The University of Toronto is one of the collaborators on a high-energy physics project that was recently named the 2013 Breakthrough of the Year by the magazine Physics World. The IceCube project, located at the South Pole in Antarctica, recently detected the first observable cosmic neutrinos with a massive detector buried under the ice.
Neutrinos are incredibly small subatomic particles that, like the neutron, have no electric charge. Unlike the neutron, neutrinos are so small that scientists have never been able to measure their mass accurately. Neutrinos are produced through radioactive decay or through nuclear reactions. The reactions in the sun’s core produce neutrinos; billions of neutrinos from the sun pass through the earth’s atmosphere every second. These neutrinos, however, have low energies compared to those produced when a star goes supernova or collapses to form a black hole. Scientists set out these extragalatic, extremely high energy cosmic neutrinos when they built the IceCube. Neutrinos could provide us with direct information about cosmic phenomena like gamma ray bursts or active galactic nuclei.
Theoretical predictions suggested that if these cosmic neutrinos could be detected, they would have a frequency of roughly one per kilometre per year. Neutrinos also travel close to the speed of light, and rarely interact with atoms and molecules. In order to have any chance of finding them, the IceCube collaborators had to construct a massive, highly sensitive detector.
Neutrino detectors typically consist of a large number of small, light-sensitive modules suspended in a clear medium, like water or ice. Most neutrino detectors, like the Super-Kamiokande detector in Japan, use water; the Sudbury Neutrino Observatory used heavy water. Suspending thousands of tiny modules in water is difficult however, and the larger the detector, the more difficult this problem becomes. IceCube scientists built the largest neutrino detector now in existence by using the ice to suspend their modules and keep them in place. Scientists used a hot-water drill to create 86 holes three kilometres deep into the ice, and then quickly installed the modules before the water froze again. The result was a network of 5,160 sensors that are buried between one-and-a-half and two-and-a-half kilometres under the surface.
When a neutrino passes through the detector, it may strike one of the atoms in the ice. The result is a new particle called a muon, which creates a small flash of blue light (known as Cherenkov light). This light is detected by the sensors in the ice, which send all of their data to a computer on the surface. The amount and intensity of the light then allow researchers to calculate the energy of the neutrino, and then to determine whether or not it might be a cosmic neutrino.
The vast majority of the neutrinos that pass through IceCube have fairly low energies, but scientists recently detected two ultra-high energy particles — neutrinos with energies so high that it is believed that they could not possibly have originated in our galaxy.
The IceCube neutrino detector represents international scientific collaboration on the grandest scale — over 250 researchers from over 40 institutions in a dozen countries are currently involved in the project, and have overcome extremely harsh conditions to build and maintain the detector. With more data, they hope that these observations will give insight into some of the most mysterious phenomena in the universe. As University of Toronto professor and IceCube collaborator Ken Clark commented: “This is the beginning of a new era for astronomy … This result opens up the ability to use neutrinos to explore our universe.”
