The hosts explain why a neutrino observatory can't work like a normal telescope: since neutrinos themselves aren't directly seen, you need a huge volume of transparent material packed with light detectors to catch the rare flashes from neutrino interactions. They trace the first attempt at this, DUMAND (Deep Underwater Muon and Neutrino Detector), a 1960s/70s project off Hawaii using photomultiplier tubes on the ocean floor, which was shut down in 1995 after struggling with background noise from ocean bioluminescence and radioactive potassium-40 in seawater. They then point to the Baikal Neutrino Telescope in Russia's Lake Baikal as a similar water-based detector still working to filter out comparable background issues at the highest energies.
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DUMAND was spearheaded by the University of Hawaii and used an array of photomultiplier tube detectors resting on the ocean floor.
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Detectors had to be tuned to higher-frequency light to avoid being swamped by visible-range ocean bioluminescence.
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Lake Baikal holds about one-fifth of the world's unfrozen surface freshwater, making it a natural large-volume site for a similar photomultiplier-grid detector.
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Water-based detectors still struggle specifically with capturing the highest-energy neutrino events due to background radiation.
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33:13So now, the problems that is clear, this is like a highly improbable event to capture. So how would we make a neutrino observatory? Because this is very different than Hubble and JWST and the other types of observatories that many folks would think about when they hear that word. That's right, because in Hubble and all of these traditional observatories, you just look for light, and light is, and then you capture the light and you're good to go. Here, we're looking for like a second messenger, right? We're not detecting the neutrino, we're detecting how the neutrino interacts with the stuff that we then look at, okay? And we need a lot of it. We need a lot of stuff because the flux of these events is so small.
33:58The first sort of attempt at doing this was do-mand, do-mand maybe, deep underwater muon and neutrino detector project. This was way back in 1960. They were trying to create a deep ocean detection off the coast of Hawaii, off the main island. This was spearheaded by the University of Hawaii. What they wanted to do was basically on the ocean floor, rest a bunch of these detectors, like in this case, there's six that are coming out of the ocean floor. Each of these detectors is going to detect this light. It's a bunch of photomultiplier tubes in some sense, which just, once a photon comes in, that's going to be converted into an electrical signal, very much like a CCD that's in our camera, but
34:42like on steroids, like a single photon. We'll set this thing off, right? And it was right off the coast of Hawaii. There were a lot of problems though. The first time you try something as crazy as this, there's going to be problems. A big thing was measurement backgrounds from ocean bioluminescence. Yeah, I was going to say. Because like the living things in the ocean themselves produce light. And if these things are that sensitive, they're just going to go off on the ocean bioluminescence. Now, all of that stuff is in the visible range, but that means that we have to tune our detectors for higher frequency light. And that removes a whole chunk of stuff that we would be otherwise seeing.
35:24The other thing was the ocean is full of potassium 40, which is a radioactive isotope of potassium. So there's radiation in the ocean itself. Right. Like the water itself has dissolved radioactive potassium. And so you get this massive background, which is just super annoying. And you're just going to spend all your time filtering out. Right yeah. So the project was canceled in 1995. It had serious engineering problems, but we learned a lot because that's, we learn a lot whenever we try new things. And now there are new detectors in bodies of water that are actually trying to do the same thing, but in a much bigger sense.
36:05My favorite one is actually the Baikal Neutrino Telescope. This is in Lake Baikal, which is the largest body of fresh water on the planet. It has something like 25% of the world's freshwater in a single lake in Russia. And they've got a very similar detector from Dumond, where in the bottom, they've got these photomultiplier tubes that are arranged in a grid, and they're trying to find neutrinos. So water-based detection is working, right? But there are problems with trying to get it to detect the really, really high energy stuff, okay? Again, because of some of the background, some of that radiation stuff, so on and so forth.
36:48So this is where our Nobel Laureate comes in. I always love talking about this story because it's one of those, there's so much deeper than what you see on the headline. People take away Antarctica and neutrino, not knowing and understanding the importance of neutrino in terms of this mapping piece. How can an observatory be under the ice? Yeah.