Posted by Ana Roldán on June 24, 2014.
When you think about Antarctica, perhaps you're thinking of a boring frozen desert at the end of the world. Well you could not be more wrong.
Antarctica is one of the most interesting places on the planet, its weather conditions, its geographic location, its particular atmosphere - those are just a couple of things that make Antarctica a treasure for the scientific community.
One of the fascinating experiments on Antarctica is the IceCube. Who would have thought that we could use all the ice that is in the white continent to create a massive neutrino detector? To explain what it does and how it does it, we must first understand what a neutrino is.
Neutrino (Italian for "the little neutral one") is a subatomic particle that follows Fermi-Dirac statistics thus going by the name fermion. When a particle follows Bose-Einstein statistics it's considered a boson. But what does this mean?
A particle following the distribution of Fermi-Dirac implies that the energy distribution of the particle in the coldest temperatures, is distributed as a step function, i.e. the particles are placed from the lowest energy level to the highest, until they have taken all levels.
But why is a neutrino so special? Well, the standard model of physics predicted that neutrinos (like neutrons) have no charge or mass and a spin of 1/2. However now, we know that neutrinos do have mass. Experiments have shown that it's about a billionth of the mass of a hydrogen atom and this changes everything.
The fact that neutrinos have a mass implies that a neutrino can transform into other types of neutrinos, well, quantum physics was always a bit confusing. But this and the fact that its interaction with other particles is almost zero and are only affected by the weak nuclear force and gravity, make them very difficult to detect and analyze.
The second problem we face if we want to know more about these interesting particles, is to know the source of the neutrinos that can be detected.
The main source of neutrinos in our solar system is our sun. There exist more extravagant sources, such as supernovae or the cosmic background radiation. It is the latter which makes the neutrinos the key to understanding the mysteries of our universe.
And here is where the largest neutrino detector in the world, IceCube, comes into action. How can we use the Antarctic ice to catch these elusive particles? IceCube has thousands of photomultipliers deployed deep into the Antarctic ice (between 1450 and 2450 meters). A photomultiplier device (PMT) can amplify and translate a single photon "hit" into a measurable electric signal. These opto-electronic sensors are deployed in lines of sixty modules each, in holes in the ice melted using a hot water drill.
As mentioned above, it is not possible to directly detect a neutrino, instead we get neutrino kinematic information detecting rare collisions that occur between a neutrino and an atom within the ice. Due to the high density of ice, almost all products of the initial collision detection will be muons.
Detecting alone is not enough and one still has to identify the source. A lot of noise comes from the background radiation which is not as interesting. Interestingly, this impacts only the top of the detector and can be therefore filtered out.
Most of the "upstream" remaining neutrinos are pretty interesing and either come from cosmic rays striking the opposite side of the earth or some unknown fraction may be of astronomical origin. A lot of statistics is used to distinguish between these two sources. The direction and energy of the incoming neutrino is estimated by means of the byproducts of the collision. Unexpected high energy or an excess from a known spatial direction indicate an extraterrestrial source.
The main goal of IceCube is to produce a map of the neutrino flow of the northern hemisphere (yes, even though we are in the south, we are detecting the neutrinos that traverse our north pole all the way down to the south). Complete with other detectors in Japan and the Mediterranean, the maps of the other hemispheres will help us learn more about our universe. In addition, the information obtained from this detector will give us new insights into the origin of gamma radiation, the theoretical radiation ejected from black holes, and even test the validity of string theory.