Editing Super-Kamiokande (section)

===SuperKGd===
[[Gadolinium]] was introduced into the Super-Kamiokande water tank in 2020 in order to distinguish neutrinos from antineutrinos that arise from supernova explosions.<ref name="Abe 2022" /><ref name=":0">{{cite web |title=How do you catch something smaller than an atom that's travelled across galaxies? |last1=Sturmer |first1=Jake |last2=Asada |first2=Yumi |date=2019-06-17 |language=en-AU |last3=Spraggon |first3=Ben |last4=Gourlay |first4=Colin |website=ABC News |url=https://www.abc.net.au/news/2019-06-17/inside-super-kamiokande-360-tour/11209104 |access-date=2019-06-25}}</ref> This is known as the '''SK-Gd project''' (other names include '''SuperKGd''', '''SUPERK-GD''', and similar names).<ref name=":1">{{cite journal |title=Current status of SK-Gd project and EGADS |journal=Journal of Physics: Conference Series |volume=718 |pages=062070 |year=2016 |last1=Xu |first1=Chenyuan |issue=6 |bibcode=2016JPhCS.718f2070X |doi=10.1088/1742-6596/718/6/062070 |doi-access=free}}</ref> In the first phase of the project, 1.3 tons of a Gd salt (gadolinium sulfate octahydrate, {{chem2|Gd(SO4)3⋅(H2O)8}}) were added to the ultrapure water in 2020, giving 0.02% (by mass) of the salt. This amount is about a tenth of the planned final target concentration.<ref name="Abe 2022" /><ref name=":0" />

[[Nuclear fusion]] in the Sun and other stars turns protons into neutrons with the emission of neutrinos. Beta decay in the Earth and in supernovas turns neutrons into protons with the emission of anti-neutrinos. The Super-Kamiokande detects electrons knocked off a water molecule producing a flash of blue Cherenkov light, and these are produced both by neutrinos and antineutrinos. A rarer instance is when an antineutrino interacts with a proton in water to produce a neutron and a positron.<ref name=":2">{{cite journal |last=Castelvecchi |first=Davide |title=Gigantic Japanese detector prepares to catch neutrinos from supernovae |date=2019-02-27 |journal=Nature |language=EN |volume=566 |issue=7745 |pages=438–439 |bibcode=2019Natur.566..438C |pmid=30814722 |doi=10.1038/d41586-019-00598-9 |doi-access=free}}</ref>

Gadolinium has an affinity for neutrons and produces a bright flash of gamma rays when it absorbs one. Adding gadolinium to the Super-Kamiokande allows it to distinguish between neutrinos and antineutrinos. Antineutrinos produce a double flash of light about 30 microseconds apart, first when the neutrino hits a proton and second when gadolinium absorbs a neutron.<ref name=":0" /> The brightness of the first flash allows physicists to distinguish between low-energy antineutrinos from the Earth and high-energy antineutrinos from supernovas. In addition to observing neutrinos from distant supernovas, the Super-Kamiokande will be able to set off an alarm to inform astronomers around the world of the presence of a supernova in the Milky Way within one second of it occurring.

The biggest challenge was whether the detector's water could be continuously filtered to remove impurities without removing the gadolinium at the same time. A 200-ton prototype called EGADS with added gadolinium sulfate was installed in the Kamioka mine and operated for years. It finished operation in 2018 and showed that the new water purification system would remove impurities while keeping the gadolinium concentration stable. It also showed that gadolinium sulfate would not significantly impair the transparency of the otherwise ultrapure water, or cause corrosion or deposition on existing equipment or on the new valves that will later be installed in the [[Hyper-Kamiokande]].<ref name=":1" /><ref name=":2" />