After two decades of planning, the world’s first kilometer-scale neutrino observatory should finally be completed by this December. Named IceCube, it will consist of an array of 5,160 optical sensors embedded within one cubic kilometer of the Antarctic ice shelf – to put the accomplishment in perspective, one of the next-largest such observatories is just 40 cubic meters in size. Its main purpose will be to try to establish, once and for all, the source of cosmic rays.
Neutrinos are the second-most abundant particles in the universe, after photons, and are created when radioactive particles decay. In violent events like super novae and gamma ray bursts, high-energy neutrinos are the result. Neutrinos have no electrical charge, and have such a low mass that they typically pass unimpeded through matter, so they don’t often make their presence known. Occasionally, however, a neutrino can strike the nucleus of an atom, creating a particle called a muon. When this happens, the muon radiates blue light.
At IceCube, the optical sensors will be monitoring the cubic kilometer of ice, and will detect the blue flares that occur when a neutrino collides with an ice atom. By observing the resulting muon, scientists can determine the direction from which the neutrino arrived, along with the cosmic ray that it was a part of.
The Antarctic ice is ideal for such a study, as it is exceptionally pure, clear, and free of radioactivity. The blue light from a muon can travel through it for over 100 meters (328 feet), in what is otherwise a pitch black environment.
Here’s the thing, though... most of the muons detected will be from neutrinos that originated not in deep space, but merely in the atmosphere above the observatory. To filter for truly cosmic high-energy neutrinos, the observatory will be looking specifically for muons that indicate their neutrino came from the north, passed through the earth, and impacted the ice atom from below.
The IceCube project is a collaboration of over 35 international research institutions, led by the University of Wisconsin. The cost of the project is estimated at US$271 million.
All images courtesy the National Science Foundation.