Edited by WANG Xiaoxia

The Jiangmen Underground Neutrino Observatory (JUNO) in south China successfully filled its 20,000-ton liquid scintillator detector and begun data taking on August 26.

Located 700 meters underground in Kaiping, Jiangmen city in Guangdong province, JUNO is the first of a new generation of ultra-large and ultra-precision neutrino experiments to reach this stage and has set multiple world records in technology. Its operation will help answer fundamental questions about the nature of matter and the universe.

Elaborate planning

Proposed in 2008 and approved in 2013, JUNO went through more than a decade of preparation and construction, tackling numerous challenges in designing and engineering.

Back into 2012, the Daya Bay Reactor Neutrino Experiment in Guangdong discovered an unexpectedly large neutrino oscillation related to the mixing angle, which brought China's neutrino experimental research from scratch to the forefront of the world.

The more advanced and larger-scale JUNO shoulders a greater mission: to tackle one of this decade's major open questions in particle physics — the ordering of neutrino masses, and thereby exploring the mysteries of the origin of the universe.

Physical sensitivity analysis specified that the experimental site should be within an area 50 to 55 kilometers away from the nuclear reactor and less than 200 meters wide.

Therefore, JUNO has been located 53 kilometers away from the Taishan and Yangjiang nuclear power plants, which is perfect for detecting the antineutrinos produced by them and measuring their energy spectrum with record precision.

The key to revealing the ordering of neutrino masses lies in the design and construction of the central detector, according to Heng Yuekun, deputy project manager in charge of JUNO's detector.

From 2013 to 2015, after considering various factors such as the detector's performance, cost, construction feasibility, risk, and lifespan, it was ultimately decided to use acrylic spheres to load the liquid scintillator, pure water as the shielding liquid, and a steel grid structure to support the entire structure.

Challenging experiment

After the detector's design was finalized, construction began, with groups of engineers and workers descending to the canyon 700 meters underground from the spring of 2015.

The JUNO experiment hall is the largest-span underground chamber in China. Its top arch has a span of 49.5 meters, making it a world-class extraordinary underground project.

Li Xiaonan, director of JUNO at the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences (CAS), has been living in Kaiping since 2013 and is in charge of infrastructure. Although Li majored in particle physics, during the seven years of construction, he also studied multiple fields such as geology, hydrology, measurement and control, and fire protection, to communicate smoothly with the contractors and overcame ensuing difficulties.

JUNO is the most challenging project I have ever conducted, Wang Yifang, a renowned high-energy physicist and an academician of the CAS, frankly stated. The challenges were not only in engineering, but also in the development of the detector's key core component.

The acrylic spheres used in JUNO were 20 times larger than the largest SNO experimental acrylic sphere in the world when the project was proposed.

The Donchamp company set up a dedicated production line and worked closely with the IHEP, breaking through multiple technical barriers to develop acrylic spheres qualified for JUNO.

Independent and collaborative innovation

The photomultiplier tube (PMT) is the most crucial component of the detector. A total of 20,000 PMTs operate simultaneously to capture scintillation light from neutrino interactions and convert it to electrical signals.

However, the technology was monopolized by Japan and acquiring it was very costly. Therefore, independent R&D of PMT was put on the agenda and in 2009, Wang and his team proposed a novel PMT solution.

After 11 years of verification, they successfully developed a PMT with independent intellectual property rights and the highest photon detection efficiency in the world. They also obtained patent license from the European Union, the United States, Japan and other countries.

In 2020, 15,000 20-inch domestically produced PMTs were delivered. Compared with imported PMTs, this saved two-thirds of the cost and rescued the domestic PMT industry, which was on the verge of bankruptcy.

While hosted by the IHEP, JUNO is a collaborative innovation of over 700 scientists from 74 research institutions in 17 countries and regions, along with thousands from the industrial sector.

The landmark achievement is the result of the international cooperation among many research groups outside China, who have brought to JUNO their expertise from previous liquid scintillator set-ups.

Marco Beretta, an Italian expert, arrived at the experimental site in July 2023 and became one of the heads of liquid scintillation. When Beretta saw the light inside the detector for the first time, his excitement knew no bound. "The birth of such a huge detector is like a paean, so wonderful," he said in eulogy.