The Super-Kamiokande neutrino detector looks like something out of a sci-fi movie that critics would deem “too unrealistic.” Picture a gigantic dome-shaped chamber with gold walls, an array of light detectors, and 50,000 tons of pure water. However, the Super-K neutrino detector is real and hidden 1,000 meters under Mount Ikeno in Japan. (Neutrinos are sub-atomic particles that travel through space and solid matter.) 
The scientists who work in this chamber are using these particles to find dying stars and discover more about our universe. While this detector is a seemingly magical place, it holds many dangers for those who work in it.
The World of Sub-Atomic Particles
Astrophysicist Neil deGrasse Tyson has dubbed neutrinos “the most elusive prey in the cosmos,” because they are so difficult to locate. He explains that the reason the chamber is buried so far in the ground is to prevent other particles from entering and conflicting with the experiments.
“Matter poses no obstacle to a neutrino,” Dr. Tyson says. “A neutrino could pass through a hundred light-years of steel without even slowing down.” 
That is why neutrinos have no trouble descending from space through rocks and earth to enter the Super-K. Neutrinos can pass through a hundred light-years of steel without stopping or slowing down.
That raises the question of why it’s important to detect neutrinos. The answer is, amazingly, neutrinos can help scientists find dying stars.
“If there’s a supernova, a star that collapses into itself and turns into a black hole,” Dr. Yoshi Uchida of Imperial College London told Business Insider. “If that happens in our galaxy, something like Super-K is one of the very few objects that can see the neutrinos from it.”
When a star begins to collapse into itself, it emits neutrinos, and the Super-K discovers this warning so the scientists can prepare to witness the upcoming supernova.
“The back-of-the-envelope calculations say it’s going to be about once every 30 years that a supernova explodes in the sort of range that our detectors can see,” said Dr. Uchida. “If you miss one you’re going to have to wait another few decades on average to see the next one.”
However, this isn’t all the Super-K does.
Sending Neutrinos Through Japan
In Tokai, on the opposite end of Japan from the detector, the T2K experiment shoots a neutrino beam through the ground for the Super-K to pick up. T2K is short for “Tokai to Kamioka.” 
Studying how neutrinos change as they shoot through matter can lead to discoveries about the universe; for example, it can show the connection between matter and antimatter.
“Our big bang models predict that matter and antimatter should have been created in equal parts,” says Dr. Morgan Wascko of Imperial College, “but now [most of] the antimatter has disappeared through one way or another.”
How the Super-K works
Although it’s buried deep under the ground, the detector is similar to the size of a 15-story building. The main chamber is filled with 50,000 tons of distilled water because neutrinos travel faster through water, especially when it’s ultra-pure.
“If an airplane is going very fast, faster than the speed of sound, then it’ll produce sound — a big shockwave — in a way a slower object doesn’t,” says Dr. Uchida. “In the same way a particle passing through water, if it’s going faster than the speed of light in water, can also produce a shockwave of light.”
What makes the chamber seem even more mystical is the 11,000 golden bulbs lining the walls. They are called Photo Multiplier Tubes (PMTs) and they are light detectors that can pick up shockwaves.
Dr. Wascko called them “the inverse of a lightbulb,” which is appropriate since they can locate minuscule amounts of light and channel them into an electrical current for the scientists to study.
The Dangers of Pure Water
The water Super-K is being filtered and re-purified constantly and shined with UV light to kill any bacteria. For the shockwaves to reach the sensor, the water has to be beyond clean, which makes it a frightening element.
“Water that’s ultra-pure is waiting to dissolve stuff into it,” said Dr. Uchida. “Pure water is very, very nasty stuff. It has the features of an acid and an alkaline.”
The scientists row across the water on rubber boats whenever the light sensors need fixing or replacement, taking care not to touch the water.
When Dr. Matthew Malek of the University of Sheffield was a Ph.D. student, he and two collogues were checking on the sensors when the gondola that takes the workers out of the tank broke. The three students had to wait in their rubber dinghy for a while, but they made the best of it, lying back and chatting.
“What I didn’t realize, as we were laying back in these boats and talking is that a little bit of my hair, probably no more than three centimeters, was dipped in the water,” said Malek.
Thankfully, the water was being drained out of the detector at the time so he didn’t have to worry about contaminating any experiments. However, the real shock came that night when he woke up at 3 a.m.
“I got up at 3 o’clock in the morning with the itchiest scalp I have ever had in my entire life,” he says. “Itchier than having chickenpox as a child. It was so itchy I just couldn’t sleep.”
He immediately connected it with the incident with the tank earlier that day and realized the pure water had irritated his scalp. Malek quickly took a shower and conditioned his hair for half an hour.
Another frightening tale about the potency of pure water occurred in the year 2000 during a routine tank draining. The researchers discovered an outline of a wrench on the floor of the chamber. Someone had dropped it there when it was filled in 1995. Five years later, the tool had completely dissolved. 
A Bigger and Better Detector
As impressive as the Super-K maybe, a more advanced version called the “Hyper-Kamiokande” has been proposed. It would be 20 times larger than the current Super-K with 99,000 light detectors. 
Dr. Waskco says, “We’re trying to get this Hyper-Kamiokande experiment approved, and that would start running in approximately 2026.”
- Trevor. Super-Kamiokande. Atlas Obscura. https://www.atlasobscura.com/places/super-kamiokande
- National Geographic. Stalking the Wild Neutrino | Cosmos: A Spacetime Odyssey. YouTube. https://www.youtube.com/watch?v=EY-WH62IXbM April 23, 2014
- T2K Experiment. About T2K. https://t2k-experiment.org/t2k/
- Isobel Asher Hamilton. A golden chamber buried under a mountain in Japan contains water so pure it can dissolve metal, and it’s helping scientists detect dying stars. Business Insider. https://www.businessinsider.com/super-kamiokande-neutrino-detector-is-unbelievably-beautiful-2018-6 October 7, 2019
- Davide Castelvecchi. Gigantic Japanese detector prepares to catch neutrinos from supernovae. Nature. https://www.nature.com/articles/d41586-019-00598-9 February 27, 2019.
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