Francis Halzen's path to IceCube began in 1987 when a glaciologist told him about Russian plans to detect radio emissions from neutrino interactions in Antarctic ice, which Halzen calculated would not work for the high-energy neutrinos he wanted. Instead, with John Learned, he proposed using Cherenkov radiation in ice and pointing the detector downward so Earth itself would filter out atmospheric and solar neutrinos, leaving only the highest-energy cosmic ones. A 1991 Greenland experiment first showed polar ice could work as a Cherenkov detector, leading to the AMANDA project, which from 1993 used hot-water drilling to sink photomultiplier tubes into the Antarctic ice and discovered that deeper ice, free of trapped air bubbles, was far more transparent than the surface layers, a finding that proved the method could work and set the stage for IceCube.
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Halzen and Learned presented their ice-Cherenkov proposal at a conference in Poland in 1988, with the detector concept oriented to look downward through the Earth.
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AMANDA stands for Antarctic Muon And Neutrino Detection Array and was built in phases between 1993 and 2000.
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The hot-water drilling technique used to bore holes for the detectors was originally developed for glaciology research before being adapted for physics.
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AMANDA succeeded in detecting atmospheric neutrinos, demonstrating the detection method worked even though these were not yet the sought-after astrophysical neutrinos.
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Surface ice appears white and blocks light because of trapped air bubbles, while deeper ice is compressed enough that the air escapes, making it much clearer.
811 words · auto-generated from the episode video
37:17How can you move faster than the speed of light through ice? (Laughs) There's a lot of working parts, right? And this is where Francis Halzen comes in. In 1987, he's giving a talk at the University of Kansas about the possibility of detecting neutrinos using water, and a glaciologist who was in the audience, he informs Halzen that there are Russian plans to detect radio emissions of charged particles produced by neutrino interactions in the Antarctic ice. They're using radio detectors, okay? Now, the idea here is that the neutrino collides with an atomic nucleus in the ice, that creates an electric spark, and that produces a radio wave. He quickly does some calculations with these colleagues, and he figures out that actually the
37:59detection threshold would be too high for the petaelectron volt neutrinos that we really wanna see. Right. Okay? So, Halzen's idea is, wait, ice is interesting though, okay? He's like, instead of radio, what if we used Cherenkov radiation? And him and his research colleague, John Learned, who was a big part of the DUMAND project, he was actually at the University of Hawaii, they come out with this proposal at a conference in Poland in 1988. This is the proposal for an ice cube-like observatory. On the right-hand side, you can already see somewhat their idea.
38:41The Earth there is upside down. Antarctica is at the top. What they're proposing is to look downward into the Earth. Okay, so if we were in Antarctica, our detector would be pointing downwards. Why? Because we want to look for neutrinos that have made it all the way through the Earth. If we look up, we're gonna get solar, we're gonna get atmospheric neutrinos, we're gonna get all sorts of stuff. But the really high-energy ones don't even care that the Earth was there. It's built-in filtering. Yeah. That's very different than the DUMAND problem. You're using the entire planet as a filter. It's pretty amazing. These two came up with this idea in 1988. Halzen takes this idea a little bit further.
39:23It's kind of just an idea. And then in Greenland, it works for the first time. So Greenland publishes, the Greenland detector publishes in 1991, an observation of muons using polar ice caps as a Cherenkov detector. They have a single sort of tube of photomultiplier tubes and they report using ice to detect Cherenkov light. Once this happens, people are like, okay, so there's a chance. There's a chance this could actually happen. You're saying there's a chance. Right yeah. So now we could actually happen. The Amanda project is proposed. Antarctic muon and neutrino detection array.
40:06And the Amanda project in Antarctica subsequently demonstrates the instrumentation that leads to IceCube. There's photomultiplier tubes that go down into these boreholes. The holes are drilled using high pressure hot water systems that are originally pioneered for glaciology. The physicists just borrowed that and said, hey, you guys have been digging for like climate change and glaciology. Let's do the same kind of tunneling, but now instead I'm gonna put all these detectors into the ice. Amanda was constructed in phases from 1993 and 2000 and Halsin was a driving force in Amanda. And in those early deployments, they run into problems. Namely,
40:46the ice that's at the very top, it's very white. Right? The reason why it's white is because there's a lot of air that's trapped in the ice. That's what's causing the diffraction, right? And you can't really see that far into the ice. Well, just the same reason why we can't see into the ice all that far or the light doesn't penetrate. Well, that means that the Cherenkov light that I'm trying to see also doesn't penetrate through the ice because of all the trapped air particles, right? They went deeper and they found that as the ice gets packed, the deeper layers don't have a lot of air because the air starts escaping because of all of that
41:30high pressure. So the deeper I go into the Antarctic ice sheet, the clearer the ice gets, okay? That was a big finding of the Amanda project. And there's a driving force behind now understanding at what depth do we need to go to in order to basically have a clean lens as an analogy. Yeah, that's a very good analogy actually. And so all of this was because of Amanda and Amanda finally detected atmospheric neutrinos and demonstrated that the method would actually work. They're able to observe high energy neutrinos, but these are still from the atmosphere and sort of solar system sources. But they're saying that now this definitely works.
42:10We just need the money. We've got a plan. We need the money. Show me the money. NSF comes in, the National Science Foundation and they make a bet.