Source: CCTV News Client
Every moment, our bodies are passed through by tens of thousands of "stealth substances", but we are completely unaware of their existence. These "stealth substances" are one of the most fundamental particles that make up the material world: neutrinos.
In Jiangmen, Guangdong, the Jiangmen Neutrino Experimental Device, a major national science and technology infrastructure, is under intensive construction. At present, the central detector plexiglass ball located 700 meters underground has completed the installation of the equatorial layer, and the construction of the entire detector is expected to be completed by the end of this year.
What are neutrinos? Why should we probe it? What does this detection device 700 meters deep underground look like? How will it capture neutrinos? What mysteries of the universe will be answered for us by studying neutrinos? Ask the end ↓
What are neutrinos? Why detect neutrinos?
What exactly are the ubiquitous neutrinos in the universe? Why should we build a large device to detect it?
Hu Tao: Neutrino is an uncharged neutral particle, the so-called micron, that is, the mass is particularly small, it is about one billionth smaller than the mass of an electron, like electrons and quarks, they belong to one of the most basic particles that make up the current material world.
Neutrinos are the smallest bricks known to build the material world, smaller than the gaps that make up our human body, and are difficult to detect, so it is also called "ghost particles". Neutrinos are everywhere, nuclear reactions inside stars, explosions of supernovae, the operation of nuclear reactors, and even the decay of radioactive material in rocks, all produce large numbers of neutrinos. However, because neutrinos interact weakly with ordinary matter, and can even pass through the entire earth without being absorbed by any matter, the ubiquitous neutrinos are the least understood of elementary particles, and there are still many unsolved mysteries.
Hu Tao, researcher at the Institute of High Energy Physics, Chinese Academy of Sciences, Jiangmen Neutrino Experiment Site Installation Manager Hu Tao: Because neutrinos are difficult to interact, and even if they do, they are weak interactions, in other words, the energy exchange is very small, so the detector must be particularly large, large enough to be able to get enough samples to detect these "ghost particles" that are difficult to capture.
700 meters deep underground What does the neutrino detection device look like?
The reason why we want to detect neutrinos everywhere around us is because they still have many unsolved mysteries. And our detector is a huge spherical device buried 700 meters underground, what does this device look like?
CCTV reporter Ren Meimei: From the entrance of the Jiangmen Neutrino Underground Laboratory to a depth of 700 meters, you need to take a cable car, which is also a "shuttle" for researchers to commute up and down every day, which can carry more than 40 people at the same time.
Take the cable car, through the 1267-meter inclined shaft tunnel, and in about 15 minutes, the reporter came to the cave 700 meters underground.
CCTV reporter Ren Meimei: The spherical detection device in the center of this huge pool is the main detector, which is a plexiglass body with a diameter of more than 30 meters, a stainless steel support frame on the outside, a photomultiplier tube and a series of detection equipment installation work is in full swing, it is like a big "eye" buried deep in the ground, helping scientists capture the traces of neutrinos.
How to install the equatorial layer of the largest circle of spherical detectors?
At present, the central detector plexiglass ball of this neutrino experimental device has been installed in the equatorial layer, so what are the difficulties in installing the equatorial layer as the largest circle layer?
CCTV reporter Shuai Junquan: At the bottom of the pool of the Jiangmen neutrino experiment, the world's largest and most sensitive liquid scintillator neutrino detector is being built above. Above the construction platform of the scaffolding, the world's largest plexiglass ball with a diameter of 35 meters is under construction, and the installation of its largest circle, the equatorial layer, is underway.
The lifting platform is an important auxiliary platform to complete the installation of plexiglass balls, and the diameter and height are variable layer by layer. The engineering personnel will complete the processes of lifting the plexiglass in place, splicing and polymerization, curing, annealing, grinding, cleaning, film and other processes layer by layer on the platform, and finally complete the overall installation of the plexiglass ball.
Ma Xiaoyan, senior engineer of the Institute of High Energy Physics, Chinese Academy of Sciences and chief engineer of Jiangmen Neutrino Experiment: Compared with other layers, the difficulty of the equatorial layer lies in the large size of each plate, which will be relatively difficult in the process of adjustment.
According to experts, the entire detector is made of a total of 263 plexiglass plates, each with a thickness of 120 mm, which is also the world's largest single plexiglass structure.
Ma Xiaoyan, senior engineer, Institute of High Energy Physics, Chinese Academy of Sciences, chief engineer of Jiangmen Neutrino Experiment: You think 120 mm is very thick, if you look at it proportionally, it is actually thinner than an eggshell, (because) in the future where we stand is a liquid scintillator, 20,000 tons of liquid scintillator in a plexiglass sphere.
How will the underground "crystal ball" catch "ghost particles"?
According to experts, the main part of the Jiangmen neutrino detector, that is, this plexiglass ball with a diameter of about 35 meters, is equivalent to the height of a 12-story building, after completion, the glass ball will be filled with 20,000 tons of liquid scintillator, and its external pool will be filled with 35,000 tons of high-purity water, equivalent to 3 million barrels of bottled water. Why fill a spherical detector with a liquid scintillator, and how would such a setup capture "ghost particles" neutrinos?
Liquid scintillator, as a medium for detecting neutrinos, its main component is actually a liquid called alkyl benzene, is the raw material of washing powder and washing liquid used in daily life, non-toxic and odorless does not have any harm to the human body, but the liquid scintillator used in the Jiangmen neutrino experiment must not only be very transparent, but also require very clean, specifically 20,000 tons of liquid mixed with dust can not exceed 0.008 grams.
Hu Tao: After the charged particle hits it, it can emit light, we call it flashing light, so we call it liquid scintillator, a ball with a diameter of 37 meters, light may be emitted in the middle, it has to pass through this scintillator to come out, if the transparency is not enough, the light will be absorbed inside, it will not be transmitted, so we need to be more transparent.
When the external neutrinos enter the detector, as long as there is a neutrino and the liquid scintillator, it will emit a faint light, which is indistinguishable by our naked eyes, but tens of thousands of photomultiplier tubes around the detector can capture it, convert the light signal into an electrical signal, and amplify the weak electrical signal by 10 million times.
Hu Tao, researcher of the Institute of High Energy Physics, Chinese Academy of Sciences and site installation manager of Jiangmen neutrino experiment: The photomultiplier tube converts it from optical signals into electrical signals, and then sends them to the computer through electronics, and after data analysis, we can get some performance of neutrinos.
Why was the neutrino detection device built 700 meters underground?
Why was the neutrino detector built 700 meters underground? Can't the detection of neutrinos be done on the ground?
Hu Tao: Neutrinos are originally difficult to detect, there are a large number of cosmic rays 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 rays.
How hard is it to build a big science installation 700 meters underground?
Experts said that to capture neutrinos, you need to be deep enough, dark enough, clean enough, 700 meters underground, can shield most of the cosmic rays. However, it is not easy to build such a huge detection device 700 meters underground, what are the difficulties?
Hu Tao: First of all, this side is in the south, when digging, there is a lot of water, now three or four hundred cubic meters of water per hour we pump up, our underground tunnels, including the underground laboratory excavation are very difficult, the second is that the temperature of the rock 700 meters underground has reached 37 degrees, this high temperature and high humidity environment is very stressful for our personnel. The third is about the plexiglass ball, which is currently the world's largest single plexiglass ball, so the whole production actually has a lot of technical problems in it.
In order to ensure the cleanliness, deformation control and operation safety of the detector, the entire underground assembly of the stainless steel grid frame does not use welding, but adopts patented high-strength stainless steel rivet connection technology, the accuracy should reach the millimeter level, and the number of rivets reaches 120,000 sets.
What scientific mysteries will be solved when the neutrino detection device is built?
What scientific mysteries will the neutrino detector, a scientific heavy weapon hidden 700 meters underground, solve for us after its completion, and what mysteries of the universe will be explored?
It is understood that after completion, the Jiangmen neutrino experiment will run for 30 years, and it is expected to produce a series of major physical achievements in neutrino mass sequence, accurate measurement of neutrino mixing parameters, and supernova, earth and solar neutrino research.
Researcher of Institute of High Energy Physics, Chinese Academy of Sciences Jiangmen Neutrino Experiment Site Installation Manager Hu Tao: The primary goal of the Jiangmen Neutrino Experiment is to hope to measure the mass order of neutrinos, which is a very important parameter for neutrino physics, including astrophysics, and the second further accurate measurement of some parameters of neutrino oscillation, hoping to get the international most advanced level, hoping to have new physical discoveries, in addition to which we may have some mechanisms in supernova explosions, physical nuclear reactions in the core of the sun, Even the study of the Earth model is likely to have some contribution.