Freezing a ghost

It’s an eerie thought that billions of neutrinos pass through your body every second. These ghostly subatomic particles have no electric charge, barely any mass and zip around the universe at the speed of light. They may come from fusion reactions in the sun or exploding supernovae in the furthest reaches of space. Scientists have long been intrigued by them, particularly the mysterious ultra-high-energy variety that arrives from deep space. But how do you catch a ghost? Freeze it in a square kilometre block of ice. That’s what an international team of scientists are doing with the US$271 million IceCube Neutrino Observatory. They drilled down two and a half kilometres into the Antarctic ice to create a giant neutrino detector. So far they have caught 28 very high-energy neutrinos, as reported in November in the journal Science

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Under the ice

Neutrinos only give themselves away when they directly hit another subatomic particle, like those in the hydrogen and oxygen nuclei of frozen water molecules. The collision creates a tiny flash of blue light that is picked up by extremely sensitive detectors buried in the pitch-black subsurface ice.

But a direct hit is rare. To increase their odds of spotting a neutrino, scientists filled the cubic kilometre of ice with over 5,000 detectors, as pictured in this artist’s impression. 

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Scientists drilled 86 holes into the ice and lowered in long strings. Each carries 60 spherical light detectors to spot tell-tale flashes in the darkness.

On 9 August 2011, IceCube detected a very-high-energy neutrino, which was carrying 1040.7 TeV of energy as it smashed into the ice, tens of times more energetic than the particles accelerated around the Large Hadron Collider. They named the particle after Sesame Street’s Bert. This infographic describes the event. Each sphere represents one light detector in the IceCube array; the bigger the sphere, the more light that detector picked up. 

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A neutrino named Ernie

Since IceCube started up in May 2010, the detector has spotted 28 very-high-energy neutrinos, as shown looking down on the detector array. The most energetic neutrino of all arrived on January 2012. It was named Ernie.

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Where in the Cosmos?

Because neutrinos rarely interact with matter, they fly straight and true through space. By tracing back their path in the detectors, physicists have started mapping where in the universe the neutrinos came from. 

More neutrinos are needed, but scientists suspect Bert and Ernie trace back to the violent regions that surround many supermassive black holes. “It’s a new window on the universe,” says Gary Hill, a member of IceCube, based at the University of Adelaide.