Neutrinos are similar to the more familiar electron, with one crucial difference: Neutrinos do not carry an electric charge. Because neutrinos are electrically neutral, they are not affected by the electromagnetic forces which act on electrons. And conversely, they have little effect on matter and are capable of passing through objects larger than the Earth with little difficulty. This makes detecting neutrinos extremely difficult.
IceCube is the world’s largest neutrino detector, encompassing a cubic kilometer of ice. The top array of the detector is 1,400 m (4,590 ft or almost 1 mile) beneath Antarctica. The high pressure at that depth drives all gases out of the water, making the ice clearer than crystal. It is also extremely dark as no light penetrates the ice to that depth. Darkness is a critical need, as the detection of neutrinos depends on seeing the small flash of blue light emitted when neutrinos pass through molecules of ice.
This flash of light is recorded by 5,160 digital optical modules (DOMs) buried in the ice. Each DOM contains a photomultiplier tube (PMT), a device that amplifies the amount of light it receives, along with its associated circuitry. A typical use of PMTs is in nightvision goggles.
Researchers drilled 86 holes in the ice up to 2.5-km deep. Each hole holds a string of 60 DOMs spaced along the kilometer depth of ice used as the sensing element. Once buried in the ice, the DOMs are no longer accessible. However, electronic service and software upgrades are handled remotely using technology similar to that developed for space missions.
The DOMs record the direction and intensity of the light as the high-energy neutrino passes through the ice. This lets researchers determine where the neutrino came from.
hile only in service a short while, the IceCube observatory has already changed the way scientists look at the generation of cosmic rays and other high-energy particles. For example, it has been a long-held belief that cosmic rays, a major source of neutrinos, were emitted from gamma-ray bursts or GRBs.
GRBs arise when a massive star many times the size of our sun goes supernova, creating a light that shines many millions of times brighter than our sun. In looking at over 300 GRBs, IceCube found no corresponding neutrino emissions, refuting the role of GRBs in the creation of cosmic rays.