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May 7, 2009

Particle Showers

A computer reconstruction of a downward-moving cosmic ray muon (an elementary particle similar to an electron) track, observed in the IceCube Neutrino Detector. The colored dots represent light sensors that detected photons emitted by the muon as it traversed the ice near the sensors. The size of the dots is proportional to the number of photons detected by the sensor, while the different colors indicate the time of detection of the photons, and consequently the direction of the muons movement. (The red sensors detected the track, and then the yellow, and then green, following the rainbow.) The track in the ice in the lower portion of the picture is close to one kilometer in length.

The IceCube detector is currently under construction at the South Pole, with IceTop--a km2 array of particle detectors--being installed directly above it. IceTop will be used to detect extended particle showers induced in our atmosphere by high energy cosmic rays. High-energy muons in showers can penetrate deep into ice. Thus, events with trajectories that pass through both IceTop and the deep detectors of IceCube are seen by both components.

More about this Image
The IceCube Neutrino Detector, a telescope currently under construction at the South Pole, will search for neutrinos from the most violent astrophysical sources: events like exploding stars, gamma ray bursts and cataclysmic phenomena involving black holes and neutron stars. IceCube is a powerful tool for searching for dark matter, and could reveal new physical processes associated with the enigmatic origin of the highest energy particles in nature. IceCube will encompass a cubic kilometer of ice and uses a novel astronomical messenger, called a neutrino, to probe the universe.

Neutrinos are produced by the decay of radioactive elements and elementary particles such as pions. Unlike other particles, neutrinos are antisocial and difficult to trap in a detector. It is the feeble interaction of neutrinos with matter that makes them uniquely valuable as astronomical messengers. Unlike photons or charged particles, neutrinos can emerge from deep inside their source and travel across the universe without interference. They are not deflected by interstellar magnetic fields and are not absorbed by intervening matter. However, this same trait makes cosmic neutrinos extremely difficult to detect; immense instruments are required to find them in sufficient numbers to trace their origin. Although trillions of neutrinos stream through your body every second, none may leave a trace in your lifetime.

Scientists use large volumes of ice below the South Pole to watch for the rare neutrino that crashes into an atom of ice. This collision produces a particle--called a "muon"--that emerges from the wreckage. In the ultra-transparent ice, the muon radiates blue light that is detected by IceCube's optical sensors. The muon preserves the direction of the original neutrino, thus pointing back to its cosmic source. It is by detecting this light that scientists can reconstruct the muon's, and hence the neutrino's, path. The picture is radically complicated by the fact that most muons seen by IceCube have nothing to do with cosmic neutrinos. Unfortunately, for every muon from a cosmic neutrino, IceCube detects a million more muons produced by cosmic rays in the atmosphere above the detector. To filter them out, IceCube takes advantage of the fact that neutrinos interact so weakly with matter. Because neutrinos are the only known particles that can pass through the Earth unhindered, IceCube looks through the Earth and to the northern skies, using the planet as a filter to select neutrinos.

Antarctic polar ice has turned out to be an ideal medium for detecting neutrinos. It is exceptionally pure, transparent and free of radioactivity. A mile below the surface, blue light travels a hundred meters or more through the otherwise dark ice. Frozen in the ice, IceCube will be the world's largest and most durable particle detector.

IceCube is an international project sponsored and conducted by the United States and several non-U.S. countries and funding agencies, with several different institutions involved in all phases of the project. Primary funding comes from the National Science Foundation through its Major Research Equipment and Facilities Construction (MREFC) program and Research and Related Activities (R&RA) grants. Scheduled completion date is 2010. To learn more, visit the IceCube website. [IceCube startup and construction and IceCube maintenance and operations supported by NSF grants ANT 02-36449 and ANT 06-39286.] (Date of Image: 2008)

Credit: IceCube Collaboration

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