IceCube – One hole done, 79 more to go
By Kristan Hutchison, Sun staff
With hot water and a hose fit for a giant, drillers melted the first hole for the world’s largest neutrino detector.
They sprayed 85C water straight down into the ice for 58 hours in January, melting a hole 2.5km deep before lowering in a string of volleyball-sized detectors.
“It was our first year and I think it was great to get a hole,” said system engineer Jeff Cherwinka. “The difference between one hole and four holes is substantial, but the difference between one hole and no holes is infinite.”
The hole is the first of 80 that will eventually perforate a cubic kilometer of Antarctic ice. When it’s done, IceCube will be nearly invisible from above – just a field of flat ice blending seamlessly into the Antarctic plateau. But 4,800 glass eyes will be frozen into the ice below, each watching for a spark of blue light indicating one of the universe’s smallest and most elusive particles had a fatal collision.
Phantom particles
Usually neutrinos whiz around the universe undetected. Though the subatomic particles are common, they have no charge; barely any mass and rarely react with anything. About 10 million neutrinos will zip through your body as you read this sentence, but only one in a lifetime will stop there, according to Francis Halzen, a University of Wisconsin-Madison professor of physics and principal investigator for the IceCube project.
This lack of interaction is both a blessing and a curse for astrophysicists. Neutrinos arrive at the Earth unchanged, like messages sent from the outer reaches of the universe. But the messages are in an invisible ink. They show up only when a neutrino crashes into a proton, creating another kind of particle called a muon. An existing experiment at the South Pole proved these blue flashes could be detected by light-sensitive optical modules frozen deep in the ice. IceCube is the next generation of that earlier experiment, the Antarctic Muon and Neutrino Detector Array (AMANDA).
With 750 optical modules in 19 holes, Amanda catches one blue flash every seven hours, Halzen said. That’s thousands of neutrinos since 1997, but in order to map the origins of neutrinos in the sky, scientists need to detect millions.
“Neutrinos travel like bullets through a rainstorm,” Halzen said. “Immense instruments are required to find neutrinos in sufficient numbers to trace their origin.”
IceCube will expand AMANDA more than tenfold and detect 300,000 neutrinos a year when it is finished in 2010.
“As the detector grows you get more and more information and the picture gets more clear,” Halzen said.
The picture he and other researchers expect to see will include gamma ray bursts, active galaxies, black holes and the dark matter researchers now say makes up 90 percent of the universe.
“One of the most exciting things about IceCube is that we just don’t know what we will find,” said astrophysicist Spencer Klein, who heads the IceCube physics analysis team for U.S. Department of Energy’s Lawrence Berkeley National Laboratory. “When you open up a new window into the universe, you open up the possibility of entirely new discoveries.”
Drilling fast and deep
First the 80 holes must be drilled. In concept, drilling ice is simple – hot water melts a hole in the ice. Making it work on the large scale needed is difficult. Even the drill used for AMANDA was too small. Four years ago the Ice Core Drilling Service in Madison, Wis., began designing and building a new enhanced hot water drill, funded by the National Science Foundation. The drill has been turned over to IceCube and this season was the first chance to test it.
The goal is to penetrate 2.4 km into the ice in 30 hours, compared to the 100 hours it took to drill each hole for AMANDA. To do that the enhanced hot water drill has a longer hose and a higher flow rate. Hot water flows through the new drill at 760 liters a minute, compared to Amanda’s 380 liters, basically doubling the drilling speed. The hose is more than 2.5km long, allowing them to drill from the top to the bottom without stopping to add sections.
The hose also had to be strong enough not to burst from the pressure required to pump water for kilometers. Made out of a heavy-duty rubber, reinforced with Aramid, the hose had to be special ordered from Italy. At 11,340kg, the hose still requires an auxiliary cable for support, even though the load is reduced to 6,000lb due to the buoyancy of the water in the hole.
“It’s a pretty high-tech tool,” Cherwinka said. “It’s one of the most critical parts of the system and we’ve only been able to find one vendor who can make it.”
Most components of the drill had been checked to see that they worked, but there was nowhere convenient with ice deep enough to actually test the drill before bringing it to the South Pole. Though they’d hoped to drill several holes the first year, just getting one done and proving the drill would work was a success, said Jim Yeck, the IceCube project director.
“We met all of the high-level milestones, including the most significant one, the installation of a string,” Yeck said.
It wasn’t easy. Almost 1 million pounds of cargo had to be shipped to the South Pole in LC-130s for the drilling to start. The drill alone took 30 flights. With only a three-month window of weather good enough to work at the South Pole, 24 cargo handlers, electricians, engineers and drillers worked in shifts. By the time the drill was assembled, the drill team had only two weeks to melt a hole and lower the first IceCube string.
Deep drilling of the first hole was not going very fast and the drill was veering off course. A thousand meters into the hole a decision was made to pull up the drill to investigate the problem. As the drill sat idle on Jan. 17, water began to freeze in the return line. While the drill crew was trying to solve this problem an experienced driller was hit by a cable and injured.
The driller was evacuated from the South Pole and in a hospital within 22 hours. Meanwhile, drilling was halted for several days to reassess its safety.
“It was a really bad day,” Cherwinka said.
The safety assessments found the problem was a cable that had been on the ground and fell into the hole. A new procedure for handling the cable was developed and practiced and then drilling operations resumed at a new location 8 meters away.
“It wasn’t a matter of ignoring something or taking a shortcut or misbehaving,” Cherwinka said. “We just didn’t recognize this particular cable on the ground as a hazard and now we do, so it’s an easily remedied problem.”
When they started drilling again there were only a few days left before the drill team was scheduled to leave the South Pole. They worked 12-hour shifts until the hole was complete Jan. 27 and began packing away the equipment.
“The drillers in particular, those guys were working really, really hard and they were up against quite a few hurdles,” said Ryan Bay, a post-doctoral fellow from University of California Berkeley who was at the South Pole working on the IceCube project. “It was a tough season, but there was a payoff in the end.”
Most of the drill crew will return for the next summer season, when the goal is to finish 10 more holes. The successful hole took 58 hours, making it twice as fast as the AMANDA holes. But with 79 holes to go the drill team needs more speed.
“We’re looking at ways to improve the speed and I definitely think it’s doable,” Cherwinka said. Though IceCube will take at least five years more to complete, the first string of optical detectors is already working alongside AMANDA’s 19 holes. If IceCube drilling stays on schedule it will take over as the world’s largest neutrino telescope next season. It was already dubbed the world’s weirdest telescope.
“Nobody who built a new instrument like this could predict what they will actually do with it,” Halzen said. “And what they do with it later is always much more interesting than what they promised.”
