 "Photo: Photo Courtesy NSF")
In one of the coldest places on Earth, UW-Madison scientists are building the world’s largest telescope to search for some of the universe’s smallest particles.
Located at the South Pole, the IceCube Neutrino Observatory will, as its name implies, look for tiny, subatomic particles called neutrinos, which are produced by nuclear reactions that take place in the cores of stars and in other deep-space phenomena.
Because neutrinos are far smaller than even a single atom, and because they have no electric charge, they can pass through large, solid objects, such as the Earth, or strong magnetic fields. As a result of these properties, they come straight from their source without being deflected or absorbed. This makes the neutrino an ideal messenger particle to convey information about events occurring far across the universe.
UW-Madison professor Francis Halzen, the lead researcher on the IceCube project, believes neutrinos have the potential to change the way astronomy is done.
“The important thing is, they’re just like light,” Halzen said. “People have done astronomy with light since they had eyes, and so, we want to do astronomy with neutrinos.”
Being able to detect neutrinos and determine where they are coming from will help researchers peer into the depths of space and hopefully expose some of the greatest mysteries confronting modern astronomy and physics, such as the nature of supernova explosions, black holes, dark matter and other deep-space phenomena.
The same properties that make neutrinos an ideal messenger particle, however, also make them extremely difficult to detect. Trillions of neutrinos stream through the human body every second, but few, if any, ever interact with atoms. And if a particle never interacts with any of the matter around it, it is essentially impossible to detect.
Fortunately for Halzen and the IceCube team, on very rare occasion a neutrino will collide directly with the nucleus of an atom and cause a nuclear reaction that emits a small amount of blue light. If scientists can detect this light, they can infer a great deal about the neutrino that created it and where it came from.
A Huge Idea
Scientists have known that neutrinos could be used in astronomy since they were discovered in 1956. One of the greatest difficulties, however, was designing a large enough detector.
“When they thought about it a little bit–a little bit in this field means 20 years–they realized they needed detectors that were a kilometer in size, and nobody knew how to do this,” Halzen said.
“You need a big block of very transparent material,” he added.
The prevailing idea for many years was to attempt to build a detector in a kilometer cube of water, deep in the ocean.
“People couldn’t make that work,” Halzen said. “So that’s when the idea came: What about using ice?”
The gigantic sheet of ice that makes up the South Pole is almost two miles thick. Thus, the ice at the bottom is under enormous pressure from the weight of the ice above it. This compresses the ice, eliminating impurities that would distort the researchers’ measurements.
“Even in a lab, you cannot make a solid that’s as transparent as the ice at the South Pole,” Halzen said.
In the 1990s, UW-Madison scientists constructed a small, proof-of-concept telescope at the South Pole, nicknamed AMANDA (Antarctic Muon and Neutrino Detector Array), to demonstrate that the technique worked.
“People thought I was crazy when I said well you go to the South Pole and drill 2.5 kilometers in the ice,” Halzen said. “That’s simpler than putting [sensors] in water off the coast of Hawaii? And the answer is yes, and we’ve proven it.”
Construction on AMANDA finished in 2000, and in 2002 the National Science Foundation awarded the university a $242 million grant to build the full-scale IceCube Neutrino Observatory.
Construction on IceCube began in 2005 with the deployment of the first string of sensors. When construction finishes early next year, IceCube will consist of a total of 86 strings made up of 60 sensors each for a total of over 5000 sensors arranged in a grid one cubic kilometer in size.
These sensors are buried roughly a mile under the surface of the ice. Construction is limited to December and January, summer in the southern hemisphere. The harsh conditions at the South Pole, with an average winter temperature of -85oF, make working outside impossible during any other part of the year.
Putting it All Together
As one can imagine, the execution of placing the sensors is not a simple process. Deployment manager Tom Ham oversees the entire process. First a 2.5-kilometer-deep hole is melted in the ice using a hot water drill that requires roughly as much power as a heavy diesel-electric locomotive.
“When you think about it, that’s a long way down just to put hot water,” Ham said. “To melt ice that deep is a big thing.”
It usually takes about 24-32 hours to drill a single hole. Once the drilling is finished, the string of 60 sensors is lowered into the hole. Each sensor is enclosed in a plastic sphere slightly larger than a basketball and was custom designed for the IceCube project. Their job is to detect the blue light emitted by neutrinos interacting with atoms in the ice and to then transmit the information to the surface.
After the string of sensors is installed, the water in the hole slowly refreezes over the course of the next week and a half to a month. The sensors become permanently secured in the ice and can therefore never be removed for maintenance or repair.
6 comments