Underground laboratory for hunting ghosts

 Where is the "ghost" sacred?

　　Our universe was born in a big bang 13.7 billion years ago. The Big Bang produced countless neutrinos in one second. In the early days of the Big Bang, because photons could not escape from gravity, neutrinos did not interact with matter and could travel unimpeded. Therefore, they have carried cosmic information earlier than light to the present. Because neutrinos are difficult to detect, they are called "ghost particles" by the scientific community.

　　Scientists believe that at the beginning of the big bang, the universe created the material world and the antimatter world at the same time, but the antimatter disappeared later. Where did it disappear? To reveal the mystery of the disappearance of the antimatter world, we must look for clues in the existing material world. Scientists generally believe that the neutrinos, which are widely present in the material world, contain important information about how the antimatter world disappeared.

Who discovered the "ghost"?

　　Because neutrinos play an important role in elementary particle theory and play a special role in astronomical observations, the study of neutrinos is difficult, but it has always attracted scientists.

　　The discovery of neutrinos came from the study of radioactivity at the end of the 19th century and the beginning of the 20th century. Researchers have discovered that a part of the energy of matter is missing during the beta decay process. Based on this, Niels Bohr, the famous leader of the Copenhagen School of physics, believes that the law of conservation of energy fails in the process of β decay.

　　In 1930, the Austrian physicist Pauli proposed a hypothesis that in the process of β decay, in addition to electrons, a new particle with zero rest mass, neutrality, and photons is emitted. Take away another part of the energy, so there is an energy loss. The interaction between this particle and matter is so weak that it is difficult for the instrument to detect it.

　　In the spring of 1931, the International Conference on Nuclear Physics was held in Rome. Among the participants were Heisenberg, Pauli, Madame Curie, etc. At that time, Pauli named this particle "neutron". At first he thought that this kind of particle originally existed. In the nucleus. In the same year, Pauli proposed in a seminar of the American Physical Society that this kind of particle does not originally exist in the nucleus, but is produced by decay. Pauli predicted that the energy-stealing "thief" was a neutrino.

　　The real neutron was discovered in 1932, and the Italian physicist Fermi renamed Pauli's "neutron" as "neutrino." In 1995, American physicists Cowan and Reins directly detected neutrinos through experiments for the first time, and they won the Nobel Prize in Physics.

"Ghost" Underground Laboratory

　　When light travels between stars, it will turn around under the influence of time and space, but neutrinos will not. Therefore, neutrino detectors are built underground to block interference from other radiation sources. At present, several countries in the world have established underground neutrino laboratories.

　　The earlier one is the Sudbury Neutrino Observatory in Canada, which was built in the 1980s and the main body was built at a depth of 1,600 meters underground. The giant plastic sphere at its core is filled with 800 tons of special substances called liquid scintillators. The entire sphere is surrounded by a "water shell" and is monitored by about 10,000 extremely sensitive light detectors, that is, photomultiplier tubes. When neutrinos come into contact with other particles in the detector, they can create light in the liquid scintillator, and the photomultiplier tube can capture this. On this basis, scientists discovered the three forms of neutrinos.

　　The world's largest neutrino observatory is the Amundsen-Scott IceCube Neutrino Observatory built in Antarctica. There are as many as 5,160 detectors located under the ice in the South Pole to search for high-energy neutrinos that originate from extreme events in the universe such as star explosions, black holes, and neutron stars. According to reports, when neutrinos break into water molecules in the ice layer, they will produce a cone-shaped glow, namely Cherenkov radiation, which is exactly what the IceCube detector wants to capture. Scientists hope to restore the path of neutrinos and identify their origin.

　　Japan’s large-scale neutrino detector is the Super Kamioka Detector, which was built in an abandoned arsenic mine at a depth of 1,000 meters at the Mozu Mine in Kamioka Town, western Japan. The huge detector contains 50,000 tons of pure water and nearly 11,200 photomultiplier tubes. Similar to IceCube, the Shengang detector also uses Cherenkov radiation to capture neutrinos. In 1998, the Shengang detector was the first to find evidence of neutrino oscillations and revealed that neutrinos also have tiny masses.

　　There are three neutrino labs buried in the mountainous area of ​​Daya Bay in China. Each of the eight cylindrical detectors contains 20 tons of liquid scintillators, and there are thousands of photomultiplier tubes around them. They are placed in pure pools to block radiation in any environment. A group of nearby nuclear reactors continuously produces huge amounts of electron antineutrinos. These antineutrinos come into contact with the liquid scintillator, emit a glow, and are captured by a photomultiplier tube.

Future application

　　Wonderful neutrinos. Now that we can generate and detect them, can we use them in our daily lives? Scientists at Fermilab in the United States used a test device to successfully use neutrinos in communications with a bit error rate of 1%. Since neutrinos can pass through matter in a straight line almost unimpeded, this communication will not be blocked by sea water and ground, nor can it be interfered, intercepted, or deciphered. Maybe one day, it can become a practical means of communication.

　　However, in the eyes of scientists, the importance of basic research is far greater than its practicality. When Madame Curie discovered the decay of the atomic nucleus more than 100 years ago, no one knew what it would mean. Today, the use of nuclear energy has profoundly changed the world. Today's extremely mysterious neutrinos are only in the ivory tower. With the continuous deepening of human understanding of the natural world, the day when more mysteries of neutrinos are revealed, it may profoundly affect the future.