Machine Design

Scanning the ice for neutrinos

A team representing 26 U.S., European, Japanese, and New Zealand institutions is building a device in Antarctica that will search for high-energy neutrinos.

A digital optical module hangs on a 2,400-ftlong electrical cable hanging in a hole drilled through Antarctic ice using a jet of hot water. The entire cable holds 60 such modules.

A schematic of IceCube shows the scale of the device.

A tank containing two digital optical modules is frozen into a block of ice. IceTop uses 160 of these tanks to study cosmic rays and calibrate IceCube, the neutrino telescope.

. The device is dubbed IceCube because its detector array will occupy a cubic kilometer, making it 20 times larger than Amanda, the Antarctic muon and neutrino detector array already in place.

IceCube's detectors, called digital optical modules (DOM), consist of glass spheres the size of a basketball housing photomultipliers which turn light into electronic signals. Neutrinos change into light-emitting muons, among other things, when they strike a hydrogen or oxygen atom in water. DOMs will peer into the ice surrounding and containing the array, looking for the telltale light. The DOMs run on about 5 W and have sampling times measured in nanoseconds.

Engineers plan to hang 60 DOMs on a cable suspended in a 2,400-m-deep hole drilled through the ice using hot-water jets. The entire array will have 70 such cables when completed in 2010. The photomultipliers detect neutrinos with energies ranging from 200 billion to 1 quadrillion (1015) eV.

On top of the IceCube array, researchers are building IceTop, 160 tanks holding two DOMs frozen in ice. A pair of tanks will connect to each IceCube cable. Researchers will use IceTop to calibrate IceCube and study high-energy cosmic rays.

Though situated at the bottom of the world, the telescope will be "aimed" north. Only neutrinos such as those emitted from black holes and dark matter will have the energy to penetrate through the Earth and be picked up by the device. IceCube will cost an estimated $272 million, with the U.S. picking up $242 million of the tab.

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