Hunting Neutrinos at the End of the Earth
How the unique properties of the South Pole allow the IceCube Neutrino Observatory to detect the elusive particles
Neutrinos are invisible, electrically neutral and nearly massless subatomic particles. Since they travel through the Universe at close to the speed of light without being absorbed or deflected by cosmic objects, neutrinos can provide us with information to further our understanding of their mysterious origins in processes such as supernovae (the explosive death of a star) and black holes. However, these properties also make neutrinos extremely difficult to detect, with roughly 100 trillion of them passing through our bodies every second, so how can we hope to find these elusive particles?
One experiment which searches for neutrinos is the IceCube Neutrino Observatory at the Amundsen-Scott South Pole Station in Antarctica. The experiment covers a volume of one cubic kilometre and cost $279 million USD, which was mainly provided by the US National Science Foundation. The IceCube Collaboration consists of around 300 scientists from 12 countries.
When a neutrino interacts with a proton or neutron, a fast-moving secondary particle is produced. This secondary particle may be detected through the emission of blue light known as Cherenkov radiation, produced when a particle passes through a medium faster than the speed of light in that medium. An ideal material for a detector would have a relatively slow speed of light and be transparent, making ice a good candidate.
The South Pole is the most suitable location for this experiment, because it is the only place on Earth with the infrastructure required for this type of research and such a large amount of transparent, pure and stable ice. Whereas most ice contains air bubbles which would interfere with the results, the thick ice at the South Pole has been tightly compressed over time, forcing out the bubbles. However, these conditions, which are ideal for a neutrino observatory, also produced logistical challenges. Construction of the observatory began in 2004, but could only take place between November and February, during the Southern Hemisphere’s summer, so it was not completed until 2010.
In one of my previous posts, I wrote about the search for dark matter using direct detection in the SuperCDMS experiment. The IceCube collaboration use a different technique, known as indirect detection, to search for the invisible dark matter particles that are thought to make up 26% of the content of the Universe. Dark matter particles respond to the gravitational force, meaning that a large number of dark matter particles should be trapped in the gravitational field of the Sun, accumulating in its core. These particles should annihilate with each other, producing particles that could decay into neutrinos that should be detected by IceCube. This would lead to an excess of neutrinos in the direction of the Sun. Since this has not yet been observed, the collaboration has been able to rule out dark matter particles with certain properties.
Like SuperCDMS, the detectors in IceCube must be shielded from radiation at the Earth’s surface, and so are buried almost a mile underground. The sensors used to detect the Cherenkov radiation are known as digital optical modules (DOMs). Any electronic problems must be solved remotely from the IceCube Lab at the South Pole, as the DOMs can no longer be accessed once they have been frozen deep within the ice.
The search for dark matter is just one of the key areas of research carried out at IceCube. Another is the search for sources of the high-energy cosmic rays which are also produced in violent and poorly understood processes, but, unlike neutrinos, are deflected by the magnetic fields of cosmic objects, so cannot be clearly observed. As well as this, the experiment is used to hunt for sterile neutrinos. The collaboration has even produced a detailed analysis of the movement of the South Pole glacier, where it was found that light travels preferentially in the same direction as the glacier. A study of the ice stability of the glacier below the experiment showed that the deformation of the ice sheets has resulted in a small amount of strain, which could limit the lifetime of the experiment.
In 2013, IceCube was named Physics World Breakthrough of the Year for making the first observations of extremely high-energy neutrinos from outside our solar system. With the unique properties of the ice at the South Pole, the experiment is one of the clearest examples of a location being carefully selected for a specific experiment, despite the logistical challenges. The effect of the glacier itself on the detection of neutrinos beautifully demonstrates the relationship between amazing science experiments and their locations.
