10 June, 2018
Super Kamiokande (SK) is preparing for the observation of supernova relic neutrinos emitted by all of the supernova explosions that have occurred since the beginning of the universe. Specifically, we aim to observe supernova relic neutrinos by adding gadolinium (Gd), a rare earth metal, to the pure water in the detector. For this reason, we plan on conducting refurbishment work during the period from June to September 2018, draining the water of the SK tank a little at a time as the work progresses.
In February of 1987, the Kamiokande detector, SK’s predecessor, detected the world’s first neutrinos from a supernova burst. Since then, no supernova explosion has occurred in or near our galaxy, so we have not observed any neutrinos from a supernova burst since then. The frequency of supernova explosions in our galaxy is rather low: once per 30-50 years. However there are hundreds of billions of galaxies in the universe, so supernova bursts are occurring once every few second in the whole universe. It is expected that the supernova relic neutrinos emitted by all of the supernova explosions since the birth of the universe to the present constitute a kind of diffuse gas which fills space; thousands of these neutrinos should pass through our hands every second, but they have not yet been detected. By observing these diffuse supernova neutrinos, we aim to understand the mechanism of supernova explosions and to clarify the history of stellar formation.
These supernova relic neutrinos should have interacted several times a year in the SK detector, but up to now could not be distinguished from various background processes and could not be identified. By adding a substance called gadolinium, a rare earth metal, to the pure water in SK, it should become possible to distinguish these interactions. As shown in the figure, a relic supernova "anti-electron neutrino" reacts with protons in water and releases positrons and neutrons. The generated positrons emit Cherenkov light. After a short delay, the neutrons are captured by Gd and emit gamma rays; these gamma rays scatter off electrons in the water, emitting additional Cherenkov light. We are trying to distinguish supernova relic neutrinos by observing the characteristic signal of two consecutive bursts of Cherenkov light from almost the same place in the tank.
(May 31, 2018: refurbishment work started)
