On Saturday, December 18, the IceCube Neutrino Observatory sank the last of 86 strings of sensitive photodetectors to a depth of almost two and a half kilometers in the ice at the South Pole. Then on December 20th the final Digital Optical Module was deployed (see video below, with Star Wars and Star Trek cliches included), marking completion of the huge neutrino telescope IceCube project.

“With the completion of IceCube, the 1970s dream of building a kilometer-scale neutrino detector has finally become a reality,” says Francis Halzen, a professor of physics at the University of Wisconsin-Madison and the IceCube Collaboration’s principal investigator. “Finally science can start with a stable instrument that already yields neutrinos with unprecedented energy and statistics.”

Under construction since 2004, IceCube encloses a cubic kilometer of clear ice, beginning one and a half kilometers beneath the surface and extending downward another kilometer. The telescope has to be this big because neutrino collisions with matter are exceedingly rare: out of uncounted trillions of neutrinos constantly passing through the ice, IceCube will observe just a few hundred a day.

Seeing them at all is only possible because when neutrinos collide with the nuclei of oxygen atoms in the ice, they turn into energetic charged particles called muons, moving in the same direction. Because these muons (and other debris from the collision) are moving faster than light can travel through ice, they radiate a shock wave of blue Cherenkov radiation visible to IceCube’s photodetectors.

Catching neutrinos with Digital Optical Modules

Sixty basketball-sized detectors are mounted on each IceCube string. Called Digital Optical Modules (DOMs), their optical parts are photomultiplier tubes (PMTs) that detect and amplify Cherenkov radiation from passing muons. Electronics convert the PMT signals to digital form. The PMTs and circuit boards are housed together in transparent glass pressure vessels.

Digital Optical Modules were originally developed at Berkeley Lab. In 1997 two prototypes were built by the Jet Propulsion Laboratory and installed in the AMANDA array (IceCube’s predecessor) at the South Pole. The University of Wisconsin, the lead institution in the AMANDA Collaboration, pioneered the hot-water drilling techniques that made deep-ice strings of sensors practical.

Berkeley Lab and the University of Wisconsin worked together with other institutions to design and build String 18, a single string of 40 prototype DOMs installed at AMANDA in 2000. Robert Stokstad and David Nygren of Berkeley Lab’s Nuclear Science and Physics Divisions developed and championed the idea of digital optical modules and led the String 18 operation, with Physics Division engineer Jerry Przybylski doing much of the hardware work. Their superb performance resulted in the DOM technology being selected for IceCube.

Berkeley Lab’s built-in electronics have performed with astonishing dependability. Ninety-eight percent of IceCube’s over 5,000 DOMs are working perfectly, and another one percent are usable – reassuring numbers, given that the DOMs now frozen in the ice will never be seen again.

“The DOMs are no more accessible than a space satellite in high orbit,” says Spencer Klein, Berkeley Lab’s group leader for neutrino astronomy, “but they’re a lot more reliable and extremely robust. They’re also performing far above specifications, which called for them to be able to resolve the timing of Cherenkov radiation flashes within five nanoseconds” – five billionths of a second. “Instead, their timing resolution is about two nanoseconds.”

In an article released today from Nature News, the final cost of the project is quoted at US$271 million. Compare this to the $1.7 billion cost of the Space Shuttle Endeavour, the orbiter built to replace the Space Shuttle Challenger, or the average cost to launch a Space Shuttle of about $450 million per mission.

In a lab on the surface of the ice, signals from DOMs on many different strings are combined into a single data stream, which is analyzed to determine the direction and energy of the neutrino events that left their tracks. For separating neutrino signals from far more copious background events, the most important discrimination is whether the signal comes from overhead or below. Muons moving upward through IceCube must come from neutrinos that have passed through the Earth. Downward-going muons produced when cosmic rays hit the ice are a million times more numerous.

Says Spencer Klein, “These physics results are just a taste of the things that we can expect from IceCube now that it is complete. After more than a decade of work, Berkeley Lab researchers can now fully enjoy the fruits of their labor, and are looking forward to a bountiful physics harvest.”

Source Articles:

"IceCube completed" from Nature News

Extensive Details on IceCube South Pole Neutrino Detector from University of Wisconsin

Into the Ice: Completing the IceCube Neutrino Observatory