JUNO Completed Liquid Filling and Begins Data Taking
On August 26, the Jiangmen Underground Neutrino Observatory (JUNO), a large underground neutrino observatory in southeastern China with INFN also contributing to its realization, has 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 a new generation of very large neutrino experiments to reach this stage. Initial trial operation and data taking show that key performance indicators met or exceeded design expectations, enabling JUNO to tackle one of this decade’s major open questions in particle physics: by exploiting the well-known phenomenon of neutrino oscillations, JUNO will clarify the ordering of neutrino masses—whether the third mass state is heavier than the second.
“The electronic component of JUNO, developed thanks to a direct collaboration between the INFN Padua Division and the IHEP laboratory in Beijing, follows a completely innovative design compared to previous generations of experiments. In fact, it was installed directly in the water inside the metal structure that supports the detector, in order to reduce electronic noise in the recorded signals and increase the detector’s sensitivity,” explains Alberto Garfagnini, head of electronics for the experiment, professor at the Department of Physics and Astronomy of the University of Padua and researcher at INFN. “This approach required a special effort, both during the design phase and component selection, as well as during mass production and installation, since in the event of malfunction the electronics cannot be repaired or replaced because the components are no longer accessible.” One of the performance tests for JUNO’s electronics before its production and installation in the experiment was conducted right in Montegrotto, near Padua.
Located 700 meters underground near Jiangmen city in the 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 enable cutting‑edge studies of neutrinos from the Sun, supernovae, the atmosphere, and the Earth. It will also open new windows to explore unknown physics, including searches for sterile neutrinos and proton decay.
“Thanks to the immense effort of the scientific Collaboration in filling and commissioning the experiment, as soon as the last drop of scintillator entered the apparatus, JUNO began its journey towards the many physics goals that characterize its program, starting with the determination of the neutrino mass ordering,” emphasizes Gioacchino Ranucci, international deputy spokesperson of the Collaboration, leader of the Italian group, and coordinator of the European groups in JUNO. “The INFN team made a decisive contribution to the promising performance observed during the commissioning of the apparatus: a contribution that covers not only the crucial purification of the scintillator, but also the realization of key parts of the electronics, the control and minimization of radioactive backgrounds, and computing. Our group’s commitment will now continue with data analysis, in which we are already actively involved.”
The scientific lifetime of JUNO will extend for at least 30 years, with a possible upgrade of the apparatus after the measurement of the mass ordering.
JUNO is a highly international project, managed in China by the IHEP Institute, with which INFN has a long tradition of cooperation. The project involves more than 700 researchers from 74 institutions across 17 countries and regions. INFN participates with its divisions in Catania, Ferrara, Milan, Milan Bicocca, Padua, Perugia, Rome III, and with the National Laboratories of Frascati. The collaboration between the INFN Padua Division and the IHEP laboratory in Beijing for the development of the electronics was recognized as a project of Major Relevance by the Italian Ministry of Foreign Affairs and International Cooperation (MAECI), and jointly by the National Natural Science Foundation (NSFC) for the period 2018–2020, supporting the mobility of Italian and Chinese researchers during the prototype validation phase and final design definition.
Characteristics of the experimental apparatus. At the heart of JUNO is a central liquid‑scintillator detector with an unprecedentedly large effective mass of 20,000 tons, housed 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. During construction, numerous unprecedented milestones were achieved, such as a high-performance PMT characterized by an innovative design, both in structure and electronic amplification. Among the other technological achievements are the development of an explosion-proof and waterproof enclosure to protect the PMTs; the high-efficiency purification system that produces radiopure scintillator with a light attenuation length greater than 20 meters; and innovative underwater electronics, with aerospace-grade reliability achieved using commercially available components.
