IceCube: the Neutrino Observatory in Antarctica
Neutrinos are the most abundant particles in nature with their number far exceeding the number of atoms in the entire universe, yet scientists know little about them despite millions passing through Earth every day. They are almost massless particles with no electric charge and can travel to Earth with no attenuation or deflection by magnetic fields. Because they travel through the universe almost undisturbed by matter they can point scientists towards the sources where they were created, with the most energised neutrinos expected to travel from the most extreme environments in the universe including massive exploding stars. These sources are able to accelerate cosmic rays to energies over a million times the energies achieved by human-made accelerators such as the Large Hadron Collider at CERN. While billons of neutrinos reach Earth every second most of them are called ‘muon neutrinos’ which are generated from cosmic rays interacting with the Earth’s atmosphere. In contrast researchers are trying to find a few dozen neutrinos generated elsewhere, deep in the universe.
the IceCube neutrino observatory
To detect these rare neutrinos, scientists have established in Antarctica the IceCube neutrino observatory, a cubic-kilometre participle detector made of Antarctic ice, located near the Amundsen-Scott South Pole Station: being at the South Pole means scientists can use the Earth to filter out the large background of atmospheric muons.
The IceCube Lab. Credit: Emanuel Jacobi, IceCube/NSF
The observatory is comprised of a surface array, IceTop, and a denser inner sub-detector, DeepCore and is the first gigaton neutrino detector ever built. Its primary task is to observe neutrinos from the most intense astrophysical sources in the universe. The in-ice component of IceCube – DeepCore – consists of 5,160 digital optical modules (DOMs) each with a 10’’ photomultiplier tube and associated electronics. The DOMs are attached to vertical ‘’strings’’ that are frozen into 86 boreholes located in a cubic kilometre. The depth of each borehole ranges from 1,450 to 2,450 metres. The strings are laid in a hexagonal grid with 125 metres spacing between, with each string holding 60 DOMs, spaced vertically apart 17 metres. In total eight of these particular strings in the centre of the array are deployed more compactly, with the distance between reduced to around 70 metres and a vertical spacing of 7 metres between DOMs. This enables scientists to study neutrino oscillations. Meanwhile, IceTop consists of stations located on top of the same number of IceCube strings. Each station has two tanks that have downwards facing DOMs. Overall, IceCube measures the interaction of neutrinos with the ice. Specifically, when neutrinos hit the ice they produce electrically charged secondary particles that in turn emit Cherenkov light, the result of travelling through the ice faster than light can. The IceCube sensors collect this light, which is digitized and time-stamped. All the sensors collect this data which is then converted into light patterns that reveal the direction and energy of the neutrinos.
Global scientific cooperation in Antarctica
The study of neutrinos in Antarctica is a truly global effort with IceCube built using a National Science Foundation (NSF) award along with assistance from partner funding agencies around the world. The Division of Polar Programs in NSF’s Geosciences Directorate and the Division of Physics continue to support the project with a maintenance and operations grant as do other institutions and funding agencies. As part of the scientific cooperation efforts the Antarctica expedition comprised of the IceCube Collaboration involves 300 physicists and engineers from the U.S., Germany, Sweden, Belgium, Switzerland, Japan, Canada, New Zealand, Australia, U.K., Korea and Denmark with the University of Wisconsin-Madison being the lead institution.
New era in particle physics
In August of this year, scientists announced for the first time an observation of the rare neutrino, confirming the discovery of particles from beyond our solar system and galaxy. This discovery, according to Vladimir Papitashvili of the NSF’s Division of Polar Programs, opens ‘’the doors to a new era in particle physics’’, while Olga Botner, spokesperson for IceCube Collaboration, believes this discovery is the key to unexplored parts of the universe and could possibly reveal the origins of the highest cosmic energies.