December 11, 2008, South Pole Station -- Tom Gaisser
What IceTop Sees
As James Roth described in his blog two days ago, IceTop consists of an array of tanks spread out on the surface above the deep neutrino telescope of IceCube. Each tank has two standard IceCube digital optical modules (DOMs) embedded in the ice as shown in the picture by James Roth of the first tank we filled this year, which is already freezing.
The DOMs in our tanks are an integral part of the IceCube system of detectors. When a charged particle enters the ice in our tank, it makes a conical flash of Cherenkov light that is reflected from the white inner walls of the tank to illuminate the DOMs in the tank. The computer in each DOM records the time and digitizes the pulse of light. If several particles hit the tank at once, the overall signal is proportionately bigger.
Serap Tilav made the nice diagram below that shows how we detect charged particles.
Figure 1. Cherenkov photons emitted by a through-going muon are reflected by the liner. After several reflections, they may either get absorbed by the tank ice or be detected by the DOMs.
Where do the particles come from? The Earth’s atmosphere is continually bombarded by a rain of energetic particles from space. They are ionized nuclei, mostly hydrogen (protons) and helium (alpha particles), but also an important fraction of heavy nuclei like iron and others. When the primary particles enter the atmosphere, they interact to produce secondary particles from the zoo of fundamental particles. The black arrow in the diagram indicates the primary cosmic-ray particle. The particles are of three types: hadrons, muons, and electromagnetic quanta (photons and electrons).
Hadrons are strongly interacting sub-atomic particles, the most familiar of which are protons and neutrons. But this class also includes a group called mesons, mainly pions and kaons. If they have sufficiently high energy, the protons and neutrons interact again and produce more secondaries. The mesons may interact and also produce more secondary particles or they may decay. When charged pions and kaons decay they produce muons, which are penetrating charged particles related to electrons but with much higher mass. Neutral pions decay to two gamma-rays. These energetic gamma-ray photons quickly generate a multiplicative electromagnetic cascade of positive and negative electrons and photons. The result is an air shower consisting of a hadronic core with an electromagnetic halo of electrons and photons and a smaller number of muons. In the diagram, red lines indicate the electromagnetic component and blue lines the muons.
Our tanks are about 6 feet in diameter, which corresponds to a cross-sectional area of about 2.7 square meters. Low-energy primaries make small showers in which only one or two particles reach the ground. If the primaries have sufficient energy, however, the whole cascade reaches the surface in a disk about a meter thick with all the particles traveling at nearly the speed of light.
Our array consists of pairs of tanks separated by 125 meters on average. In order to hit several detectors, the primary cosmic particle has to have a kinetic energy equivalent to about one million times the equivalent rest mass of a single proton. When such a shower crosses the detector the DOMs fire in a sequence of times from which the direction of the incident particle can be determined. If the event is vertical, all the particles hit the ground at the nearly the same time (although there is a slight conical shape to the shower front). But a shower generated by an inclined particle will hit detectors on the near side first and sweep across the array. The energy of the event is related to the amount of light produced in the tanks by the particles in the shower. So the two main quantities we measure are the energy and direction.
High-energy muons in showers can penetrate deep into the ice. Thus events with trajectories that pass through both IceTop and the deep detectors of IceCube are seen by both components of IceCube. The picture here shows the reconstruction of such a coincident event. Its energy is about one billion times the equivalent of the rest mass of the proton. It probably contains about 2000 muons that penetrate more than a mile into the ice. Such events are rare but interesting! Color indicates time, with red early and blue/green late. It takes about 6 microseconds for particles traveling at the speed of light to get from IceTop to the deep part of IceCube.
