How does the "Crystal ball" buried 700 meters underground capture the magic Neutrino? Ask to the end

Source: CCTV News ClientEvery moment, our bodies are penetrated by tens of thousands of 'invisible substances', yet we are completely unaware of their existence. These "stealth matter" is one of the most basic particles in the material world: Neutrino

Source: CCTV News Client

Every moment, our bodies are penetrated by tens of thousands of 'invisible substances', yet we are completely unaware of their existence. These "stealth matter" is one of the most basic particles in the material world: Neutrino.

In Jiangmen, Guangdong, the national major scientific and technological infrastructure, Jiangmen Neutrino experimental device, is under intense construction. At present, the central detector located 700 meters underground, the organic glass sphere, has been installed in the equatorial layer, and the construction of the entire detector is expected to be completed by the end of this year.

What is Neutrino? Why detect Neutrino?

What is the ubiquitous Neutrino in the universe? Why do we need to build a large device specifically to detect it?

Hu Tao, the on-site installation manager of Jiangmen Neutrino Experiment, a researcher of the Institute of High Energy Physics of the Chinese Academy of Sciences: Neutrino is a neutral particle without charge. The so-called micro means that its mass is very small. It is about one billionth less than the mass of the electron. Like electrons and quarks, it is one of the Elementary particle that make up the material world today.

Neutrino is the smallest brick in the physical world known to mankind, which is even smaller than the gap between the particles that make up our body and is difficult to detect, so it is also called "ghost particle". Neutrino are everywhere. A large number of Neutrino are produced by the Nuclear reaction inside stars, the explosion of supernovae, the operation of nuclear reactors, and even the decay of radioactive materials in rocks. However, because the interaction between Neutrino and ordinary matter is very weak, and even can pass through the whole earth without being absorbed by any matter, the ubiquitous Neutrino are the least understood Elementary particle, and there are still many unsolved mysteries.

Hu Tao, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, Jiangmen Neutrino Experiment Site Installation Manager: Because Neutrino are difficult to interact, and even if it happens, it is Weak interaction. In other words, there is very little energy exchange, so the detector must be made very large, so that it is possible to get enough samples to detect these elusive "ghost particles". What we are building is a huge A spherical detector buried deep underground.

What does the Neutrino detector look like 700 meters deep underground?

The reason why we want to detect the ubiquitous Neutrino around us is that they still have many unsolved mysteries. And our detector is a huge spherical device buried 700 meters underground. What exactly is this device under construction?

Ren Meimei, a reporter from CCTV: From the entrance of the Neutrino Underground Laboratory in Jiangmen to the depth of 700 meters underground, you need to take a cable car. This cable car is also a "shuttle" for researchers to get on and off the work every day, which can carry more than 40 people at a time.

Taking a cable car and passing through a 1267 meter inclined shaft tunnel, the reporter arrived at a 700 meter underground rock cave in about 15 minutes.

CCTV reporter Ren Meimei: The spherical detector in the center of this huge pool is the main detector. It is an organic glass body with a diameter of more than 30 meters. On the outside is a stainless steel support frame. The installation of a series of detection equipment, such as Photomultiplier tube, is under way. It is like a big "eye" buried deep in the ground, helping scientists capture the traces of Neutrino.

How to install the maximum equatorial layer of a spherical detector?

At present, the central detector of the Neutrino experimental device, the plexiglass ball, has been installed in the equatorial layer. What are the difficulties in installing the equatorial layer as the largest circle?

Shuai Junquan, a reporter from CCTV: At the bottom of the pool for the Neutrino experiment in Jiangmen, the world's largest and most sensitive liquid scintillator Neutrino detector is being built above. Above the construction platform of the scaffold, the world's largest diameter 35 meter organic glass ball is being constructed, and its largest circle layer - the equatorial layer - is being installed.

The lifting platform is an important auxiliary platform for completing the installation of organic glass balls, with a diameter and height that can be changed layer by layer. The engineering personnel will complete the lifting and positioning, splicing and polymerization, curing, annealing, polishing, cleaning, and coating processes of organic glass layer by layer on the platform, and ultimately complete the overall installation of organic glass balls.

Ma Xiaoyan, chief engineer of Jiangmen Neutrino Experiment, senior engineer of Institute of High Energy Physics, Chinese Academy of Sciences: Compared with other layers, the difficulty of equatorial layer lies in the large size of each plate, which will be relatively difficult in the process of adjustment and installation.

Experts introduce that the entire detector is composed of a total of 263 pieces of organic glass panels, each with a thickness of 120 millimeters, which is currently the largest single organic glass structure in the world.

Ma Xiaoyan, chief engineer of Jiangmen Neutrino Experiment, senior engineer of Institute of High Energy Physics, Chinese Academy of Sciences: You think 120mm is very thick. If you look at it proportionally, it is actually thinner than the eggshell. (Because) In the future, this place where we stand is a liquid scintillator. Twenty thousand tons of liquid scintillators are inside a plexiglass ball.

How will the underground "Crystal ball" capture "ghost particles"?

According to experts, the main part of the Jiangmen Neutrino detector, that is, the plexiglass ball with a diameter of about 35 meters, is equivalent to the height of 12 floors. After completion, 20000 tons of liquid scintillators will be filled in the glass ball, while the pool outside it will be filled with 35000 tons of high-purity water, equivalent to 3 million barrels of water. Why should a liquid scintillator be injected into the spherical detector, and how will such a setting capture the "ghost particle" Neutrino?

Liquid scintillator, as the medium for detecting Neutrino, its main component is actually a liquid called alkyl benzene, which is the raw material of detergent and liquid detergent used in daily life. It is non-toxic, tasteless and harmless to human body. However, the liquid scintillator used in the Jiangmen Neutrino experiment should not only be very transparent, but also be very clean. Specifically, the dust mixed in 20000 tons of liquid should not exceed 0.008g.

Hu Tao, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, Jiangmen Neutrino Experiment Site Installation Manager: After charged particles hit it, it can emit light. We call it flicker light, so we call it liquid scintillator. A ball with a diameter of 37 meters may emit light in the middle. It needs to pass through the scintillator. If the transparency is not enough, the light will be absorbed inside, and it will not be transmitted. So we need to be more transparent.

When the outside Neutrino enters the detector, as long as one Neutrino interacts with the liquid scintillator, it will emit weak light, which is indistinguishable to the naked eye. However, tens of thousands of Photomultiplier tube around the detector can capture it, convert the optical signal into electrical signal, and amplify the weak electrical signal by 10 million times.

Hu Tao, site installation manager of Jiangmen Neutrino experiment, researcher of Institute of High Energy Physics, Chinese Academy of Sciences: Photomultiplier tube converts it from optical signal to electrical signal, and then sends it to computer through electronics. After data analysis, we can get some properties of Neutrino.

Why is the Neutrino detection device built 700 meters underground?

Why should the Neutrino detector be built 700 meters underground? Can't Neutrino be detected on the ground?

Hu Tao, the on-site installation manager of Jiangmen Neutrino Experiment, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences, said: Neutrino is originally difficult to detect. There are a large number of Cosmic ray from the universe on the ground, and this Cosmic ray hitting our detector will also produce false Neutrino signals. So we put the detector underground, so that the rock layer 700 meters underground can block these Cosmic ray.

How difficult is it to build a large scientific installation 700 meters underground?

Experts said that in order to capture Neutrino, an environment that is deep enough, dark enough and clean enough is needed. The vast majority of Cosmic ray can be shielded 700 meters underground. However, building such a huge detection device 700 meters underground is not an easy task. What are the difficulties?

Hu Tao, the on-site installation manager of Jiangmen Neutrino Experiment, a researcher at the Institute of High Energy Physics of the Chinese Academy of Sciences: First of all, this is in the south. When digging, there was a lot of water. Now we pump up 300 or 400 cubic meters of water every hour. It is very difficult to dig our underground tunnels, including the underground laboratory. Second, the temperature of the rock 700 meters underground has reached 37 degrees, This high temperature and humidity environment places great pressure on our personnel's work. The third one is about organic glass balls, which are currently the largest single organic glass balls in the world, so there are actually many technical difficulties involved in the entire production.

In order to ensure the cleanliness, deformation control, and operational safety of the detector, the entire stainless steel grid underground assembly does not use welding, but uses patented high-strength stainless steel rivet connection technology, with an accuracy of millimeter level, and the number of rivets reaches 120000 sets.

What scientific puzzles will be solved when the Neutrino detection device is completed?

The Neutrino detector, a scientific heavy weapon buried 700 meters underground, will solve some scientific puzzles for us and explore some mysteries of the universe when it is completed?

It is understood that the Jiangmen Neutrino experiment will run for 30 years after its completion, and it is expected to produce a series of major physical achievements in Neutrino mass sequence, accurate measurement of Neutrino mixing parameters, and research on supernovae, Earth and solar Neutrino.

Hu Tao, the on-site installation manager of the Neutrino experiment in Jiangmen, a researcher of the Institute of High Energy Physics of the Chinese Academy of Sciences, said that the primary goal of the Neutrino experiment in Jiangmen is to measure the quality order of Neutrino, which is a very important parameter for Neutrino physics, including astrophysics. The second is to further accurately measure some parameters of Neutrino oscillation, hoping to achieve the most advanced level in the world, and hope to have new physical discoveries, In addition, we may have some contributions to the research on the mechanism of supernova explosion, the physical Nuclear reaction of the Solar core, and even the Earth model.

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