Construction has now begun for a telescope whose light-collecting “mirror” will be buried more than a mile beneath the South Pole ice cap. Dubbed IceCube because its array of detectors covers a cubic kilometer of ice, this telescope is designed not to capture starlight but to study the high-energy variety of the ghostlike subatomic particles known as neutrinos. Originating from the Milky Way and beyond and traveling to Earth virtually unobstructed, these high-energy neutrinos serve as windows back through time and should provide new insight into questions about the nature of dark matter, the origin of cosmic rays, and other cosmic issues.

This is the first IceCube string of DOMs to be lowered down into a mile-long hole drilled through the Antarctic ice by jets of hot water. That far below the surface, blue light travels more than a hundred meters through the otherwise dark ice.

Berkeley Lab is one of 26 institutes participating in the IceCube collaboration. Our researchers were responsible for the unique electronics package inside the digital optical modules (DOMs) that will enable IceCube to pick out the rare signal of a high-energy neutrino colliding with a molecule of water. A DOM is a pressurized glass sphere the size of a basketball that houses an optical sensor, called a photomultiplier tube, which can detect photons and convert them into electronic signals that scientists can analyze.

“Each of these DOMs is like a server that you can log onto and download data from, or upload software to,” says Robert Stokstad of the Nuclear Science Division (NSD), who heads the Institute for Nuclear and Particle Astrophysics (INPA) and is the leader of Berkeley Lab’s IceCube effort.

Equipped with onboard control, processing and communications hardware and software, and connected in long strings of 60 each via an electrical cable, the DOMs can detect neutrinos with energies ranging from 200 billion to one quadrillion (1015) or more electron volts. In the past few weeks, the first IceCube cable, with its 60 DOMs, was lowered down into a hole drilled through the Antarctic ice using jets of hot water. Plans call for a total of 4,200 DOMS to be put in place over the next five years. The Antarctic summer season, during which the weather is “mild” enough for work on IceCube to proceed, lasts only from mid-October to mid-February. After that, winter sets in and the climate is much too harsh for any outdoor work.

When completed, IceCube’s total volume of detectors will be about 20 times greater than that of its predecessor, another South Pole high energy neutrino telescope called AMANDA (Antarctic Muon And Neutrino Detector Array) which has 680 optical sensors.

“The South Pole might seem like an unusual place to build neutrino telescopes, but the Antarctic ice is very clear and very stable, and has relatively low background radiation levels,” says NSD astrophysicist Spencer Klein, who heads the physics analysis team for Berkeley Lab’s IceCube effort.

The South Pole might seem like an odd place to build neutrino telescopes, but the Antarctic ice is very clear and stable, and features relatively low background radiation. IceCube will be installed to the far left of the station shown here.

Neutrinos are one of the most common and mysterious particles in the universe. Produced by the decay of radioactive elements and certain elementary particles, they carry no electrical charge and scarcely a hint of mass, which means they are unaffected by magnetic fields and rarely interact with other forms of matter. Able to escape from anything other than a black hole, their pathway to earth is essentially a straight line from their point of origin. Because these neutrinos are the only known particles able to pass through Earth untouched, scientists can point telescopes like IceCube and AMANDA to the northern skies and use the planet as a filter.

While there are extensive ongoing studies of the neutrinos emitted out of thermonuclear reactions in the core of the sun, as well as antineutrinos from nuclear reactors, IceCube is designed to study the neutrinos spawned in the most violent of astrophysical events, i.e., supernovas, gamma-ray bursts, and cataclysmic phenomena involving black holes and neutron stars. In studying these high-energy neutrinos, scientists hope to be able to produce a map of the neutrino sky.

Trillions of neutrinos pass through each square centimeter of Earth’s surface every second without a trace of impact. However, every so often, a neutrino does collide with an atom. This rare collision generates a muon, a heavy electron-like subatomic particle that, as it passes through ice or water, emits flashes of bluish light called Cherenkov radiation. IceCube’s DOMs can detect this light and scientists, by measuring the intensity and arrival time of the light at multiple DOMs, can reconstruct the directional path of the muon and determine the type, direction and energy of the neutrino that helped create it. This is critical for separating a muon generated by a cosmic neutrino from the millions more muons generated by cosmic rays in the atmosphere.

Azriel Goldschmidt, Robert Stokstad and Spencer Klein of the Institute for Particle Astrophysics at Berkeley Lab were part of the team responsible for the electronics inside the Digital Optical Modules (DOMs) that will enable IceCube to pick out the rare signal of a high-energy neutrino colliding with an atom of ice.

On the surface of the ice, located where the IceCube detectors emerge from the frozen depths, there is another array of detectors called IceTop. This past season, eight of the 160 tanks that will make up the completed IceTop were installed. Each tank is about two meters in diameter and will hold two IceCube DOMs frozen in water. A pair of tanks will be connected to each IceCube cable. IceTop will be used to calibrate IceCube and to study high-energy cosmic rays.

“For practical reasons, it was important that the DOMs be built around an integrated circuit that could give us fast sampling times (pulses on a nanosecond timescale) with low power demands,” says Azriel Goldschmidt, another NSD astrophysicist who has been working on DOM data analysis. “Each DOM runs on only about five watts of power.”

The DOM integrated circuit was custom made at Berkeley Lab based on architecture developed by Stuart Kleinfelder, formerly of the Engineer-ing Division and now at UC Irvine. More than 30 Berkeley Lab scientists and engineers are involved in the IceCube project. Project leaders, in addition to Stokstad, Klein and Goldschmidt, include William Edwards of the Engineering Divi-sion, the project manager, and David Nygren of the Physics Division, one of the world’s foremost experts in particle detection.

Says Klein, “One of the most exciting things about IceCube is that we just don’t know what we will find. When you open up a new window into the universe, you open up the possibility of entirely new discoveries.”

Construction of IceCube is projected to cost $272 million. The National Science Foundation is providing $242 million for the project, and an additional $30 million will come from foreign partners.