Video: World’s largest neutrino detector takes shape under ice
PEERING into a hollow drilled into the Antarctic ice, I suddenly feel rather shaky. “Your biggest danger is the hole,” I’d been warned. “Don’t turn your back on it.”
So I’m careful not to. One slip could be fatal when the hole in question is 2.5 kilometres deep. Here at the South Pole, drillers are pumping hot water into the ice to bore holes that almost reach the bedrock. As this hole nears completion, a team of scientists and engineers prepares what looks like a giant string of pearls to lower into it. In fact, it’s a kilometre-long string of shiny, bulbous detectors designed to look for flashes of light emitted by neutrinos smashing into the ice. This is just one of 80 such strings that will make up the – an audacious attempt to turn a cubic kilometre of clear Antarctic ice into the world’s largest neutrino detector.
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Physicists are hoping IceCube will detect extremely energetic “astrophysical” neutrinos – something never done before. This will help to pinpoint sources of high-energy cosmic rays, shed light on mysterious celestial objects such as microquasars, reveal any dark matter at the centre of the sun, and see further and at higher energies than with any other telescope. “Neutrinos are the only particles with which we can look at high energies very deeply into the universe,” says physicist Albrecht Karle of the University of Wisconsin, Madison.
Although only half complete, IceCube is already bigger than any other neutrino telescope. As I walk the short distance from this latest hole back to my lodgings at the US Amundsen-Scott South Pole station, I appreciate just how hard it must be to build an experiment on the coldest, driest and highest continent on Earth.
“Building an experiment on the coldest, driest and highest continent on Earth does not come easy”
Even in summer, temperatures at the pole average about -25°C, and the wind chill often makes it feel more like -40°C. Breathe in too quickly and the cold air can sear your lungs, leaving you coughing for days. And walking, let alone working, is exhausting when you are dressed in the heavy clothing designed for extremely cold conditions. Then there is the altitude. Unusual atmospheric conditions can reduce the air pressure so much that the South Pole feels higher than its 2800 metres – enough to cause acute altitude sickness. “It is a tough place to work,” says IceCube physicist Gary Hill.
So why build a neutrino telescope at the South Pole? The answer lies with neutrinos’ slippery nature. They interact so rarely with matter that they are incredibly difficult to detect. To have any chance of spotting signs of one, you need to monitor an enormous volume of clear material, such as water or ice.
IceCube detects neutrinos that interact with an atomic nucleus in the ice to produce a muon with so much energy it travels faster than light does in ice. This speeding muon creates a shock wave that takes the form of blue Cerenkov light, which is picked up by light detectors called digital optical modules.Physicists have used a similar technique to detect neutrinos from nuclear reactions in the sun, from the 1987A supernova and from cosmic rays. But the real prizes are the astrophysical neutrinos, which have up to a billion times more energy and reach us from the dust-shrouded depths of the universe.
It is because so few of these high-energy neutrinos come our way that the size of the detector matters. “Back in the 1970s, we all agreed that if we built a detector a cubic kilometre in size, we should be able to start doing neutrino astronomy,” says John Learned of the University of Hawaii, Manoa. He led the effort from 1976 to 1995 to build the pioneering Deep Underwater Muon and Neutrino Detector (DUMAND). His team started submerging strings of light detectors deep in the Pacific Ocean. A lack of funds and serious technological challenges eventually sank the project, but DUMAND spawned other experiments, including the Lake Baikal neutrino detector in Siberia and the ANTARES telescope in the Mediterranean. However, all of these detectors are far smaller than a cubic kilometre (see Diagram).
So where to find a cubic kilometre of transparent material? None of the candidate locations was at all convenient, but one did have a clear advantage: Antarctica. In the 1990s, Bob Morse and Francis Halzen of the University of Wisconsin, Madison, and colleagues started work on the Antarctic Muon and Neutrino Detector Array (AMANDA).
The concept is simple: drill holes in the ice using pressurised hot water, lower strings of sensors into them, and let the holes refreeze. The South Pole seemed ideal, as the ice sheet is kilometres thick and offers a stable working platform. There is no need for ships or underwater vehicles, and no worries about the strings being buffeted by undersea currents. Plus, there are no concerns about slime growing on detectors or light pollution from radioactivity in seawater or bioluminescent creatures. What could go wrong?
The ice, for one thing. “The stuff you can read about ice in textbooks is wrong,” says Halzen. The Antarctic ice was supposed to be free of air bubbles below a depth of 400 metres. But when the first AMANDA strings were lowered down to 1 kilometre, the physicists were in for a shock. The ice was full of bubbles, which caused the Cerenkov light produced by neutrino interactions to scatter after travelling very short distances. This made it hard to discern the original direction of the neutrinos.
So the team tried drilling deeper, and lowered another set of AMANDA strings to a depth of 2 kilometres. Down there, the pressure from the ice above had turned the bubbles into crystals of air hydrate clathrate. These have nearly the same refractive index as ice, so the scattering is much lower.
AMANDA proved that an ice-based detector works: it has spotted large numbers of atmospheric neutrinos, which are created when cosmic rays hit the upper atmosphere. However AMANDA failed to glimpse any astrophysical neutrinos before its reign as a stand-alone experiment ended in 2006.
The stage was set for IceCube. With its 80 strings, carrying a total of 4800 light detectors to monitor a cubic kilometre of ice, the telescope is nearly 100 times the volume of AMANDA. But IceCube sought clearer ice and longer strings than had been used with its precursor. Would it be better to drill deeper into the Antarctic ice sheet?
To find out, the researchers lowered three of AMANDA’s short strings to a depth of 2300 metres. They discovered that the ice was indeed clearer. Next, Buford Price and Ryan Bay of the University of California, Berkeley, and colleagues carried out a detailed analysis of the ice using a tool called a dust logger. As the device is lowered down a hole in the ice, it fires laser light and analyses the reflections. This showed that there are fine layers of dust within the ice, deposited over tens of thousands of years – some due to ancient volcanic eruptions. Below a depth of about 2100 metres, however, they found very little dust.
So the team decided that the new strings should be lowered to 2450 metres, where the ice is more than 60,000 years old. It was an extraordinarily ambitious plan, and it met with intense scepticism. “Everybody told us that IceCube was a cute idea that wouldn’t work,” says Halzen, IceCube’s principal investigator. Happily, the sceptics have been proved wrong.
After trudging 1.5 kilometres on bulldozer-compacted ice paths cutting across the South Pole station’s runway, I reach “drill camp”. It is here that water is heated and pressurised, and fed into fat insulated hoses that snake over the ice sheet towards the makeshift tower where the drillers are boring the latest hole half a metre wide.
Inside the tower, drillers are blasting 750 litres per minute of nearly boiling water at a thundering pressure of 70 atmospheres through a narrow nozzle, instantly melting the rock-hard ice. Weighted with more than half a tonne of steel, the drill sinks straight down as the ice melts beneath it, and it will have deviated by less than a metre from vertical by the time it reaches the bottom. The drill takes 24 hours to descend 2450 metres, 10 hours to come back up and uses nearly 20,000 litres of fuel per hole – every drop of which has to be flown in on US military cargo planes.
Despite the costs, hot water is the only way to drill such holes at the South Pole. “Using water in one of the coldest environments on Earth is a horrible combination, but it works,” says Dennis Duling, a veteran driller who has been with the team since AMANDA.
During the night, a second team lowers a string of digital optical modules (DOMs) into the hole. They have to work swiftly, because the hole begins to freeze within hours of being drilled. Computer simulations show that in less than a day and half the hole will have narrowed too much for the 33-centimetre-wide detectors to pass through. One by one, the team attaches DOMs to a weighted cable, and lowers them into the hole. As each one disappears, a shout of “DOM ahoy!” rings out.
Over four seasons, the drilling teams have mastered the process to the point that they can now deploy a string within 9 hours. “To drill holes 2500 metres deep seemed like quite a crazy idea,” says Karle, “but now it almost seems like an industrial process.” The IceCube team has sunk a record 18 strings this season and, with 40 strings in place so far, IceCube is half done. “Unless something bad happens – like an accident that closes us down – we should be finished by 2010,” says Halzen.
“To drill holes 2500 metres deep in the ice seemed a crazy idea. Now it is almost an industrial process”
That’s not long to wait, but the IceCube researchers don’t have to. The physics has already started. “Last May we started taking data with 22 strings and it looks fantastic,” says Karle. IceCube is already observing atmospheric neutrinos slamming into the ice and there is euphoria at the South Pole.
The ice is so clear at depths below 2000 metres that IceCube’s kilometre-long strings can track a streaking muon and reconstruct the path of the incoming neutrino with unprecedented accuracy. Physicists are especially interested in muons that are travelling upwards through the ice because these have come from neutrinos that have passed through the Earth before interacting in the detector, rather than from cosmic rays slamming into the atmosphere (see Diagram).
If IceCube starts seeing high-energy astrophysical neutrinos with such precision, it could illuminate many mysterious objects in the universe, such as active galactic nuclei, microquasars and ultraviolent blazars – compact energy sources powered by enormous black holes that spew out intense jets of particles.
Although these celestial objects also emit penetrating gamma rays and cosmic rays, it is the neutrinos that offer us the best chance of opening up the sky to high-energy astronomy. That’s because neutrinos flit effortlessly through virtually everything in their path, while high-energy gamma rays from astrophysical sources are absorbed by the cosmic microwave background radiation and so never reach us. As for cosmic rays, the majority are deflected by magnetic fields as they travel, so their incoming direction rarely reveals their actual point of origin.
While IceCube is expected to work for decades, there is concern about its long-term stability. At the South Pole, the surface ice is slowly sliding towards the ocean. But no one knows what’s happening near the bottom, where the IceCube strings lie. “In the worst case, the bottom is frozen stuck to the bedrock and the top is moving at 9 metres per year, so there is a shear,” Bay says.
Bay has attached inclinometers to several strings, which will map the movement of the ice over the next few years. “If the bottom of our strings is in very strongly shearing ice, it may impact the longevity of the detector,” he says. Yet even as the deep ice is being mapped, the IceCube team is suggesting a staggering expansion at the surface (see “Son et lumière”).
If all goes well with IceCube, Halzen plans to go beyond astronomy. It should detect 50,000 high-energy atmospheric neutrinos each year, compared with just 7500 bagged by AMANDA in six years. This treasure trove of high-quality information could point towards new physics. Much smaller neutrino detectors in Japan and Canada have revealed that the neutrinos have mass, contrary to our best theory of particles and their interactions. Their findings are the most compelling evidence we have for physics outside of the standard picture. Will IceCube’s neutrinos provide us with more detailed clues? With more neutrinos at higher energies, you can start asking the questions, says Halzen.
It is already clear that IceCube has turned the icy wastes of the South Pole into a neutrino physicist’s field of dreams. “Everybody thinks that something will be seen at a cubic-kilometre scale,” says Halzen. “I’m not responsible for what is emitting neutrinos in the universe. We developed the technique, we built it, and we’ll see what comes.” But he too is optimistic. “There’s a feeling that the moment of truth has come.”
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Son et lumière
It’s going to be a light, sound and radio show at the South Pole, with normally shy neutrinos pushed into starring roles. Even as IceCube – designed to look for flashes of light – is being completed, plans are afoot to detect both the sound and radio waves generated by neutrinos smashing into the ice.
During the past two Antarctic summers, the IceCube team has added extra detectors to measure how radio and sound waves travel through the ice. The idea is to look for neutrinos that carry staggering amounts of energy, called “GZK neutrinos”. They are produced when ultra-high-energy cosmic rays travelling across the universe interact with the cosmic microwave background. Finding them would verify our theories of cosmic-ray physics at the highest energies.
Physicists predict that when a GZK neutrino with an energy of about 1020 electronvolts smashes into the ice, it will produce the radio equivalent of a sonic boom. The technique of listening to such signals was pioneered by the Radio Ice Cerenkov Experiment, which lowered radio detectors into holes dug for IceCube’s predecessor, AMANDA. And last year an experiment called ANITA flew suspended on a balloon over the Antarctic ice sheet, listening for radio waves resulting from a GZK-neutrino interaction. ANITA’s antenna failed to picked any up, though the analysis of its observations is not yet complete and it is expected to fly again in December 2008. “The trouble is that the experiment can’t stay up there forever,” says John Learned, a neutrino physicist at the University of Hawaii, Manoa.
He and ANITA’s principal investigator, Peter Gorham, have teamed up with the IceCube collaboration and hope to expand the neutrino detector at the South Pole. “Conceptually it is like putting ANITAs on the ice around IceCube,” says Learned. Since radio waves travel kilometres through the ice, the antennas can be spaced on the surface about a kilometre apart. “Eventually we would like to cover 1000 square kilometres,” says Learned. “If we do that, we have got to see those high-energy GZK neutrinos.”
