The Jiangmen Underground Neutrino Observatory (JUNO) has completed its innermost acrylic sphere and is assembling the outer stainless steel structure and photomultiplier tubes (PMTs), meaning that the JUNO construction has entered its final stage. All installation tasks are expected to be completed by the end of this November, according to the High Energy Physics (IHEP) of the Chinese Academy of Sciences this week.

Located 53 kilometers from the Yangjiang and Taishan Nuclear Power Plants in southern China's Jiangmen City of Guangdong Province and 700 meters underground, JUNO is a multipurpose neutrino experiment designed to determine neutrino mass ordering and precisely measure oscillation parameters.

Neutrinos are tiny, massive particles with no charge. With this facility, scientists hope to uncover more insight on the mass of neutrinos, how they can change identities while traveling in a process called oscillation and solve more mysteries about the perplexing elementary particle, which is both extremely abundant and difficult to spot.

"By studying neutrinos, we can understand why the universe has become what it is today, and what will be the future of the universe," said WANG Yifang, chief scientist of JUNO and director of IHEP.

The core detector of JUNO is a 13-story-tall spherical acrylic vessel with an unprecedented energy resolution of 3%. It has a wall thickness of 120 millimeters and weighs more than 600 tonnes. It is the largest single spherical acrylic vessel in the world, presenting production and construction challenges never before encountered.

As a container for the liquid scintillator used in neutrino detection, the acrylic sphere will operate submerged in pure water, requiring the structure to withstand approx. 3,000 tons of buoyancy over extended periods.

Upon installation, the vessel will first be filled with ultrapure water for cleaning and protection from deformation by water pressure outside. Later, liquid scintillator will replace the water little by little. The acrylic vessel will eventually be filled with 20,000 tonnes of liquid scintillator, the target substance for capturing neutrinos.

The JUNO project was begun in 2013, with civil construction starting in 2015. The facility will begin data collection in August 2025 according to the plan, with an operational life estimated at 30 years.

Since the establishment of the JUNO international collaboration group in July 2014, it has evolved into a large global collaboration consisting of 750 scientists and engineers from 74 research institutions across 17 countries and regions. The collaborative group jointly shares funding contributions and research results as well as jointly develops technologies.

Once completed, JUNO is expected to be the world'largest and most sensitive neutrino detection project and become one of the world's three centers for neutrino research, along with Japan's Hyper-K and DUNE in the U.S., according to IHEP.

Stainless steel structure of JUNO detector (Image by IHEP)

JUNO detector (Image by IHEP)

Acrylic sphere of JUNO detector (Image by IHEP)