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The name “neutrino” may be relatively familiar to those of us living in Japan. The name “neutrino” may be relatively familiar to those of us who live in Japan, as it has produced two Nobel laureates in physics, Masatoshi Koshiba and Takaaki Kajita, who made major discoveries related to neutrinos by observing them using the giant research facilities “Kamiokande” and “Super-Kamiokande. Nevertheless, neutrinos have an elusive image, as they are also called “ghost particles. Neutrinos are one of 17 subatomic particles that have been found so far. In fact, the universe is filled with an average of 300 neutrinos per 1 cc of space, and they are passing through the earth and our bodies at this very moment. However, we do not sense any of them, and they are called ghost particles because they are difficult to detect.

On the other hand, when we look at the forces that govern the universe, there are currently four possible forces: gravity, electromagnetism, the strong force, and the weak force. Neutrinos, however, do not interact with most matter, because they have no electric charge, so the electromagnetic force does not work, and although they are subject to gravity and the weak force, they are not subject to the strong force. Therefore, neutrinos can penetrate the earth and our bodies. This is why they are called ghost particles. Subatomic particles called “quarks” and “leptons” make up matter.

Protons and neutrons, which make up the nucleus of an atom, are made of quarks. On the other hand, neutrinos, which are leptons, are classified into three types: electron neutrinos, muon neutrinos, and tau neutrinos, corresponding to the electron, muon, and tau particles, which are also leptons (there are three types of antineutrinos as well). This classification is based on an attribute of neutrinos called “flavor. Initially, neutrinos were thought to have no mass, again like ghosts. However, neutrinos have the property of changing type as they fly, and this is called “neutrino oscillation. This neutrino oscillation is considered evidence that neutrinos have mass, and this discovery led to the Nobel Prize in Physics for Takaaki Kajita.

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Neutrinos are also classified according to their source: atmospheric neutrinos from the atmosphere, solar neutrinos from the interior of the sun, supernova neutrinos from supernova explosions, neutrinos from the Earth’s interior, and neutrinos produced in nuclear reactors and accelerators. In 1987, a supernova “SN 1987A” appeared in the Large Magellanic Cloud, and Kamiokande observed neutrinos originating from the supernova. The discovery of the supernova neutrino led to Masatoshi Koshiba’s Nobel Prize in Physics and marked the beginning of neutrino astrophysics.

Kamiokande captured only 11 supernova neutrinos, but its successor, Super-Kamiokande, was built to capture even more neutrinos. Super-Kamiokande is filled with 50,000 tons of pure water, and neutrinos enter its tank. Most neutrinos pass through the tank without incident, but on the rare occasions when they collide with water molecules, they produce a faint blue light called “Cherenkov light. The 13,000 optical sensors (photomultiplier tubes) installed on the inner wall of the tank observe the Cherenkov light and detect neutrino events.

Super-Kamiokande has contributed to the elucidation of neutrinos in the past, but in 2020, 13 tons of gadolinium (gadolinium sulfate octahydrate), a type of rare earth, was added to the pure water in the tank to start observations as a new device. This has made it possible to observe “supernova background neutrinos” in particular with high sensitivity. Since the birth of the universe, neutrinos emitted by supernova explosions have been diffused and accumulated in the universe. Supernova background neutrinos are such neutrinos drifting in the universe, and their observation is expected to advance the understanding of supernova explosions and the synthesis of elements in the universe.

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However, supernova background neutrinos are even more difficult to observe because of their low energy compared to other neutrinos. Dissolving gadolinium in pure water can improve their observational performance. The new Super-Kamiokande, improved with the addition of gadolinium, will not only be useful for observing supernova background neutrinos. It also has the potential to predict supernova explosions. Stars that are predicted to go supernova, such as massive stars, experience increasingly violent nuclear reactions in their final stages, according to an article published in AAS Nova/Astrobaites, titled “A Beginner’s Guide to Predicting Supernovae.

(Pre-supernova Alert System for Super-Kamiokande)”), neutrinos are emitted when silicon is produced in a fusion reaction (also called silicon fusion or silicon burning process) just a few hours before a supernova explosion. The improved Super-Kamiokande with gadolinium fusion has the potential to observe those neutrinos. In the case of the famous Betelgeuse, silicon fusion is said to begin about 10 hours before it goes supernova, so astronomers can receive an alert within 10 hours before the supernova explosion. This means that telescopes and detectors can be pointed at the candidate supernova star during that time.

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