The Detector of Jiangmen Neutrino Experiment Center

The detector of the Jiangmen Neutrino Experiment Center is located 700 meters underground in Jiangmen, Guangdong Province. Its core is a 35.4-meter-diameter organic glass sphere—the world's largest single-piece structure of its kind. Inside this massive "glass sphere" are densely packed photomultiplier tubes (PMTs) that appear ordinary on the surface but are actually highly technologically advanced; they are the key to capturing neutrinos.

Within this glass sphere, which resembles a mysterious world, the PMTs act like sharp "eyes." The total number of these PMTs reaches 45,000, including 20,000 20-inch ones and 25,000 3-inch ones, together forming a high-sensitivity "net that covers all directions." When neutrinos pass through the detector, only a tiny fraction interact with the liquid scintillator (LS) inside the sphere, producing faint photons. These photons are precisely the targets captured by the PMTs.

The working principle of PMTs is remarkably sophisticated. They convert optical signals into electrical signals. First, when the photons generated by the interaction between neutrinos and the liquid scintillator are detected by the PMTs, they are transformed into electrons through the photoelectric effect. Subsequently, these electrons undergo secondary electron emission under the influence of a multiplying electric field. After multiple such multiplication and amplification processes, a measurable electrical signal is finally output.

To ensure the safety and accuracy of the experiment, each PMT is equipped with a protective device. This is because the PMTs not only need to have high photon detection efficiency but also must be able to operate in a 44-meter-deep water tank for a long time without implosion. Additionally, to eliminate signal interference from non-neutrino sources, the Jiangmen Neutrino Experiment employs anti-coincidence detectors and ultrapure water as shielding layers, thereby reducing interference from cosmic rays and other radiations. Through these precise designs and the application of advanced technologies, the Jiangmen Neutrino Experiment can accurately measure the neutrino mass hierarchy and other important parameters, providing crucial data for understanding the fundamental physical processes of the universe.

Photomultiplier tubes (PMTs) are important products among photon-counting devices and are light-detecting instruments with extremely high sensitivity and ultra-fast time response. They find wide applications in numerous fields. For instance, they can be found in equipment such as photon counters, ultra-weak light detectors, instruments for chemiluminescence and bioluminescence research, ultra-low energy ray detectors, spectrophotometers, polarimeters, colorimeters, illuminometers, dust counters, turbidimeters, densitometers, thermoluminescence dosimeters, radiation calorimeters, scanning electron microscopes, and biochemical analyzers.

A PMT is a vacuum device composed of a photoemissive cathode (photocathode), focusing electrodes, electron multiplier stages, and an electron collector (anode). Typical PMTs can be divided into two types based on the incident light reception method: end-window type and side-window type. When light irradiates the photocathode, the photocathode emits photoelectrons into the vacuum. These photoelectrons enter the multiplication system under the influence of the electric field of the focusing electrodes, undergo multiplication and amplification through further secondary emission, and finally, the amplified electrons are collected by the anode and output as signals.

PMTs have different operating modes, including DC operation mode and pulse operation mode. The DC operation mode is suitable for long-term or repetitive measurement of weak light events; the pulse operation mode, on the other hand, is applicable for short-term or one-time measurement of weak light events. When PMTs are used to measure impact events that have fast time processes, large changes in light intensity, and occur only once, they must be operated in the pulse mode. This can improve the dynamic range of the PMTs and increase the signal amplitude through certain measures.

In terms of structure, in the electron optical focusing system, the focusing stage D+ and the cathode K converge the electrons emitted by the photocathode into a beam, which then passes through a membrane hole and strikes the first multiplier stage. Under the excitation of high-speed primary electrons, the first multiplier stage emits several secondary electrons. These secondary electrons reach the second multiplier stage under the action of the electric field, triggering more secondary electron emission. This process continues until D10. Finally, the multiplied photoelectrons are collected by the anode, outputting a photocurrent that forms a voltage V0 across the load RL. In terms of cathode structure, the potential distribution on the cathode surface is uniform (due to the additional spherical photocathode and cylindrical electrode), and the trajectory lengths of electrons emitted from different directions (the center and edge of the cathode) differ slightly, resulting in almost the same travel time. The anode usually adopts a grid-like structure.

In the Standard Model of particle physics, neutrinos are one of the fundamental particles that make up the material world. The most prominent characteristic of neutrinos is that they barely interact with matter and have extremely strong penetrating power, making it very difficult for scientists to detect them in experiments. Nevertheless, neutrinos are ubiquitous in our lives. Every second, 300 trillion solar neutrinos pass through our bodies. The bananas we eat constantly produce neutrinos, and even the human body itself continuously generates neutrinos through the decay of "potassium-40." However, due to their super-strong penetrating ability, neutrinos can pass through the human body, the Earth, and the Sun without any obstruction, just like "ghosts"—earning them the nickname "ghost particles."

Although neutrinos themselves cannot be directly detected, when a large number of neutrinos pass through a detector, a very small portion of them have a probability of being captured by the working material of the detector. This interaction produces observable photons, which are then collected and amplified by PMTs, thereby indirectly realizing the observation of neutrinos. However, detecting neutrinos is extremely challenging. Modern large-scale neutrino experiments are often of enormous scale and require a large amount of working material to "wait in ambush" (like waiting for a hare by a tree stump). Take the Jiangmen Neutrino Experiment as an example: 20,000 tons of liquid scintillator can only detect 60 reactor neutrinos, 4 atmospheric neutrinos, 1 geoneutrino, and 90 "boron-8" solar neutrinos every day. In contrast, the number of cosmic rays (which serve as background noise) reaches 100,000—even after placing the detector 700 meters underground, where the intensity of cosmic rays has been reduced by 200,000 times.

Neutrino detection is inseparable from PMTs, and in particular, large-sized PMTs can achieve higher photon detection efficiency, helping scientists identify faint neutrino signals. In the international market, a Japanese company once held a monopoly in the manufacturing of large-sized PMTs. Especially for 20-inch PMTs, the price was exorbitant, at 3,000 US dollars each. For Japan's "Super-Kamiokande Neutrino Experiment," which ordered over 11,000 such large-sized PMTs, the cost of PMTs alone exceeded 200 million RMB.

However, for the Jiangmen Neutrino Experiment conducted by Chinese scientists, 20,000 20-inch PMTs are required. If all of them were purchased from Japan, the cost would be extremely high. Therefore, before the construction of the Jiangmen Neutrino Experiment began, the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS) took the lead and formed a cooperative team with units including North Night Vision Technology Co., Ltd. of China North Industries Group, Xi'an Institute of Optics and Precision Mechanics of CAS, CNNC Control System Co., Ltd., and Nanjing University to tackle key technological challenges collectively.

They overcame multiple technical difficulties, such as the preparation technology of high-quantum-efficiency photocathodes, microchannel plates, large-sized glass shells, and packaging technology for vacuum optoelectronic devices. Finally, they developed prototype tubes whose key technical indicators—including quantum efficiency, collection efficiency, and single-photoelectron peak-to-valley ratio—have reached the international advanced level. This achievement has broken through the manufacturing technology of 20-inch PMTs. The new-type PMTs have realized localization production, possess complete independent intellectual property rights, and have significantly enhanced the innovation capabilities and international competitiveness of domestic enterprises in the field of extra-large vacuum electronic devices. North Night Vision Technology Co., Ltd. has also established a dedicated production line for manufacturing large-sized PMTs. These 20-inch PMTs can not only be used in the Jiangmen Neutrino Experiment but also play a role in other scenarios that require measuring faint optical signals, such as the Large High-Altitude Air Shower Observatory (LHAASO) in Sichuan, which is used to detect cosmic rays. China's large-sized PMTs have achieved the world's highest photon detection efficiency, broken the international monopoly, and obtained patent authorizations in the European Union, the United States, Japan, and other regions. This is a successful example of cross-border cooperation between the scientific community and industry, promoting industrial technological progress through large-scale scientific facilities, and has laid a solid foundation for the emergence of more similar achievements in the future.