Cosmic Rays

The Earth is immersed in a "sea" of high-energy nuclei known as cosmic rays. In the late 1800's people realized that there was an ionizing radiation present in the our surroundings.  But it was not until Victor Hess performed a series of high-altitude balloon flights beginning in 1905, that we learned that the origin of this radiation was beyond the Earth's atmosphere. Hess won the Nobel prize for this discovery in 1936. At present we know that cosmic rays are composed of all nuclei, from the simple hydrogen nucleus (a proton) to the iron nucleus and beyond (transuranic elements have been observed in cosmic rays). The energy spectrum of cosmic rays has been measured up to 10²¹ eV (electron-volts) and is shown below. A well hit tennis ball has roughly the same energy as the highest energy cosmic rays, but here it is packed into a single atomic nucleus.

Extensive Air Showers

When a high-energy cosmic ray enters the atmosphere it loses its energy via interactions with the nuclei that make up the air. At high energies these interactions create particles. These new particles go on to create more particles, etc. This multiplication process is known as a particle cascade. This process continues until the average energy per particle drops below about 80 MeV (million electron-volts). At this point the interactions lead to the absorption of particles and the cascade begins to die. This altitude is known as shower maximum. The particle cascade looks like a pancake of relativistic particles traveling through the atmosphere at the speed of light. Though the number of particles in the pancake may be decreasing, the size of the pancake always grows as the interactions cause the particles to diffuse away from each other. When the pancake reaches the ground it is roughly 100 meters across and 1-2 meters thick. If the primary cosmic ray was a photon the pancake will contain electrons, positrons, and gamma rays. If the primary cosmic ray was a nucleus the pancake will also contain muons, neutrinos, and hadrons (protons, neutrons, and pions). The number of particles left in the pancake depends upon the energy of the primary cosmic ray, the observation altitude, and fluctuations in the development of the shower. This particle pancake is known as an extensive air shower (or simply an air shower).