Astronomy under the ice: Scientists use Antarctic ice to study some of the tiniest particles in the cosmos

by Timothy Oleson
Monday, November 25, 2013

Deep below the glacial surface at the South Pole, where the Antarctic ice is crystal clear yet pitch black, a 3-D array of more than 5,000 custom-built and precisely positioned sensors, each about the size of a basketball, lies frozen in place. The sensors keep watch for thousands of momentary flashes of blue light that zip by every second, some the result of collisions between neutrinos — nearly massless subatomic particles with no electrical charge — and the relatively large atomic nuclei in the frozen water. Fittingly dubbed IceCube, the array is the world’s newest and largest operational neutrino observatory, and its aim, despite its subterranean setting, is to illuminate interstellar phenomena that are millions, and even billions, of light-years away.

Construction of the full observatory was finished in late 2010, culminating roughly a decade’s worth of planning and development. IceCube’s completion is regarded as a massive technical achievement in its own right, and it has been collecting prized data since 2005. But with its full suite of sensors finally in place, IceCube is now capable of detecting elusive neutrinos with greater sensitivity than ever before. And scientists hope its biggest contributions — determining the sources of deep-space cosmic rays, which will help resolve the astrophysical origins of the cosmos, observing the inner workings of supernovae and black holes, and deciphering dark matter’s mysteries — are yet to come.

The Ideal Messenger

Neutrinos — kin to electrons in the subatomic family of leptons — are born in natural nuclear decay processes, in nuclear reactors and in interactions between cosmic rays and atoms in the atmosphere. They are produced by the sun, supernovae and maybe by black holes. And still farther from home, cosmicĀ  neutrinos are ejected with tremendous energy by far-off sources that remain mysterious to physicists.

Trillions of neutrinos stream through Earth in any given second, the vast majority without notice due to their weak interaction with other matter and fleetingly small mass — less than half that of an electron and roughly 1,000 times less than a proton.

Yet these same attributes that make them so hard to detect is the reason why neutrinos are “an ideal candidate” for helping observe distant phenomena beyond the reach of optical telescopes, says Darren Grant, a particle physicist at the University of Alberta in Edmonton, Canada. A neutrino is “like a messenger from space, coming directly from its source,” Grant says. And determining the location and nature of these sources is the main purpose of IceCube.

High-energy cosmic neutrinos have remained especially elusive, however, leaving stumped astronomers to theorize for decades about their deep-space origins in the absence of physical evidence. Current theory suggests that violent phenomena such as quasars, black holes or gamma ray bursts, along with cosmic rays, accelerate the particles to astounding energies — exceeding energies produced at the world’s most powerful accelerator, the 7-trillion-electron-volt Large Hadron Collider, by more than a million times. Cosmic rays, predominantly consisting of the nuclei of hydrogen, helium and other elements that travel through space, are easier to detect because they are larger and carry an electrical charge. However, they are easily deflected by magnetic fields that erase evidence of their origins. Neutrinos are rarely absorbed by intervening matter and are not deflected by magnetic fields, thanks to their neutral charge, so they preserve information about the location and energy of the events that spawned them.

“If you can definitively make a measurement of where the highest-energy neutrinos are coming from in the universe, and you can start to pin down how some of these processes work, it revolutionizes the entire field of astrophysics,” Grant says. “Ultimately, the neutrinos are coming from the very early stages of production when these things turn on.”

Building an IceCube

By the 1970s, people had discovered that they need very large detectors with kilometer-scale dimensions to distinguish cosmic neutrinos in statistically significant numbers from those originating inside our own atmosphere or solar system, says Francis Halzen, principal investigator for the IceCube project and a physicist at the University of Wisconsin at Madison, the lead institution for the project. The earliest attempts to detect extraterrestrial neutrinos involved relatively small instruments buried underground. Subsequent generations improved the detection capacity for neutrinos by using sensors immersed in relatively homogenous and optically clear water. But they still were not big enough.

IceCube’s fundamental improvement over earlier detectors is its size, Halzen says. It is made up of 86 strands with 60 sensors apiece frozen in place between 1,450 and 2,450 meters below the surface near the South Pole’s Amundsen-Scott research station. All told, the 5,160 sensors occupy a cubic kilometer of ice volume, making it the first neutrino observatory to meet the theoretical size requirement for detecting significant numbers of cosmic neutrinos.

Building a neutrino observatory in the Antarctic ice was thought to be “a pretty wild idea when it was first proposed,” says Albrecht Karle, also a physicist at the University of Wisconsin at Madison and associate director of IceCube. Deploying sensitive instrumentation at such depths in a glacier is no simple feat. To do so, the team designed a 5-megawatt hotwater drill to melt 60-centimeter-diameter holes in the ice to the requisite 2,500-meter depth. After drilling each of the 86 holes — accomplished over seven austral summer field seasons — a single strand of sensors was delicately lowered into the melted ice, which then refroze around the strand.

Why go through all this trouble instead of submerging the detector in a large body of water as was done with earlier instruments, as well as with the IceCube-contemporary ANTARES (Astronomy with a Neutrino Telescope and Abyss environmental RESearch) project in the Mediterranean Sea? Both ice and water work, Karle says, but “what seemed like the more unlikely and difficult choice turned out to be the one that was much more predictable.” Once the sensors are frozen into the ice, he says, they are very stable, whereas the push and pull of currents and the decay of radioactive potassium-40 in seawater, which contribute to a larger background signal, complicate detection in water-based detectors. Furthermore, the highly compressed deep ice offers excellent optical clarity. There is a trade-off, however, with deploying detectors in the ice: Once they’re in, they’re in. They cannot be recovered for repair or tinkering.

Construction of IceCube began in 2004 and followed on the heels of the earlier AMANDA (Antarctic Muon and Neutrino Detector Array) project, which began in the 1990s and served as a proof-of-concept for IceCube. When IceCube’s last sensor strand was installed just over a year ago, the final bill — footed primarily by the National Science Foundation along with contributions from Germany, Sweden and Belgium — was a little over $270 million, modest compared to most large-scale particle physics projects, Halzen says.

Detecting Deep Space Phenomena with DOMs

IceCube’s sensors, known as digital optical modules or DOMs, are encased inside 1.3-centimeter-thick glass-walled spheres to withstand the crushing pressure of the overlying ice. Each module contains more than 1,000 parts, including a photomultiplier assembly and digitizing electronics to process and relay data to the surface. The DOMs are actually designed to detect neutrinos indirectly by spotting evidence of muons, also members of the lepton family, that emerge from collisions between neutrinos and much larger atomic nuclei and are accelerated along the same path as incoming neutrinos. As it rockets through the ice at nearly the speed of light, a muon produces a trail of blue light known as Cherenkov radiation that acts like an arrow pointing back in the direction of the neutrino’s origin.

The trick is to distinguish muons coming from the relatively few high-energy neutrinos from a sea of background muons, generated constantly by showers of lower-energy atmospheric neutrinos and other subatomic interactions. To help differentiate cosmic neutrinos from the background, IceCube’s DOMs are positioned to “look down” through the planet to the Northern Hemisphere. In effect, Earth’s interior acts as a filter. Because muons decay over distances of only a few kilometers, any muons that arrive at the detector from below must arise from neutrinos that have passed through the planet.

Neutrinos are further differentiated based on their energy: In general, the more energy a neutrino has, the more energy it imparts to a muon and the more blue light is emitted and detected. Still, sorting the data gathered by the detector is not simple, and a high degree of precision is required. Complex algorithms are needed to discern the direction and arrival times of thousands of muons per second. “If you mistake a muon coming down [from above] for a muon coming up [through Earth] one in a milliontimes, you have no experiment,” says Halzen, adding that IceCube’s current success rate is about one mistake for every hundred million muons.

IceCube began operating at partial capacity in April 2005, less than a year after the first DOMs were deployed, and “in the last several years, we have been taking quite accurate data. We have almost 50,000 neutrinos accumulated per year,” Karle says. Mostly, he says, these are “consistent with neutrinos that are generated in the atmosphere of Earth from cosmic rays. And now we are intensifying our search with the full detector for neutrinos of astrophysical origin.”

Already, IceCube has helped improve long-standing theoretical models that predict how violent phenomena such as gamma ray bursts behave, Grant says. Nonetheless, no definitive evidence has been found so far to pinpoint the origins of the cosmic neutrinos that IceCube was designed to detect. Such evidence could still be years away, even with the full detector in operation. The number of cosmic neutrinos that are expected to produce a muon within sight of the detector is miniscule, and it takes time to collect them in meaningful quantities.

As the search for cosmic neutrinos continues, so do many other projects at IceCube. Of the roughly 250 participating scientists, Halzen says, fewer than one-third are working on neutrino astronomy. Some are looking at lower-energy neutrinos to shed light on dark matter, which along with dark energy comprises about 95 percent of the universe. Some are working out details of “bread and butter atmospheric neutrino physics.” And some are trying to use the particles to map Earth’s internal structure.

For his part, Halzen is particularly interested in another avenue: observing neutrinos as they pour from supernovae within our own galaxy. “Although it’s in every textbook, there’s no evidence for it. It’s just very sensible speculation,” he says. “If supernovae don’t accelerate galactic cosmic rays, nobody has a sensible alternative. It’s just like the Higgs [boson]. If you rule that out, nobody else has a really great alternative to it.”

The anticipation of actually solving some of these mysteries is building, although the timeline for discoveries is uncertain. In astronomy and astrophysics, “it’s become increasingly clear that many phenomena are transient phenomena, so what you might not see this year or in the next five years, might happen later,” Karle says. The detector’s useful life expectancy is unknown — perhaps a couple of decades — but IceCube’s scientists are confident in its potential. For now, all eyes are on the immediate future. With the final strands of sensors running since last spring, the first dataset from the full detector — data are analyzed one year at a time — should be available this May. “I’m sure good things will come out of it,” Halzen says. After all, the biggest reach in IceCube’s capabilities is “yet to come.”

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