On Saturday, Dec. 18, scientists at the IceCube Neutrino Observatory lowered their last strand of particle detectors into the ice beneath them. The garland of 86 basketball-sized optical sensors completed a frozen array of 5,160 detectors that will search for neutrinos 1.5 km beneath the surface at the South Pole.
"With the completion of IceCube, the 1970s dream of building a kilometer-scale neutrino detector has finally become a reality," said Francis Halzen, a professor of physics at the University of Wisconsin-Madison and the IceCube collaboration's principal investigator.
Each day trillions of neutrinos pass through the cubic kilometer the array covers. They come from the sun, from cosmic rays interacting with Earth's atmosphere, and from more distant sources such as exploding stars. As neutrinos rarely interact with matter, the detectors will catch signals of just a few hundred of them per day.
When neutrinos collide with the nuclei of oxygen atoms in the ice, they turn into muons and set loose a shower of other particles. These particles move faster than light can travel through ice, so they send out a shock wave of blue Cherenkov radiation, which photodetectors in the IceCube array can see.
The image to the right represents what a high-energy neutrino would look like moving upward through the detectors. The circles are individual detectors, and their size represents the amount of light they would detect from Cherenkov radiation. The colors move from warmer to cooler depending on when each detector would receive a signal.