What kind of experiment is done 2,400 meters underground? | Science and Technology of a Great Country

Audit expert: Chao Wei

Associate Professor, Beijing Normal University, PhD in Theoretical Physics

The "artificial sun" broke the record again, the domestically-made large passenger aircraft C919 achieved commercial operation, Xinjiang's saline-alkali land had a bumper harvest of seafood, the medical heavy ion accelerator was put into clinical application, the first domestically-made large cruise ship "Aida Magic City" successfully completed its sea trial, and the world's deepest underground laboratory was waiting for a "visit" from dark matter... In 2023, my country's science and technology continued to make new breakthroughs, bringing new possibilities and opportunities to human life and development.

On the occasion of the 74th anniversary of the founding of the People's Republic of China, Digital Beijing Science Center has brought you a special column "Great Power Science and Technology Style", allowing us to review the scientific and technological achievements made this year under the leadership of the Party.

Imagine, 2,400 meters underground, what can we see?

In such an "abyss", hidden deep in the Liangshan Yi Autonomous Prefecture of Sichuan Province, a group of scientific researchers are concentrating on searching and capturing with the faint flashes of amplified nuclear signals beside instruments that are running non-stop.

This is the China Jinping Underground Laboratory (CJPL).

Source: Astronomy Online

Jinping Underground Laboratory is the first dark matter laboratory in China and the deepest, largest and best-equipped underground laboratory in the world . It is located in the Jinping Mountain Tunnel 2,400 meters underground in Liangshan Prefecture, Sichuan Province. It can use the thick rock layer of the mountain to block the interference of cosmic rays, creating a unique environment for laboratory construction.

Source: Qiguan State-owned Assets

In order to ensure the accuracy of laboratory construction, the project construction party has taken measures such as "waterproofing and radon suppression" to try to reduce the impact on the original extremely low radiation environment of the cavern. At the same time, a large combined solid shielding device, a polyethylene shielding cabin, was also built to achieve lower radiation background requirements.

Some time ago, the second phase of this laboratory was completed, and the laboratory cabin officially entered the clean construction stage. In the future, it will also provide a very important research platform for my country's basic scientific experiments.

During the laboratory clean construction phase, the internal cave wall has completed waterproof and radon suppression construction. Source | Observer.com

Seeing this, you may still be a little confused. What kind of high-tech is this laboratory? What is the dark matter it studies? What is its significance? Without further ado, let's quickly follow the editor to learn more.

1Why is the construction so deep?

Jinping Underground Laboratory can be said to be "standing in the abyss and looking up at the stars". What are you looking up at? Dark matter. Astronomy enthusiasts have more or less heard of it. Dark matter is actually a kind of existence that does not reflect or absorb light. It is different from common forms of matter. It is invisible and intangible, and does not participate in any electromagnetic interaction or strong interaction . It can be said that it accounts for the largest mass in the universe, but it is the weakest "light".

If the entire universe is compared to an ocean, so far, due to limited understanding and observation methods, humans have only been exploring a small part of this "ocean".

In order to capture this mysterious and "shy" dark matter, more than 20 detection experiments have been carried out internationally since the 21st century. People believe that whoever unveils its veil first will promote another major leap in human understanding of the material world and the universe.

At present, dark matter detection methods can be roughly divided into three categories: "going to the sky", "going underground" and "artificial dark matter": "going to the sky" refers to indirect detection experiments , which are usually conducted in space to reduce interference from the Earth's atmosphere; "going underground" refers to direct detection experiments , which are usually conducted in underground laboratories to reduce interference from cosmic rays; "artificial dark matter" refers to accelerator experiments, which produce dark matter particles through the collision of ordinary particles, mainly carried out on the Large Hadron Collider at the European Center for Nuclear Research.

The Jinping Underground Laboratory in China adopts an "underground" research method. In order to remove irrelevant interfering particles in the universe to the greatest extent, this mysterious space in Liangshan is also equipped with multiple layers of protective "armor".

Rocks block cosmic rays, source | Screenshot of Focus Interview video

The first layer is a natural rock barrier; the second layer is a 50-centimeter-thick concrete structure for the cavern safety support structure; the third layer is a 1-meter-thick polyethylene for neutron shielding; the fourth layer is a 20-centimeter-thick lead house for gamma ray shielding; the fifth layer is a 20-centimeter-thick boron-containing polyethylene for absorbing thermal neutrons; the sixth layer is a 10-centimeter-thick copper house for shielding gamma rays from external lead and boron-containing polyethylene. These devices shield almost all conceivable radiation sources .

So, as the deepest underground laboratory in the world, why was Jinping Underground Laboratory built in Liangshan Prefecture? Can't it be built elsewhere?

2Why should it be built in Liangshan Prefecture, Sichuan Province?

There are many mountains in China, and the reason why Liangshan Prefecture, Sichuan Province was chosen as the location of the laboratory is because of "the right time, the right place and the right people."

In 2003, the planning team of the Dark Matter Laboratory, a group of people from the Department of Engineering Physics of Tsinghua University, searched for a suitable place to build a laboratory nationwide. While watching the news, the team members suddenly discovered that a large-scale Jinping Hydropower Station project was being built in Jinping Dahewan, Yalong River, Liangshan, Sichuan. The rock layer here is thick enough and has natural geographical advantages. In addition, there is a Jinping Mountain traffic tunnel connecting the two power stations . If a laboratory is built here, it will also facilitate the research team's needs to enter and exit.

Therefore, the Tsinghua University team went to Sichuan to communicate and cooperate with the hydropower station construction unit, Yalong River Basin Hydropower Development Co., Ltd., and after many negotiations, the deal was finally successfully finalized.

Cooperation ceremony, source | Jinping Underground Laboratory official website

Therefore, in 2009, the construction of the first phase of the dark matter laboratory started in the middle of the Jinpingshan Traffic Tunnel, and it was officially put into use in 2010, filling the gap of the lack of deep underground laboratories in China.

Source | Jinping Underground Laboratory official website

In 2014, due to the high-quality cooperation in the first phase of the project, the Tsinghua University team continued to cooperate with the Yalong River Hydropower Project construction team to start construction of the Jinping Underground Laboratory Phase II project; in 2015, the civil construction of the laboratory Phase II project was completed; in 2020, the "Extremely Deep Underground Extremely Low Radiation Background Frontier Physics Experimental Facility" based on the construction of the laboratory Phase II and included in the "13th Five-Year Plan" national major scientific and technological infrastructure plan was officially started and is expected to be put into use in 2024.

3 High-tech behind the lab

Jinping Underground Laboratory is the underground laboratory with the thickest rock cover in the world. The cosmic ray flux inside the laboratory is about one billionth of that on the ground , providing a very "clean" testing environment for dark matter experiments.

In addition, Jinping Underground Laboratory has a lot of hard-core high-tech, including waterproof and radon suppression engineering, extreme clean design, ultra-low background measurement, high-purity germanium detection , etc.

Jinping Mountain is located on the main stream of the Yalong River. The area is rich in water and has a rainy climate, so the water content in the rock mass is very high. In addition, the nitrogen precipitated from the rock layer of the mountain will also have a great impact on the detection of dark matter. Therefore, in order to provide a good experimental environment for dark matter research and reduce the interference of unnecessary factors, it is necessary to carry out waterproofing and nitrogen suppression engineering.

Laboratory address, source | Jinping Underground Laboratory official website

Water and nitrogen are everywhere in the deep earth environment, and it took the construction unit a lot of time to completely isolate them. The construction party designed a complex structure with up to 10 layers, adopted an integrated drainage and drainage construction process, drove anchor rods into the outer layer of the rock wall, laid drainage boards, and sprayed rubber waterproof coatings, sprayed cement mortar, polyurethane mastic, etc. After more than 6 months of testing and corrections, they finally successfully blocked water and nitrogen outside the laboratory .

Jinpingshan Tunnel, Source | Xinhua News Agency

In the construction of the laboratory, airtight doors are installed at all connections, and purification channels for human and material flows are designed with full consideration. High-efficiency filtration seals are used, and groundwater is used for temperature control to ensure that the temperature inside the laboratory remains at a comfortable 25°C. At the same time, the ambient humidity is below 65%, providing a clean and tidy research environment for experimental personnel and equipment.

In addition, in order to ensure the accuracy of the experimental results, the laboratory used GeTHU to perform ultra-low background measurements . All substances have background radiation. By selecting high-quality low-background materials and accurately controlling the amount of experimental materials, researchers can ensure that the experimental environment will not be affected by radiation from other substances.

GeTHU series spectrometer, source | Jinping Underground Laboratory official website

Under the protection of heavy "armor" in the Jinping Underground Laboratory, in the low-temperature liquid nitrogen environment, lies a 1 kg high-purity germanium experimental detector for directly detecting dark matter. It is made of "germanium", the world's purest crystal material .

The manufacturing process of high-purity germanium detectors is very complex and has a high technical threshold. Only three European and American companies have mastered this technology. Therefore, Chinese scientists started from scratch and successfully developed the manufacturing technology, which greatly promoted the level of dark matter research in China.

The research team is conducting material radioactivity background measurements. Source: Science and Technology Daily

It took decades for Jinping Underground Laboratory to be established, built and completed. Although we have not yet figured out the nature of dark matter, Chinese scientists have been working silently.

We must bring light to the darkness and build a powerful national weapon rooted in the center of the earth. Although we are still waiting for the "glimmer" of dark matter collision, scientific and technological construction cannot be achieved overnight. Exploring the unknown and finding certainty in uncertainty, perhaps this is the charm of science.