JUNO Experiment
In ‘historic milestone’, massive underground facility in Guangdong province has begun collecting data to track mysterious neutrinos
China launched the Jiangmen Underground Neutrino Observatory (Juno), the world’s largest detector for elusive “ghost particles” called neutrinos, in southern Guangdong province on Tuesday.
The massive facility has started collecting data for its mission to track the mysterious particles with no electrical charge, very little mass and moving at near-light speed.
The Institute of High Energy Physics of the Chinese Academy of Sciences said Juno had successfully completed filling its 20,000 tonne liquid scintillator detector – a giant acrylic sphere measuring 35 metres (115 feet) in diameter.
The detector is housed at the centre of a 44 metre-deep water pool, located 700 metres below ground. It is meant to measure the mass of different types of neutrinos produced by two nuclear power plants in Taishan and Yangjiang, 53km away.
Particle physicist Wang Yifang, who serves as Juno’s spokesman, called the completion of the filling of the Juno detector and starting of data-taking “a historic milestone”. The Chinese Academy of Sciences proposed the international project in 2008.
“For the first time, we have in operation a detector of this scale and precision dedicated to neutrinos,” said Wang, who directed the Institute of High Energy Physics from 2011 to 2024.
“Juno will allow us to answer fundamental questions about the nature of matter and the universe,” he added.
Juno is designed to have a scientific lifespan of up to 30 years, the institute said, with a viable upgrade pathway to search for neutrino-less double-beta decay, a theoretical radioactive decay process that would prove the neutrino particle is Majorana in type.
It enables the institute to test whether neutrinos are Majorana – particles that are their own antiparticle – and address questions in particle physics, astrophysics and cosmology that can shape popular understanding of the universe.
Juno’s chief engineer, Ma Xiaoyan, said the facility’s construction entailed “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,” she explained.
“Their teamwork and integrity turned a bold design into a functioning detector, ready now to open a new window on the neutrino world.”
Underground construction of Juno began in 2015, and by late 2021, the team started installing the detector. Its completion last December was followed by a phased filling campaign.
“Within 45 days, the team filled 60,000 tonnes of ultra‑pure water, keeping the liquid‑level difference between the inner and outer acrylic spheres within centimetres,” Ma added.
“Over the next six months, 20,000 tonnes of liquid scintillator were filled into the 35.4 metre‑diameter acrylic sphere while displacing the water.”
The institute said the team achieved “stringent requirements for ultra-high purity, optical transparency and extremely low radioactivity”.
As for Juno’s operation, thousands of photomultiplier tubes “capture scintillation light from neutrino interactions and convert it to electrical signals”, it added, referring to a flash of light produced when a substance is hit by radioactive material.
Physicist Gioacchino Ranucci of the University of Milan, Juno’s deputy spokesman, said the facility was “a result of the fruitful international cooperation ensured by many research groups outside China, bringing to Juno their expertise from previous liquid scintillator set-ups”.
“The worldwide liquid scintillator community has pushed the technology to its ultimate frontier, opening the path towards the ambitious physics goals of the experiment,” Ranucci added.
The project involved more than 700 researchers from 74 institutions in 17 countries and regions. Nearly 300 of the researchers are from Europe, including France, Germany and Italy.
Juno “is the first of a new generation of very large neutrino experiments to reach this stage”, the institute said.
The Deep Underground Neutrino Experiment in the United States and the Hyper-Kamiokande, a neutrino observatory in Japan, are both scheduled to be up and running in 2027-28.