Working staff install equipment into the JUNO veto detector system which is designed for cosmic muon detection and background reduction. Photo: Courtesy of IHEP

The liquid injection phase of the Jiangmen Underground Neutrino Observatory (JUNO), a key national science and technology project located in South China's Guangdong Province, officially began on Wednesday, the Global Times learned from the Institute of High Energy Physics of the Chinese Academy of Sciences (CAS) on Wednesday.

Multi-stage filtered ultra-pure water, is being injected into the detector pool at a flow rate of 100 tons per hour. This marks the final critical milestone in the construction of the JUNO experiment. 

Neutrinos, the lightest of the 12 elementary particles in the material world, are electrically neutral and travel at a speed close to that of light. Since the Big Bang, they have permeated the universe, emerging from nuclear reactions inside stars, supernova explosions, the operation of nuclear reactors and the radioactive decay of substances in rocks, according to Xinhua News Agency.

Since neutrinos rarely interact with ordinary matter, they pass through our bodies, buildings, and Earth unnoticed, hence earning the nickname "ghost particles." Due to their elusive nature, neutrinos are the least understood fundamental particles, requiring enormous detectors to capture faint traces, according to Xinhua.

The liquid injection will occur in two stages. During the first two months, ultra-pure water will fill the spaces inside and outside the acrylic sphere of the central detector. In the following six months, the ultra-pure water inside the sphere will be replaced with liquid scintillator. 

The entire liquid injection process is expected to be completed by August 2025, after which the facility will officially begin operation and data collection. 

The CAS experts told the Global Times in a previous interview that the core detector of JUNO is a liquid scintillator detector (central detector) with an effective mass of 20,000 tons. It is located at the center of a 44-meter-deep water pool within an underground experimental hall, 700 meters below ground. The detector's primary support structure is a 41.1-meter-diameter stainless steel frame.

This structure supports a 35.4-meter-diameter acrylic sphere, 20,000 tons of liquid scintillator, 20,000 20-inch photomultiplier tubes (PMTs), 25,000 3-inch PMTs, front-end electronics, and other detector components. 

Compared to the current best international standards, JUNO's central detector has achieved a 20-fold increase in liquid scintillator volume, a threefold increase in photoelectron yield, and an unprecedented 3 percent energy resolution. 

JUNO's primary scientific objective is to determine the neutrino mass hierarchy while conducting various other cutting-edge research projects. Upon completion, JUNO will become one of the global centers for neutrino research, forming a tripod in conjunction with Japan's Hyper-Kamiokande and the US's Deep Underground Neutrino Experiment. 

"Compared with these two countries, the JUNO holds a first-mover advantage and features a unique experimental design in terms of physics," said the experts. "As an international collaborative project led by China, JUNO will further strengthen China's leading position in this field."