier tubes (17,612 large 20-inch (51cm) diameter tubes, and 25,600 3-inch (7.6cm) tubes filling in the gaps between them), immersed in a water pool instrumented with 2,400 additional photomultiplier tubes as a muon veto.[9][15] Construction was finished and operation began in 2025.[12] The detector is deployed 700m (2,300ft) underground, helping reduce background and enabling it to detect neutrinos with excellent energy resolution.[4] The overburden includes 270m of granite mountain, which reduces the cosmic muon background.[16]
The much larger distance to the reactors (compared to less than 2km 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.
Physics
Predicted oscillation probability of electron neutrinos (black) oscillating to muon (blue) or tau (red) neutrinos, as a function of distance from source. Existing short-baseline experiments measure the first small dip in the black curve at 500km/GeV; JUNO will observe the large dip at 16000km/GeV. For reactor neutrinos with an energy of ≈3MeV, the distances are ≈1.5km and ≈50km, respectively. This plot is based on assumed mixing parameters; the measured shape will differ and allow the actual parameters to be computed.
The main approach of the JUNO Detector in measuring neutrino oscillations is the observation of electron antineutrinos ( ν e) coming from two nuclear power plants at approximately 53km distance.[16] 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.[16]
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 Neutrino oscillation §Propagation and interference). 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.
Daya Bay and RENO measured θ13 and determined it has a large non-zero value. Daya Bay will be able to measure the value to ≈4% precision and RENO ≈7% after several years. JUNO is designed to improve uncertainty in several neutrino parameters to less than 1%.[17]
12Stock, Matthias Raphael (December 2023). Status and Prospects of the JUNO Experiment. The 17th International Workshop on Tau Lepton Physics. Louisville, Kentucky. arXiv:2405.07321.
