Deep beneath the city of Jiangmen, in southern China's Guangdong Province, lies a large research infrastructure used for the study of neutrinos. Neutrinos are the oldest and most primitive elementary particles that have existed since the beginning of the universe. Studying neutrinos opens the door for humans to understand the universe and existing matter in the world. Our reporter Liu Jiaxin paid a visit to this large detector – the Jiangmen Underground Neutrino Observatory, before it's officially put into operation.

It's a tunnel heading towards 700 meters underground, with the goal of understanding how our universe came to be. Among the most fundamental particles that make up the world, neutrinos are the least understood.

They are everywhere, with 300 particles per cubic centimeter. Yet they are nowhere, because they penetrate everything and are essentially unnoticeable. To detect neutrinos, scientists opt to place detectors in low-interference environments such as underground and underwater.

The detectors need to be large enough to have more opportunities for neutrinos to react in the detector's medium; the medium needs to be transparent enough so that the scintillation light caused by the neutrinos interactions can reach the optical sensors smoothly.

All these principles have led to the construction of the Jiangmen Underground Neutrino Observatory, or JUNO. The core detector of JUNO is a liquid scintillator detector with an effective mass of 20,000 tons. As of October, the innermost acrylic sphere has been completed.

WANG YIFANG, IHEP Director, JUNO Chief Scientist and Project Manager "In the future, when we start the experiment, the whole water pool will be filled with purified water and with a cover on the top of it. A ball like this is the most, let's say, efficient way to build a structure. It's just like egg where you think shell is strong enough to sustain a lot of pressure. A thin layer of acrylic with a spherical structure can sustain a lot of water pressure."

It took over 300 million U.S. dollars and more than 10 years to build JUNO. The project will be completed by November. A series of unprecedented technical problems have been solved throughout the construction process, including developing new photomultiplier tubes, or PMTs, that serve as key tools in converting optical signals into electrical signals.

WANG YIFANG, IHEP Director, JUNO Chief Scientist and Project Manager "You see the yellowish cover. Photons comes in, hit this yellow kind of material. They will convert photons to electrons. Every single photon can generate a pulse with 10 million electrons. And then the signals will be taken out by cables to electronics. And we restore them in the computer to do analysis."

LIU JIAXIN, Jiangmen, Guangdong Province "I'm among the most fortunate to witness this large research infrastructure under construction. By the end of this year, everywhere I'm standing will be filled with water for the operation of the detection. It's a big project and a project with super precision. Every parameter has to be correct to ensure that the detector can operate normally under various circumstances, even in an event of an earthquake."

JUNO is a joint effort, bringing together 750 international minds from 74 research institutions across 17 countries and regions. And once completed, JUNO will become one of the world's three centers for neutrino research, alongside Japan's Hyper-K and DUNE in the U.S.
Liu Jiaxin, CGTN, Jiangmen, Guangdong Province. （CGTN）