Simulating Light Production in IceTop

Don't judge a book by its cover: "cosmic rays" are not rays at all. The phrase is actually kind of a catch-all term for high-energy charged particles striking our atmosphere. They're mostly protons, but can be heavier nuclei as well.

Cosmic rays were discovered in 1912 by Victor Hess. He was trying to test the theory that most of the Earth's radioactivity came from the ground, so he took three detectors up to 5300 meters in a hot air balloon, expecting a decrease in the ionization rate. Instead, he found an increase, indicating the primary source of radiation was coming not from below, but from above. Hess thought that he was observing high energy photons, hence the name "cosmic rays".

When a cosmic ray strikes a molecule in the atmosphere, it produces a cascade of lighter particles. Initial collisions produce exotic particles like pions, but by the time the shower has reached the ground, these particles have decayed, and we're generally left with several common byproducts -- muons, electrons, neutrons, neutrinos, and gamma rays.

Since cosmic rays interact in our atmosphere, it seems that all of our cosmic ray detectors should be in balloons so we can detect them before they create particle showers. So why build detectors on the ground? Statistics: as the energy of the incoming cosmic rays increases, their frequency decreases. There are two important points of study on the spectrum where the slope of flux as a function of energy changes -- the "knee" and the "ankle". These points of interest have fluxes of roughly 1 particle per square meter per year and 1 particle per square kilometer per year, respectively.

Detecting one particle per year obviously is not going to provide enough data, so we need to take an alternate approach. To maximize the flux detected, we use very large ground detectors to indirectly detect cosmic rays that interact in the atmosphere. Instead of detecting the incoming particle itself, cosmic ray detectors look for the byproducts produced in a particle shower and use them to infer information about the original. IceTop is one such detector.

Cosmic Ray Spectrum

IceCube was not built as a cosmic ray detector. It was designed to detect neutrinos, and, when finished, will have 4200 detectors buried deep in the ice (anywhere from 1450 to 2450 meters) dedicated to this task. These detectors (Digital Optical Modules, or DOMs) use photomultiplier tubes to detect light produced by collisions between neutrinos and particles in the ice. As you can see in the picture to the right, the DOMs are located on strings arranged over a roughly hexagonal area. When IceCube is finished, this area will be about 1 square kilometer.

What you can't see is that at the top of every string are two tanks built to detect cosmic rays. These tanks combined form IceTop, a partner project with IceCube. IceTop takes advantage of IceCube's large surface area and grid-like pattern to detect cosmic ray showers. IceTop's arrangement allows for the observation of cosmic rays with energies ranging from 10^14 to 10^17 eV, which is right around the "knee".