The Detection of Cosmic Rays

Cherenkov Radiation

As the extensive air shower traverses the earth's atmosphere the relativistic charged particles in the pancake emit Cherenkov light. Cherenkov light is the electromagnetic equivalent of a sonic boom. It occurs when a charged particle travels through a medium faster than light can travel through that medium. Remember, although nothing can travel faster than light in a vacuum, light slows down when it enters a medium. There is no such limitation on particles. The speed of light in a medium is just the speed of light in a vacuum divided by the index of refraction of the medium.

Detecting an Extensive Air Shower

This leads to two different methods that can be used to detect the passage of an extensive air shower: one can look for the particles in the pancake directly, or one can look for the Cherenkov light generated by the particles in the atmosphere. The figure below illustrates both techniques.

On the left is an air Cherenkov telescope (ACT).
These are large mirrors that focus the Cherenkov light generated by the air shower onto an array of PMTs, which form an image of the air shower. Properties of the image are used to distinguish between air showers generated by gamma-ray primaries and nuclear primaries. Though very few particles may survive to the ground, the Cherenkov light will reach the ground. Thus, air Cherenkov telescopes can detect lower energy cosmic rays than extensive air shower arrays. However, since they are optical instruments they can only operate on clear moonless nights and they can only view a small piece of the sky at a time.

On the right is an extensive air shower array (EAS array).
An EAS array has traditionally been composed of a sparse array of plastic scintillators. The scintillators detect the passage of charged particles that travel through them. They are very inefficient detectors of the gamma rays in the EAS. Since gamma rays outnumber electrons and positrons by a ratio of roughly 4:1 and the scintillator covers less then 1% of the total area of the array, traditional EAS arrays have rather high energy thresholds. Unlike ACTs EAS arrays can operate under all conditions, night or day, and can view the entire overhead sky continuously. By using buried counters they can detect the muons in air showers generated by cosmic-ray nuclei. However, this method of distinguishing between gamma rays and nuclear cosmic rays is not as efficient as the imaging method used by ACTs. The table below indicates the advantages and disadvantages of each type of instrument.

Another way