Climbing the World Science and Technology Summit at 2400 meters underground, Tsinghua University | Material | Technology

A New Journey of Building a Strong Country with Talents

China Jinping Underground Laboratory, Tsinghua University Dark Matter Experimental Research Team:

Climbing the World Technology Summit at 2400 meters underground

Guangming Daily reporter Yang Tongtong

Jinping Mountain in Liangshan Yi Autonomous Prefecture, Sichuan is towering and beautiful, surrounded by the Yalong River at the foot of the mountain.

At a vertical distance of 2400 meters from the mountaintop, there is currently the deepest underground laboratory in the world with rock cover - China Jinping Underground Laboratory.

There is a group of science and technology workers in the laboratory who like to call their research work "waiting for rabbits.".

The rabbits they are waiting for are extraordinary! This is the mysterious substance in the universe - dark matter. Dark matter, although never seen, has been identified by scientists through numerous astronomical observations, accounting for approximately 85% of the total mass of matter in the entire universe. You should know that the ordinary matter we currently see, such as stars and planets, only accounts for about 15% of the total mass of matter in the entire universe.

Underground, it can effectively shield the "noise" interference of cosmic rays on dark matter detection. In order to detect dark matter, this group of technology workers work day and night deep in the mountains and in the darkness to carry out scientific research and research, leading the world in the field of direct detection of dark matter.

Members of the dark matter experimental group are testing a new type of high-purity germanium detector. Information images

Trekkers in the Dark Matter World

In mid June 2023, Cheng Jianping, the director of Jinping Underground Laboratory and a double hired professor at Tsinghua University, visited the construction site of the second phase of the laboratory project. Looking at the laboratory that was about to be completed, Cheng Jianping was extremely excited and said, "This will greatly promote China's major basic and cutting-edge research in particle physics and nuclear physics, and become a paradise for particle physicists in China and even the world to explore the unknown."

In 2003, facing the urgent need to improve the level of basic research in China, Cheng Jianping led a team of more than ten people, aiming at the forefront of world science and determined to engage in direct detection research of dark matter. This will take 20 years.

At first, there were no underground laboratories in our country, and they had to frequently run to underground laboratories abroad for research. However, the experimental space given to them was only 2 square meters large. In order to find suitable experimental sites for dark matter research in China, Cheng Jianping's team has conducted research on various underground spaces nationwide for several years, but none of them meet the experimental conditions. By chance, they saw the news of the completion of the Jinping Mountain traffic tunnel, and an idea flashed through their minds - "We have the venue!"

Read out the electronic group members analyzing electronic signals. Information images

So they collaborated with Yalong River Basin Hydropower Development Co., Ltd. to jointly build a laboratory. Zeng Zhi, the head of the low background group at Jinping Underground Laboratory and a researcher at the Department of Engineering Physics at Tsinghua University, was also one of the earliest members to participate in laboratory construction. In the past 15 years, he has visited the underground laboratory of Jinping countless times and times, and the situation of staying for several consecutive months due to tight tasks often occurs.

"The steel bars, cement, aggregates, and experimental equipment in the laboratory all need to meet the requirements of low radiation background. We have carefully screened and measured every detail." Zeng Zhi said that the cosmic ray flux in the laboratory is only about one hundred million times that of the ground level. With the expansion of the laboratory from 4000 cubic meters in the first phase to 300000 cubic meters in the second phase, it has become the deepest, largest, and purest deep ground laboratory in the world, and is an internationally first-class cutting-edge physics experimental platform.

Over the past 15 years, the team has gathered more and more scientists and researchers who are passionate about science and have a strong curiosity about the universe to conduct direct detection research of dark matter in the underground laboratory of Jinping.

The first physical result of direct detection of dark matter in China has been obtained, successfully excluding the area claimed by a certain experimental group in the United States to "detect dark matter", and building the international first point electrode high-purity germanium array experimental system for direct immersion refrigeration in liquid nitrogen... The team has achieved a series of internationally leading scientific research results, enabling China's dark matter research to achieve a leap from "following" to "parallel", and then to "leading" locally.

Work together to overcome the bottleneck problem

In the past, there was always a hurdle in the minds of team members, which greatly limited their footsteps in scientific research.

That is the core device for detecting dark matter - high-purity germanium detectors have long relied on imports. "We want to develop our own high-purity germanium detector!" Since 2010, Li Yuanjing, Deputy Director of Jinping Underground Laboratory and Researcher of the Department of Engineering Physics at Tsinghua University, led the team to embark on the path of conquering detectors.

Starting from scratch is not easy! Due to the discovery of dark matter, high sensitivity is required for high-purity germanium detectors. Li Yuanjing and his team need to be able to prepare high-purity germanium crystals and explore the entire process of detector fabrication. "Every step forward is a hurdle for us, and there are countless such hurdles," said Li Yuanjing.

In 7 years, Li Yuanjing and his team overcame the bottleneck technology of high-purity germanium detectors and developed China's first 500g level high-purity germanium detector. Since then, the development of high-purity germanium detectors has entered the "fast lane", and the 1kg and 1.5kg scales have also been introduced one after another.

"To conduct scientific research, one must have the courage to break through the darkness before dawn. Once the first ray of dawn is reached, there will be a bright light ahead." Li Yuanjing said with great anticipation, "Next, our goal is to complete a high-purity germanium detector at the hundred kilogram level."

At the underground laboratory in Jinping, team members are highly encouraged to make bold assumptions and try bravely. With the continuous expansion of the scale of high-purity germanium detectors in the laboratory, more and more pulse signals are captured, posing greater challenges to Xue Tao, the head of the readout electronics group in the Jinping underground laboratory and associate researcher in the Department of Engineering Physics at Tsinghua University.

Members of the low background group are conducting surface treatment and dimensional measurements on the background testing materials. Information images

"Faced with massive signals, we need to increase the readout bandwidth, convert more pulse signals into data signals and store them in computers, while also avoiding accidentally deleting useful signals hidden in them when abandoning electronic 'noise'." Xue Tao told reporters about the difficulties we face.

They came up with a bold idea to increase the read bandwidth from 1 gigabit per second to 40 gigabit per second, and used neural networks combined with digital filtering to study new real-time online processing algorithms and conduct preliminary screening of waveform data.

At first, Xue Tao was quite hesitant about this idea. With such a big step, could it be realized? "Let's go do it!" With the strong support of the Jinping underground laboratory, Xue Tao led his team to overcome one difficulty after another.

"We've found it!" On a late night in March 2023, Xue Tao's phone received an exciting message. "For many years, we have been dreaming of finding useful signals hidden within the 'noise' of electronics, providing more useful data for the analysis of suspected dark matter signals in the future. This time, we have finally succeeded," said Xue Tao.

Handing the baton over to young people

"The Jinping Underground Laboratory, led by Tsinghua University, not only aims to launch scientific research achievements but also shoulders the mission of cultivating talents." Li Yuanjing said, "The main force of our team is these young researchers born in the 1980s and 1990s. Cultivating young people is also an important achievement, and for this, we have given them many opportunities to take on important responsibilities."

In January 2017, Tian Yang, who had just completed his postdoctoral research abroad, returned to Tsinghua University and, under the guidance of Li Yulan, the head of the detector team and the chief researcher of the Engineering Physics Department at Tsinghua University, took on the next important task - to change the cooling method of high-purity germanium detectors from vacuum chamber liquid nitrogen direct cooling to liquid nitrogen bare immersion.

For this important task, Tian Yang felt a lot of pressure, but full of energy and drive. "The competition for dark matter detection is fierce internationally, and we must seize every second," he said. Staying up late or even staying up all night is a common occurrence for Tian Yang. He still remembers the night in December 2018 when he first placed the adjusted high-purity germanium detector in liquid nitrogen. He and his team stared at it until 2am and couldn't bear to leave.

In the past 5 years, Tian Yang discovered and solved problems, finally enabling the high-purity germanium detector to meet future experimental requirements. Now an associate researcher, he is looking forward to applying his research results to the liquid nitrogen thermostat in the second phase of the Jinping underground laboratory as soon as possible.

China Jinping Underground Laboratory Phase II Large Low Temperature Liquid Nitrogen Thermostat. Information images

In early July 2023, Yang Litao, a member of the dark matter experimental group at the Jinping Underground Laboratory and an assistant researcher at the Department of Engineering Physics at Tsinghua University, came to the laboratory to install and debug high-purity germanium detectors on site.

Since pursuing her doctoral degree, Yang Litao has been engaged in research on dark matter detection under the guidance of Yue Qian, the head of the Science Department of Jinping Underground Laboratory, the chief researcher of the Department of Engineering Physics at Tsinghua University, and Ma Hao, the head of the dark matter experimental group and associate professor of the Department of Engineering Physics at Tsinghua University. At that time, it was urgent to develop a data analysis method that could distinguish suspected dark matter signals. Yang Litao set herself a "military order": "Don't do it, don't cut your hair, don't shave."

In the underground laboratory of Jinping, Yang Litao uses the day to debug and returns to her dormitory at night to write algorithms, maintaining a high-intensity work state almost every day. During this time, when everyone saw Yang Litao with her hair over her shoulders, they couldn't even recognize her, "she didn't look like a young man from the 90s at all.".

Two years later, when Yang Litao appeared in front of the team dressed neatly and neatly, everyone knew that he had done it!

In the team, there are still many persistent young researchers like Tian Yang and Yang Litao, who have gradually grown into backbone forces of scientific research and innovation through the experience of major projects.

"We can confidently hand over the relay baton to them, believing that they will make greater breakthroughs and innovations!" said Cheng Jianping.

Guangming Daily

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