Lasso looks like this! There are 1188 mounds …
The plane departed from Tianfu Airport in Chengdu, Sichuan, and arrived over Daocheng Airport in Ganzi Prefecture more than an hour later. At this time, overlooking from the plane porthole, a huge "disk" spread out on the earth.
This "disc" is the High Altitude Cosmic Ray Observatory (LHAASO), also known as "Lasso". It is the highest altitude, the largest scale and the most sensitive gamma-ray detection device in the world. It is located on Haizi Mountain at an altitude of 4,410 meters in Daocheng County, Sichuan Province, covering an area of about 1.36 square kilometers.
“‘ Lasso ’ The task is to catch ‘ from outer space. Sprinkle ’ Cosmic rays to the earth allow scientists to discover the unknown world in the depths of the universe and solve the mystery of the origin of cosmic rays. " Cao Zhen, chief scientist of Lasso and researcher of Institute of High Energy Physics, Chinese Academy of Sciences, introduced.
On May 10th, after 8 years of pre-research and 4 years of construction, "Lasso" ushered in key nodes, successfully passed the national acceptance and was officially put into operation. The Acceptance Committee believes that the completion and operation of "Lasso" has made it one of the three major international experimental facilities for particle astrophysics, which is of great significance for promoting major original breakthroughs in this field and promoting the development of frontier interdisciplinary related disciplines and international cooperation.
350,000 tons of pure water waiting for cosmic rays
Looking down from the air at the big disk of the cable, the most striking thing is the square in the center, which is the array of water cherenkov detectors.
Panoramic aerial photograph of "Lasso". Photo courtesy of Institute of High Energy Physics, Chinese Academy of Sciences
It is mainly used to detect very high-energy gamma photons, and consists of three pools, two of which are square pools with a side length of 150 meters and the other is a rectangular pool with a length of 300 meters and a width of 110 meters. The total area of the three pools is 78,000 square meters, which is equivalent to 2.5 water cubes. The huge pool contains 350,000 tons of pure water.
When cosmic rays reach the earth’s atmosphere, many secondary particles will be produced. When these particles cross the pure water in the pool, they will emit very weak blue light, which is Cherenkov light.
In order to "capture" the very weak Cherenkov light, scientists installed many crystal clear "glass bubbles" at the bottom of the pool.
"These ‘ Glass bubble ’ Is the detector unit. They are called photomultiplier tubes, which can convert weak Cherenkov optical signals into electrical signals to analyze cosmic rays. " Chen Mingjun, the general manager of the array of Lasso Shuicherenkov detectors, pointed to the "glass bubble" model and introduced it to reporters.
For Chen Mingjun team, maintaining the transparency of pure water has become a major challenge.
"When water is injected into such a large pool, it is very easy to be polluted. At first, we couldn’t clean it. This is the most troublesome problem for us. We also have nothing to learn from, and other projects will not use so much water. " Chen Mingjun said, "In order to clean the water, the team explored for two years."
"We have repeatedly tried various schemes and constantly adjusted the chlorine concentration. Sometimes we are really desperate. Fortunately, our colleagues encourage each other." Finally, Chen Mingjun and his team successfully solved this problem by adjusting chlorine dosage and increasing water circulation flow.
Muzi detector under 1188 mounds
Boarding the "lasso" array viewing platform, many regularly arranged mounds caught the reporter’s eye. These mounds are about two people high, each covering an area of about 36 square meters, and the interval between mounds is 30 meters.
Cao Zhen introduced that there are 1,188 mounds in the "Lasso" array, and a precise detector is buried in each mound — — Muzi detector.
Science and Technology Daily reporter Lu Chengkuan photo
The detectable area of each Muzi detector is 36 square meters. 1188 detectors spell out a sensitive detection area of about 40,000 square meters.
This is the largest Muzi detector in the world. "Only when the detection area is large enough can we accurately screen out ultra-high energy gamma photons." Xiao Gang, the overall head of the "Lasso" Muzi detector array, said.
Although there are great benefits, there are also great difficulties.
The main part of the detector is buried under 2.5 meters deep soil, so it is impossible to maintain it frequently, which requires it to have very high reliability.
In order to make the detector "durable", researchers have spent a lot of time, and the light pre-research took 8 years.
"We first make a reasonable plan in design, and put equipment such as power supplies that are easily damaged on the mound, and equipment that is not easily damaged will be buried in the soil." Xiao Gang stood by the mound and said, "But design alone is not enough. Although the equipment buried in the soil is not easy to break down, once there is a problem, it will be very troublesome to repair."
In order to do a good job in quality control, Xiao Gang’s team also adopted a series of technical means.
"For example, the water bag filled with ultra-pure water is a key component of our detector. Its quality requirements are very high, and it must be absolutely sealed. It cannot enter any pollutants to affect the detection efficiency." For example, Xiao Gang said, "To this end, we have specially found a rare gas — — Sulfur fluoride is used to test the tightness of the water bag. "
"The detection efficiency of this rare gas is very high. You can know which screw is not tightened by using it." Xiao Gang said with a smile, "This ensures the reliability of the water bag."
Detection sensitivity that challenges the theoretical limit
With the continuous efforts of the research team, "Lasso" has "clairvoyance" and "clairvoyance".
Here, detectors are everywhere.
In the "lasso" array, in the gap between the mounds, the reporter also found many green or white "small boxes". These "small boxes" are about 1.5 meters long and 1 meter wide, and they are called electromagnetic particle detector arrays. The distance between them is 15 meters and they are arranged in a zigzag pattern.
The electromagnetic particle detector array, the largest detector array in the "cable", is mainly used to detect the secondary electromagnetic particles produced when cosmic rays reach the atmosphere.
"This array consists of a total of 5,216 detectors. We have used it for more than 10 years from pre-research to design, installation and debugging." Cao Yu said.
High altitude cosmic ray observatory (LHAASO) (panoramic photo of UAV) taken on April 21st. Xinhua News Agency reporter Jin Liwang photo
In Cao Zhen’s view, the "lasso" that condenses the painstaking efforts of many researchers has not lived up to their hard work and expectations.
"Lasso" carried out observation work during the construction period. With unprecedented detection sensitivity, during the trial operation, "Lasso" made a number of breakthrough major scientific achievements: a large number of ultra-high energy cosmic accelerator candidate celestial bodies were discovered in the Milky Way, and the highest energy photons observed by human beings were recorded, which opened the era of "ultra-high energy gamma astronomy"; The ultra-high energy brightness of the standard candlelight crab nebula was accurately measured, and 1 trillion electron volts gamma radiation was found, which challenged the theoretical limit.
Up to now, about 215 periodical papers and 156 conference papers have been published based on the Lasso project.
Report to the 20th CPC National Congress of the Communist Party of China pointed out that we should speed up the implementation of a number of major national scientific and technological projects with strategic overall situation and forward-looking, and enhance the ability of independent innovation.
Looking forward to the work prospect after the official operation, Cao Zhen said confidently: "As a major national science and technology infrastructure, ‘ Lasso ’ In the future, more high-level research results will be produced to help science and technology run out of self-reliance ‘ China speed ’ ！”