electra-supply.comNeutrinos, one of the fundamental particles that make up our material world, are also the most abundant particles in the universe. Their most notable characteristic is their extremely weak interaction with matter, allowing them to pass effortlessly through human bodies, the Earth, and even the Sun. Additionally, they possess an incredibly light mass, moving at speeds close to that of light. Many mysteries still surround neutrinos, including the nature of their mass, their origins, the pattern of their mass hierarchy, and whether they contribute to the asymmetry of matter and antimatter in the universe. The study of neutrinos holds the potential to uncover new physics beyond the Standard Model and is vital for understanding cosmic evolution, star formation, and supernova mechanisms. Since their existence was first confirmed in 1956, research in the field has garnered four Nobel Prizes. However, “catching” neutrinos remains a significant challenge! Advancing the scale and sophistication of detection instruments to gather more precise data is a key focus in neutrino research.
From Daya Bay to Jiangmen
In 2003, researchers from the Institute of High Energy Physics under the Chinese Academy of Sciences proposed a concept: utilizing the large number of neutrinos produced by the reactor group at Daya Bay to search for a third oscillation mode of neutrinos. Neutrinos can transform from one type to another during their flight, a phenomenon known as neutrino oscillation, which serves as evidence that neutrinos possess mass.
Construction of the Daya Bay Reactor Neutrino Experiment commenced in 2007. The facility consists of a surface control room and five underground laboratories, with the deepest underground lab located 320 meters below the surface. By the end of 2011, the Daya Bay reactor neutrino experiment was operational ahead of schedule with six detectors. On March 8, 2012, Wang Yifang, then director of the Institute of High Energy Physics, announced that the experiment had discovered a new type of neutrino oscillation and had measured its oscillation rate. This discovery is viewed as a new insight into the fundamental parameters of nature. Following this, the Daya Bay experiment continued to operate with high quality, achieving significant results, including an improvement in the precision of measuring neutrino oscillation amplitude from 20% in 2012 to 2.8%.
By the end of 2020, the Daya Bay reactor neutrino experiment was officially decommissioned. As China’s first-generation neutrino experimental facility, it achieved numerous world-leading research results, including “precise measurements of the reactor neutrino energy spectrum” and “the best constraints on low-mass sterile neutrinos.” Currently, China’s new generation large-scale neutrino experiment, the Jiangmen Neutrino Experiment (JUNO), is under active construction.
On October 10, 2024, during a clear autumn day, a reporter visited the Jiangmen Neutrino Laboratory in Guangdong. The landscape was dominated by white buildings arranged harmoniously on flat land, surrounded by green mountains. At the shaft entrance, descending in a cage for about five minutes brought the team to the experimental hall located 700 meters underground.
Why choose Jiangmen, Guangdong, for the next-generation neutrino experiment? Wang Yifang explained that the Jiangmen Neutrino Experiment aims to determine the neutrino mass hierarchy by measuring reactor neutrino oscillations. The greater the thermal power of the reactor, the more neutrinos are produced, leading to higher experimental precision. The experimental station needs to be located 50 to 55 kilometers from the reactor to align with the maximum oscillation value; distances to each reactor must be equal to prevent oscillation effects from canceling each other out. The area near Jiangmen’s Kaiping city perfectly meets these stringent conditions, being surrounded by the reactor groups in Yangjiang and Taishan, which provide the world’s highest total thermal power effective for measuring mass hierarchy. Scientific analyses have defined the permissible location for the experimental station within a 200-meter-wide area located 50 to 55 kilometers from both nuclear power plants, maximizing the sensitivity for measuring the neutrino mass hierarchy. “It’s quite fortunate to find such a suitable location,” Wang remarked.
Why is the core detection equipment built 700 meters underground? According to Cao Jun, director of the Institute of High Energy Physics, cosmic rays can create false signals in neutrino detection equipment. Thus, an underground setting shields most cosmic ray interference, which is why almost all neutrino experiments are conducted underground.
Building such a large facility 700 meters underground poses significant challenges. The Jiangmen Neutrino Experiment project was initiated in 2013, with construction of the underground caverns beginning in 2015. This cavern is the largest underground dome in China, spanning a vault of 49.5 meters. “We encountered numerous challenges during the construction of the cavern. One major issue was the high water yield of the rock, complicating excavation and drainage, which significantly increased construction difficulty,” Wang explained.
In response to the world-class engineering challenges of controlling the deformation of the cavern rock and conducting safe and efficient construction under high-water conditions, a technical task force was formed, including the construction team and technical experts, who carried out extensive research and devised reasonable construction plans. Eventually, the underground cavern was successfully completed by the end of 2021.
The Organic Glass Sphere Hidden 700 Meters Underground
Exiting the cage, the reporter arrived at the entrance of the experimental hall. After undergoing dust treatment, the door slowly opened, revealing a massive white sphere resting in a cylindrical water pool. “This is the core detection device of the Jiangmen Neutrino Experiment—the central detector,” Cao Jun explained. Situated in the center of a water pool 44 meters deep, the central detector features a stainless steel mesh shell with a diameter of 41.1 meters, which serves as the main supporting structure, holding a 35.4-meter diameter organic glass sphere, 20,000 tons of liquid scintillator, and tens of thousands of photomultiplier tubes, along with various electronic components and cables. The pool must also be filled with 35,000 tons of ultra-pure water during the detector’s operation.
The organic glass sphere of the Jiangmen Neutrino Experiment is composed of 263 12-centimeter thick curved panels and upper and lower chimneys, weighing about 600 tons, making it the largest single piece of organic glass in the world. “In proportion to its 35.4-meter diameter, the 12-centimeter thick walls of the organic glass sphere are as thin as an eggshell,” Cao noted. To enhance sensitivity and accuracy, the organic glass was produced using a unique formula and process, with naturally radioactive uranium and thorium content below one trillionth to ensure high transparency and low background radiation. Protective measures were implemented during the piecing together of the sphere surface using membrane materials and water-soluble paper. Furthermore, to combat aging, which could lead to surface cracking, the research team developed multiple methods to slow down the aging process, ensuring the safe operation of the detector. After construction, the sphere will be filled with 20,000 tons of liquid scintillator, surrounded by 35,000 tons of ultra-pure water, creating varying internal and external pressures that place significant demands on the assembly process.
How will such a massive structure bearing 20,000 tons of liquid scintillator and 600 tons of organic glass remain stable? That’s where the stainless steel mesh shell comes in. Constructed from pre-fabricated H-beams bolted together with 120,000 high-strength bolts, it stands as the largest single stainless steel main structure in the country. “During the design of the stainless steel mesh shell, we achieved several technical invention patents. Notably, a rivet technology related to national standards has been approved and published, filling a gap in the domestic arena,” Cao stated. Moreover, during operation, the organic glass sphere will sit in ultra-pure water, needing to consistently withstand approximately 3,000 tons of buoyancy, with these forces transmitted through the organic glass nodes, connecting rods, and stainless steel nodes to the main stainless steel mesh structure. Extensive design optimizations and hundreds of trials have enabled each node to attain extremely high load-bearing capacities.
Neutrinos are incredibly light and fast, with an extremely weak interaction with matter—so how does the central detector capture them? The liquid scintillator plays a crucial role. As large numbers of neutrinos pass through the detector, a tiny fraction will interact with the liquid scintillator, emitting extremely faint scintillation light that can be detected by the photomultiplier tubes, thereby capturing the neutrinos. “For a sensitive target material for neutrino detection, the liquid scintillator mainly consists of alkylbenzene, a key ingredient in everyday products like hand soap and laundry detergent; however, the liquid scintillator used in the Jiangmen experiment must be extremely clean,” Wang emphasized. Additionally, the liquid scintillator must exhibit high light output, excellent transparency, and ultra-low radioactive background, making its preparation a highly challenging task.
To tackle these challenges, the liquid scintillator team at the Jiangmen Neutrino Experiment developed a purification system characterized by high cleanliness, high sealing, and high efficiency, employing systems that include aluminum oxide, distillation, and water extraction to eliminate radioactive impurities and inert gases, thereby improving transparency and optical performance. “We have successfully obtained liquid scintillator with optical transmission attenuation lengths exceeding 20 meters, achieving a world-leading level and meeting cleanliness standards,” Cao reported.
The “Web” for Capturing Neutrinos
When neutrinos emit faint light in the liquid scintillator, a series of “eyes”—photomultiplier tubes densely packed inside the stainless steel mesh shell—spring into action. Observing the site, the reporter noted the large uninstalled photomultiplier tubes, each about half a meter in diameter. A total of 20,000 of these 20-inch photomultiplier tubes will be installed on the central detector, alongside an additional 25,000 3-inch tubes, creating a comprehensive network for capturing neutrinos.
“Photomultiplier tubes are the most critical component of the neutrino detector; neutrino signals are detected through these tubes, which convert light signals from neutrino interactions with the liquid scintillator into electrical signals, amplifying them by millions before conducting further analysis on computers,” Wang described. The construction of this national treasure has, objectively, spurred the development of China’s photomultiplier tube industry. Few companies globally can produce the necessary photomultiplier tubes, and those that do often fall short in performance while being prohibitively expensive. Therefore, scientists at the Institute of High Energy Physics undertook preliminary research on photomultiplier tubes and actively pushed for domestic production. They invented a new configuration and electronic amplification method for photomultiplier tubes, collaborating with relevant enterprises to ultimately develop photomultiplier tubes that meet international standards for collection efficiency and key technical metrics, with complete ownership of intellectual property rights, successfully breaking the international monopoly in this field.
The vacuum glass shell constituting the photomultiplier tube is a typical brittle material, and there is a risk of implosion for long-term operation within a 44-meter deep water pool. The shockwave from such an implosion could detonate surrounding photomultiplier tubes, triggering a chain reaction that could damage the entire assembly—incidents like these have occurred internationally. “To mitigate this risk, we developed an underwater explosion-proof system that provides protective devices for each photomultiplier tube,” Cao revealed as he pointed to a sample photomultiplier tube. The protective device features a semi-ellipsoidal transparent organic glass shield capable of withstanding over 50 meters of water pressure while accommodating installation gaps as small as 25 millimeters and achieving precision greater than 0.4 millimeters along with over 98% light transmission through water. The rear section comprises a stainless steel protective cover and connecting structure, ensuring strength while not obstructing the experiment’s light signals. This explosion-proof system effectively slows the filling of high-pressure water into the vacuum area, thereby significantly reducing the intensity of shockwaves and preventing chain reactions.
Capturing neutrinos is not enough; accuracy is equally vital, necessitating an anti-coincident detection system to filter out signals from non-neutrinos. Yang Changgen, the head of the anti-coincidence system at the Jiangmen neutrino experiment, explained that the 35,000 tons of water in the pool needs purifying in a water purification chamber. The ultra-pure water can be utilized as a cosmic ray detector to eliminate interference from cosmic rays and also serves as a shielding layer against natural radioactivity present in surrounding rock. Additionally, a particle tracking detector located above the pool can measure the precise direction of cosmic rays, enhancing the accuracy of neutrino detection by better eliminating false signals.
The Jiangmen Neutrino Experiment is now in its final stages of construction: the innermost organic glass sphere has been assembled, and the outer stainless steel mesh and photomultiplier tubes are gradually coming together, with all installation tasks expected to be completed by the end of November. This will be followed by the filling of ultra-pure water and liquid scintillator, with operations slated to officially begin in August 2025, expected to continue for approximately 30 years. Wang Yifang noted that the Jiangmen Neutrino Experiment has an array of scientific objectives, including determining the neutrino mass hierarchy, accurately measuring three neutrino oscillation parameters, and in an upgrade planned for 2030, measuring the absolute mass of neutrinos. The experiment is also set to conduct internationally leading research on solar neutrinos and terrestrial neutrinos, with hopes of achieving significant results in areas such as supernova neutrinos and proton decay.
(Reporter: Qin Weili)
Source: Guangming Daily
