A group of people placed a super-large crystal ball in the mountains of Guangdong, planning to capture "cosmic ghosts" 700 meters underground

The winning works of the 2023 "China Science Popularization Star Creation Competition"

Author: He Jingwen

What is 700 meters? It is as high as dozens of floors, and as long as about 7,000 Xiangpiaopiao cigarettes connected together. What if it is 700 meters down? You will need to take a cable car through a long inclined shaft to reach it.

In the deep mountains of Guangdong, a group of people planned to dig a big hole 700 meters underground, place a super-large crystal ball, inject 20,000 tons of the most transparent liquid into the crystal ball, and cover the wall of the ball with dazzling golden devices. If you walk into the cave entrance in the early stage of construction, you will find that the entrance of the cave is filled with thick fog. Walking into the thick fog, the heat wave mixed with water vapor hits your face. The long tunnel is like a big steamer, and the heavy humidity suffocates people's skin. The sunlight is quickly blocked behind you. The small roller coaster-like track cable car will carry you to the depths of the ground - all of the above are not scenes from the magic world or fantasy novels, but the experimental base for Chinese scientists to capture "cosmic ghost particles".

Schematic diagram of the detector structure of the Jiangmen Neutrino Experiment Source: Jiangmen Collaboration Group

What are cosmic ghost particles?

The "ghost particle" is actually called a neutrino.

In 1930, physicist Pauli hypothesized a type of uncharged, weakly interacting particle and named this possible particle the neutrino. In 1956, the neutrino was successfully detected by two American physics prize winners, and this achievement won the Nobel Prize in Physics in 1995.

Why are neutrinos important? Because they make up "1/4" of our material world.

Mountains and rivers, rocks and soil, every life that has ever lived on Earth, and even the stars in the universe - all the matter we know, visible and invisible, is made up of 12 basic particles. Among these basic particles, there are three different types of neutrinos, namely electron neutrinos (e), muon neutrinos (μ) and tau neutrinos (τ).

There are three types of neutrinos. Image from the Internet

Neutrinos are very lazy, electrically neutral, and do not react easily with matter. When 10 trillion neutrinos pass through the earth, only one neutrino will interact with the earth. In fact, trillions of neutrinos pass through our bodies at a speed close to the speed of light every second, but we can't feel it at all, which is also due to this characteristic of neutrinos.

"The most important physics discovery in Chinese history"

Neutrinos can not only become invisible, but also transform. The scientific community calls the mutual conversion between the three types of neutrinos "neutrino oscillation". To use another vivid metaphor, the three types of neutrinos, e, μ, and τ, are like your math, Chinese, and foreign language teachers. Imagine that your math teacher is teaching a class and suddenly becomes a Chinese teacher; the Chinese teacher is teaching and suddenly becomes an English teacher. This change is neutrino oscillation.

The SNO experiment in Canada and the Super-Kamiokande experiment in Japan respectively found the pattern of math teachers becoming Chinese teachers and Chinese teachers becoming English teachers. These two experiments jointly won the 2015 Nobel Prize in Physics.

So, the next thing to explore is, can’t a math teacher directly become an English teacher?

Is there a third mode of neutrino oscillation?

Around 2003, eight experimental schemes were proposed to detect the third oscillation mode of neutrinos, including the Daya Bay experiment in China. In the end, three schemes were implemented, namely China's Daya Bay, South Korea's RENO and France's Double Chooz. Whoever can first measure the third oscillation mode of neutrinos will win this global competition in physical science.

The Daya Bay experiment has its unique advantages - since neutrinos are not easy to capture, if scientists want to study them, they need a very strong neutrino source to continuously produce neutrinos, and there is a powerful neutrino source near the Daya Bay experiment detector.

The central detector of the Daya Bay Neutrino Experiment . Image source: Institute of High Energy Physics, Chinese Academy of Sciences

Neutrino sources are both natural and artificial. For example, the sun is a super large neutrino source. In addition, supernova explosions and cosmic rays passing through the atmosphere will produce a large number of neutrinos. The earth also emits neutrinos. Even we ourselves are a neutrino source, because the human body contains potassium, and the decay of potassium isotopes will produce neutrinos. In addition to these natural neutrino sources, there are also artificial neutrino sources such as reactor nuclear fission and accelerator beam target shooting.

The Daya Bay experiment site is located not far from the Daya Bay Nuclear Power Plant reactor, which can well receive the neutrinos produced by the nuclear fission of the reactor. Because of this special feature of Daya Bay, the Americans decided to abandon the two domestically designed plans and joined the Chinese Daya Bay experiment team with $34 million in construction funds and scientists.

At that time, all countries were racing against time to build experimental stations and carry out data collection work in order to publish this major physics achievement before other countries.

In August 2011, South Korea's RENO began to collect data, while China's Daya Bay experiment originally planned to build 8 detection equipment, but at this time, only 6 had been built. South Korea's early start in collecting data meant that they could announce their results at any time. The research team of China's Daya Bay experiment acted decisively and decided to start the detection equipment in advance!

Two months later, China's Daya Bay experiment measured the third oscillation mode of neutrinos and published this important result. Science magazine rated it as one of the top ten scientific breakthroughs in the world that year and considered it "the most important physics discovery in China's history."

Fully localized! The new story will begin 700 meters underground

In 2020, the Daya Bay experiment retired with honor, but there are still many unsolved mysteries about neutrinos.

In fact, as early as the construction of the Daya Bay experiment, the scientific research team began to think about neutrino research after Daya Bay. After repeated discussions and verifications by scientific researchers, Dashishan in Kaiping City, Jiangmen, Guangdong was finally selected as the new neutrino experimental base, named "Jiangmen Experiment". Its main task is to sort the masses of three types of neutrinos. Using this experiment, scientists can also study neutrinos emitted by supernovas, the earth and the sun, look for inert neutrinos, study proton decay, and so on.

To study neutrinos, it is necessary to shield the interference of cosmic rays and light as much as possible so that the detector can better detect the traces of neutrinos. The detector is built 700 meters underground to use the rocks and land to shield other cosmic rays. Of course, it is not enough to just shield other cosmic rays, you also need "eyes" to capture the traces of neutrinos.

The principle of capturing neutrinos is very simple: researchers inject "liquid scintillator" into a huge crystal ball 700 meters underground. If neutrinos react in this liquid, the liquid will fluoresce; then, the "hunting eyes" scattered around the sphere - the detection device "photomultiplier tube" will capture these weak light signals and convert them into electrical signals, which are saved by the data acquisition system and provided to researchers for analysis and research.

"Hunting Eye" photomultiplier tube Photo by Liu Yuexiang

Photomultiplier tubes are the core detection devices of China's Daya Bay and Jiangmen experiments. When the Chinese team proposed the Daya Bay experiment in 2003, China could only produce 2-inch photomultiplier tubes, which were small in diameter, not sensitive enough, and had poor batch stability, far from meeting the experimental requirements. At that time, Japan's Hamamatsu Corporation almost monopolized the production of high-end photomultiplier tubes internationally. The nearly 3,000 8-inch photomultiplier tubes used in the Daya Bay experiment were purchased by the United States from Japan.

For the Jiangmen experiment, if it wants to measure the neutrino mass ranking, the existing photoelectric detection devices in the world, including the products of Hamamatsu Corporation in Japan, cannot meet the requirements of the new experiment. In addition, Hamamatsu's photomultiplier tubes are expensive, more than twice our expected cost. We do not have the initiative to negotiate the price. Whether we can develop high-performance photomultiplier tubes has become the key to the success of the Jiangmen experiment.

Around 2009, the Chinese scientific research team decided to start developing China's own photomultiplier tubes as soon as possible to meet experimental requirements, break the monopoly, reduce experimental costs, and gain the initiative in external procurement.

At that time, Wang Yifang, the chief scientist of the Daya Bay experiment, and several colleagues designed a concept diagram of a photomultiplier tube, whose design idea was completely different from the traditional scheme commonly used internationally. After repeated discussions and verifications, they concluded that this idea was very bold, but it was feasible in principle!

Organic glass ball lifting platform Source: Institute of High Energy Physics, Chinese Academy of Sciences

At the end of 2011, a cooperation group led by the Institute of High Energy Physics of the Chinese Academy of Sciences and composed of several units including Northern Night Vision was established, and the preliminary research of new photomultiplier tubes was officially launched.

Over the past few years, the cooperation team has traveled almost half of China, looking for companies willing to work together on the research and development, exploring new process conditions and formulas from scratch, changing workers' operating habits, and overcoming technical difficulties including maintaining high quantum efficiency over the entire area, developing large glass shells, and vacuum packaging. In 2015, before the stipulated deadline, China finally developed a photomultiplier tube prototype with key technical indicators that reached the international advanced level, and had completely independent intellectual property rights, laying a solid foundation for entering engineering and mass production.

High-efficiency 20-inch photomultiplier tubes have never been mass-produced internationally, and there is no assurance about the reliability of the product. To ensure the safety of the supply chain, the research team finally decided that China's Northern Night Vision Company would undertake the production of 15,000 photomultiplier tubes and Japan's Hamamatsu Company would undertake the production of 5,000 photomultiplier tubes. In the end, the Jiangmen cooperation group measured that the average detection efficiency of the photomultiplier tubes produced by Northern Night Vision was 30.1%; and that of Hamamatsu in Japan was 28.5%, both of which were better than the experimental requirement of 27%.

It is worth being proud of that all the main materials of the Jiangmen neutrino detector are domestically produced with high quality, which greatly reduces the cost of the experiment.

Stainless steel container for the most transparent liquid. Photo by Liu Yuexiang

Why do we need to do basic scientific research?

From Newton's laws to quantum mechanics, from black holes to gravitational waves, all of humanity has always shared the fruits of basic science. In this case, why do we need to do basic scientific research ourselves when we can just wait for others to do it?

In this regard, Wang Yifang gave his answer in an interview with Guokr Science: "For all technological inventions and scientific achievements, the first discoverer has certain advantages. If you only enjoy the achievements of others, then others will look down on you and rob you of your wealth. If you master the most cutting-edge knowledge, you will naturally have the most cutting-edge technology. With the most cutting-edge technology, the rest will naturally follow."

The history of mankind is extremely short in front of the vast universe. Even in front of the Earth, which only exists for a few billion years, it is as short as one second in a day. But in such a short second, mankind has created a brilliant civilization and continues to explore the ultimate answer to the universe.

"Understanding the magical and wonderful world of particle physics is one of the highest signs of the development of human civilization," said Wang Yifang. The Jiangmen Neutrino Experiment is one of the most important basic scientific experiments in China, and it is another question China has sent to the vast universe. Knowing more about the mysteries of neutrinos may allow us to take this solemn and solid small step in our journey of exploring the vast universe.

The Jiangmen Neutrino Experiment broke ground in 2015 and is scheduled to be completed this year. The scene described at the beginning of the article no longer exists. The ventilation ducts have well ensured that the temperature and humidity of the base meet the experimental requirements.

700 meters underground, we look forward to hearing the voice of China.

[Thanks to Ms. Zhao Jie, Associate Researcher of Neutrino Research at the Institute of High Energy Physics, Chinese Academy of Sciences, for reviewing and providing suggestions for this article]