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26.08.2025 17:03

JUNO Completed Liquid Filling and Begins Data Taking

Katharina Pultar Kommunikation und Presse
Johannes Gutenberg-Universität Mainz

New type of detector enables deeper understanding of the universe

PRESS RELEASE OF THE JUNO COLLABORATION

The underground “Jiangmen Underground Neutrino Observatory (JUNO)” near Jiangmen city in the Guangdong Province, which was constructed with the participation of researchers from the PRISMA+ Cluster of Excellence at Johannes Gutenberg University Mainz (JGU), 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: the ordering of neutrino masses—whether the third mass state (ν₃) is heavier than the second (ν₂).

Prof. Yifang Wang, a researcher at the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences and JUNO spokesperson, 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.”

A neutrino detector 700 meters underground

Located 700 meters underground, 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.

Proposed in 2008 and approved by the Chinese Academy of Sciences and Guangdong Province in 2013, JUNO began underground construction in 2015. Detector installation started in December 2021 and was completed in December 2024, followed by a phased filling campaign. Within 45 days, the team filled 60,000 tons of ultra‑pure water, keeping the liquid‑level difference between the inner and outer acrylic spheres within centimeters and maintaining a flow‑rate uncertainty below 0.5%, safeguarding structural integrity. Over the next six months, 20,000 tons of liquid scintillator were filled into the 35.4‑meter‑diameter acrylic sphere while displacing the water. Throughout, stringent requirements on ultra‑high purity, optical transparency, and extremely low radioactivity were achieved. In parallel, the collaboration conducted detector debugging, commissioning, and optimization, enabling a seamless transition to full operations at the completion of filling.

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.

Prof. Xiaoyan Ma, JUNO Chief Engineer, remarked: “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.”

The JUNO collaboration

JUNO is hosted by the IHEP and involves more than 700 researchers from 74 institutions across 17 countries and regions. Six research groups from Germany – with support from the German Research Foundation (DFG) – participated in the experiment, including the working groups of Prof. Michael Wurm and Prof. Livia Ludhova at Johannes Gutenberg University Mainz, which belong to the PRISMA+ Cluster of Excellence. Prof. Ludhova, member of the JUNO Executive Committee, says: "JUNO is the result of many years of international collaboration. Our teams have contributed important building blocks to the current success: with the OSIRIS pre-detector to ensure the radioactive purity of the scintillator during detector filling, with studies on sensitivity, and with the analysis of the first data now taken during commissioning, all in close collaboration with our colleagues in China. It is very satisfying to see how our combined expertise has now come together in a detector that will serve the global physics community for decades to come."

JUNO is designed for a scientific lifetime of up to 30 years, with a credible upgrade path toward a world‑leading search for neutrinoless double‑beta decay. Such an upgrade would probe the absolute neutrino mass scale and test whether neutrinos are Majorana particles, addressing fundamental questions spanning particle physics, astrophysics, and cosmology, and profoundly shaping our understanding of the universe.

More images:

1) https://download.uni-mainz.de/presse/08_prisma+_JUNO_central_acrylic_sphere_and_...
The central acrylic sphere and PMTs
Image/©: JUNO collaboration

2) https://download.uni-mainz.de/presse/08_prisma+_JUNO_top_tracker_above_water_poo...
Top tracker above the water pool
Image/©: JUNO collaboration

3) https://download.uni-mainz.de/presse/08_prisma+_JUNO_neutrino_event_1.jpg
A neutrino event seen by JUNO (1)
Ill./©: JUNO collaboration

4) https://download.uni-mainz.de/presse/08_prisma+_JUNO_neutrino_event_2.jpg
A neutrino event seen by JUNO (2)
Ill./©: JUNO collaboration

Read more:

https://press.uni-mainz.de/from-china-to-the-south-pole-joining-forces-to-solve-... – Press release “From China to the South Pole: Joining forces to solve the neutrino mass puzzle” (25.02.2020)

https://press.uni-mainz.de/particle-physicists-from-mainz-university-participate... – Press release “Particle physicists from Mainz University participate in JUNO neutrino experiment” (20.01.2015)

https://press.uni-mainz.de/german-research-foundation-approves-new-research-grou... – Press release “German Research Foundation approves new research group to determine neutrino mass hierarchy” (04.01.2016)

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Wissenschaftliche Ansprechpartner:

Professor Dr. Dr. Livia Ludhova
GSI Helmholtzzentrum für Schwerionenforschung GmbH
Institute for Physics / PRISMA+ Cluster of Excellence
Johannes Gutenberg University Mainz
Staudingerweg 7
55128 Mainz
E-Mail: lludhov@uni-mainz.de
https://neutrino.gsi.de/
https://www.livialudhova.com/

Professor Dr. Michael Wurm
Institute for Physics / PRISMA+ Cluster of Excellence
Johannes Gutenberg University Mainz
Staudingerweg 7
55128 Mainz
E-Mail: michael.wurm@uni-mainz.de
https://www.etap.physik.uni-mainz.de/research-groups/ex-juno/

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Weitere Informationen:

https://juno.ihep.cas.cn/ – Website of the Jiangmen Underground Neutrino Observatory (JUNO)
https://www.prisma.uni-mainz.de/ – Website of PRISMA+ Cluster of Excellence

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The JUNO detector seen from outside
Quelle: JUNO collaboration
Copyright: JUNO collaboration

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Interior view of the OSIRIS pre-detector
Quelle: JUNO collaboration
Copyright: JUNO collaboration

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Journalisten, Wissenschaftler
Physik / Astronomie
überregional
Forschungsprojekte, Kooperationen
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