Sudbury is a pretty cool place to live if you like science and cosmological mysteries (and it’s very cool in winter! 🙂 ). We have Science North, a great science education resource, research into mining in space, years ago we had NASA testing lunar rovers up here, and, possibly the most exciting, we are home to the Sudbury Neutrino Observatory, which played a key role in discovering the Sun’s missing neutrinos (and also a key role in Robert Sawyer’s book, Hominids!).
Which is why a couple of weeks ago, I had the chance to hear a talk right here in town by Dr. Stanley Yen, one of the world’s leading researchers on neutrinos, and originally from Sudbury. He gave an overview of the history of neutrino research, as well as discussing his own projects, including how he built a neutrino detector out of a bunch of scraps, in a cave. And no, that’s not just a gratuitous Iron Man movie reference, that’s what he actually did! (Check out this recording of his presentation.)
But first, what the hell is a neutrino? They’re dropped into science fiction shows and books casually enough (including mine sometimes) . We have observatories like the one in Sudbury dotted around the world, but what are they even “seeing” buried two kilometers underground?
A neutrino is a subatomic particle first suggested by Wolfgang Pauli to explain how beta radioactive decay worked. The particle was initially thought to have only limited interaction (if any) with regular matter, be electrically neutral, and have zero mass.
At the time, the suggestion was considered quite improbable, with Pauli’s paper on the subject being rejected as “too remote from reality.” But slowly, experimental evidence was gathered and the particles were finally detected in 1942. But the particle’s qualities were so bizarre, they were often referred to as “ghost particles” because of the difficulty of detection and their low interaction with other particles.
According to theory, the nuclear fusion reactions inside a star like our Sun should produce neutrinos that hit the Earth’s surface (and everything on it) at the rate of around a trillion particles per two square centimeters! And yet, so elusive are the particles, that they will happily whiz straight through us, the planet, and vanish off into the depths of space with barely any interaction!
But scientists are clever people, and in the 1960s the Homestake experiment made the first measurement of solar neutrinos. There was one problem though: the results picked up only about a third of the expected results.
If the results were accurate, there was something fundamentally wrong with the Sun, which seemed unlikely, so the hunt was on for the “missing” neutrinos. It was to take thirty years. And during this time, theories arose that neutrinos may come in different types or “flavors” and that the earlier detectors were only able to pick up one of these.
The 1990s were cool, right? Nintendos, Tamagotchis, inline skates, light-up sneakers, pocket-pagers, Nokia cellphones and, of course, the Corvette ZR-1! 🙂 But the 90s also saw the development of the neutrino detector at Snolab, here in Sudbury.
This was built 2100 meters below the ground at the bottom of the Creighton mine, a location chosen to minimize the amount of background radiation events that would mask the neutrino signal. And after years of searching, the team announced that not only had they detected neutrinos from the Sun but had also detected the right amount. The new detector was capable of detecting all three up- until-now theoretical types: the Electron Neutrino, Muon Neutrino, and Tau Neutrino.
You see, it turned out that neutrinos had mass after all. It was incredibly small, far smaller than any of the other known subatomic particles, but it was enough that it could distinguish between the three types. And not only that, the three types could oscillate as they traveled–changing from one type to another.
Since then, these “ghost particles” have become a fundamental part of the scientific study of the universe and what is happening at the center of stars. What’s more, they can be used to detect supernovas–when a large star collapses and then blows up in a giant explosion that can be seen even from one HALOgalaxy to another.
And that brings us full circle to Dr. Yen. He and his colleagues designed and built a dedicated supernova neutrino observatory at the Sudbury Neutrino Laboratory, called HALO (Helium and Lead Observatory). This can detect the unique signature of the earliest moments of a supernova explosion, allowing other observatories to be targeted on the same area.
If HALO had been built using brand new materials, it would have cost hundreds of millions of dollars, but the team ingeniously re-used components from earlier, decommissioned, experiments to build the detector at a fraction of the price. Quite literally, from scraps in a cave!
There are several other detectors in operation around the world, and they have come together to form the Supernova Early Warning System (SNEWS), which can give early detection of any supernova in our galaxy and other ones close by.
How cool is that! What science and technology research is happening where you live? Let me know in the comments!