IceCube project drills 18 holes, begins collecting data on atmospheric neutrinos
Posted February 29, 2008
Construction of the world’s largest, and perhaps most unique, telescope is 50 percent complete.
Drillers deployed the 18th string of digital sensors for the IceCube Neutrino Observatory array on Jan. 25, 2008. That means there are now 40 strings of digital optical modules (called DOMs) buried up to 2,500 meters into the ice around the South Pole.
The goal is to bury as many as 80 such strings, each carrying 60 DOMs, within a cubic kilometer of ice.
“We’ve made excellent progress for such a large experiment,” said Albrecht Karle, a professor at the University of Wisconsin-Madison, in early January as the 13th hole was under way at the time.
The multi-million-dollar international project is a big experiment to understand one of the smallest particles in the universe — high-energy neutrinos. Neutrinos are subatomic particles with almost no mass and no electrical charge that are created by certain kinds of radioactive decay, such as what takes place in the sun or from a supernova.
As the story goes, something like a billion neutrinos will shoot right through your body by the time you finish this sentence. Oh, there goes another billion … They almost never interact with matter, traveling directly from their source, and can typically pass entirely through the Earth unobstructed. Most of them are solar neutrinos from the sun.
The international team of scientists behind IceCube is interested in understanding neutrinos from more exotic, extragalactic origins — from black holes or supernova. The sources of those neutrinos could lead to a better understanding of other brainy astrophysics concepts as cosmic rays, which are energetic particles from space.
“One of the primary goals of IceCube and high-energy neutrino astronomy is to find the origin of these high-energy particle accelerators in the universe,” Karle said. “These accelerators are quite amazing. They produce particles of energy a billion times higher than what we can generate in terrestrial laboratories.”
But perhaps the biggest prize from understanding neutrinos would be the discovery of dark matter. No one has ever captured dark matter or tasted it — let alone seen it — but most scientists are convinced that this non-luminous material exists and makes up most of the mass of the universe, clumping around visible matter. And some theories even say that neutrinos could make up at least a portion of dark matter.
(Incidentally, an equally mysterious concept called dark energy, a sort of anti-gravitational force accelerating the expansion of the universe and shaping the evolution of galaxies, actually makes up about 70 percent of the universe.)
What else could IceCube discover? Karle shrugs. “Perhaps some yet undiscovered exotic phenomena, perhaps we’ll see some fundamental laws of physics violated,” he said, seeming to relish the idea.
“IceCube is a discovery experiment, so we are doing things for the first time,” Karle noted. “We’re looking at the universe in a new way, so there’s a chance for surprises.”
IceCube doesn’t directly detect neutrinos, which travel nearly at the speed of light. Instead, like a sports photographer who captures the dynamic collision of a bat on a baseball, the detector spots the rare, head-to-head collisions between a neutrino and an atom within the ice. This subatomic car wreck creates a particle called a muon. In the transparent ice, the muon radiates a blue light that IceCube’s DOMs can detect.
The muon preserves the direction the original neutrino traveled, a cosmic breadcrumb that scientists can use to trace back to the source.
Unfortunately, for the scientists, nearly all the collisions represent neutrinos from near-Earth sources. Only a fraction will come from extra-galactic origins, hence the cubed kilometer size of the array.
Construction of the detector is no mean feat in itself. The IceCube camp, located a kilometer or so from the main South Pole Station building, looks like a derailed train on ice — a collection of rectangular buildings that house water tanks, generators and high-pressure heaters.
A cobweb of hoses slung between the buildings transports thousands of gallons of water through the system, which includes a Rod well, a sort of underground aquifer of water in the ice created by a heated coil. Engineers designed the system to recapture most of the water required to melt each hole, according to Australian Alan Elcheikh, IceCube’s lead driller.
Computers monitor the entire operation, tracking everything from the amount of fuel used to drill each hole (an average of 7,400 gallons last year) to the rate of drilling (about 2 meters per minute on the way down), as well as water temperature and flow.
“It’s a home-grown control system,” Elcheikh said.
The hot-water hose itself is so heavy that the drillers must tape a secondary cable to it for support as it descends slowly into the ice. A visit to the two-storey-high tower where the operation takes place found two of the drillers — Graham Tilbury and Eric “Bear” Coplin — attending to the hose and cable. Tilbury deftly cut off the thick tape as the hose and cable slowly ascended through a notch cut into a manhole-sized cover, which protects the ice hole from an errant hardhat dropping down.
An assistant at the College of Marine Science at the University of South Florida, Tilbury said he took a leave of absence from his job for the opportunity to work in Antarctica on the IceCube team. “IceCube is a tremendously cool project,” he said.
Most of this year’s drillers, Elcheikh said, have previous experience with the project or working elsewhere on the Ice. Coplin, for instance, worked several seasons at McMurdo Station before switching to IceCube.
This year’s team set a blistering pace for ice-hole drilling. Those 18 holes represent a personal best for the project, despite a late start. Improvements in technique have whittled the time required to drill a hole down to 35 hours, Karle said. Deploying a string of DOMs takes another 10 hours, he added.
“They’re an exceptionally good crew,” he said of the 30 drillers, who work in three, 10-person shifts around the clock once a hole begins. “Maybe this is the No. 1 reason for the improvement.”
Construction is scheduled to continue through the 2010-11 summer season at South Pole. However, parts of the detectors are already working, with 22 strings taking data since May. Analysis is still ongoing, Karle said, but IceCube is already detecting one atmospheric neutrino per hour.
Those aren’t the Holy Grail neutrinos yet. “It’s too early to say anything about cosmic neutrinos,” Karle said.
IceCube isn’t the only neutrino detector on the planet. ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch), is a sort of complementary neutrino telescope currently under construction in the Mediterranean Sea off the coast of Toulon, France.
ANTARES will attempt to capture neutrino collisions in the water as they travel through the planet from the southern hemisphere, while IceCube is actually oriented toward the northern hemisphere as neutrinos pass through the Earth.
Karle said that, while it is important to have such multiple experiments to confirm results, there is also a spirit of competition involved.
“We want to be the ones to make the discovery — absolutely,” he said. “It clearly is a race. And, at the moment, we are clearly leading the field. We want the first important discovery.”
NSF-funded research in this story: Francis Halzen, University of Wisconsin — Madison.
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