The Jiangmen Underground Neutrino Observatory (JUNO) is a medium baseline[2][3] reactor neutrino experiment under construction at Kaiping, JiangmeninGuangdong province in Southern China. It aims to determine the neutrino mass hierarchy and perform precision measurements of the Pontecorvo–Maki–Nakagawa–Sakata matrix elements. It will build on the mixing parameter results of many previous experiments. The collaboration was formed in July 2014[4] and construction began January 10, 2015.[5] The schedule aimed to begin taking data in 2023.[6] Funding is provided by the Chinese Academy of Sciences, but the collaboration is international. As of March 2024, the US$376 million JUNO facility is slated to come online at the end of 2024.[7]
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.[8]: 9 Instead it was moved west to a site (Jingji town, Kaiping, Jiangmen)[5] located 53 km from both of the Yangjiang and Taishan nuclear power plants.[8]: 4
The main detector consists of a 35.4 m (116 ft) diameter transparent acrylic glass sphere containing 20,000 tonnes of linear alkylbenzene liquid scintillator, surrounded by a stainless steel truss supporting approximately 53,000 photomultiplier tubes (17,000 large 20-inch (51 cm) diameter tubes, and 36,000 3-inch (7.6 cm) tubes filling in the gaps between them), immersed in a water pool instrumented with 2000 additional photomultiplier tubes as a muon veto.[9] As of 2022, construction of the detector is well underway.[10] Deploying this 700 m (2,300 ft) underground will detect neutrinos with excellent energy resolution.[3] The overburden includes 270 m of granite mountain, which will reduce cosmic muon background.[11]
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.
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 500 km/GeV; JUNO will observe the large dip at 16000 km/GeV. For reactor neutrinos with an energy of ≈3 MeV, the distances are ≈1.5 km and ≈50 km, 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 53 km distance.[11] 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.[11]
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%.[12]
