Air Shower Schematic: A schematic diagram of particles in an extensive air shower (EAS), approaching an array of detectors at the speed of light.
Credit: Courtesy George Clark
At the time it was known that primary cosmic rays, which arrive from outer space, consist of the bare nuclei of atoms, roughly 90 percent hydrogen, 9 percent helium, and 1 percent heavier elements, accelerated to high energies by electric force in processes that are still not well understood. It was also known that their distribution in energy extends to at least 1017 eV at which energy they are very rare – a few per acre per year. (1 eV is the energy gained by an electron accelerated by a potential difference of one volt.) But the location of their origins was a mystery because observations had so far revealed no preferred arrival directions. The interstellar magnetic field, though very weak compared to the geomagnetic field that turns a compass, is strong enough to twist the trajectories of even very energetic cosmic rays, thereby destroying evidence of where they come from. However, the amount of twist decreases with energy, so it seemed that better measurement at higher energies might yield significant clues.
When a very energetic cosmic ray hits an air atom near the top of the atmosphere, it initiates a spreading cascade of secondary particles that travel with nearly the limiting speed of light and arrive at ground level as an "extensive air shower (EAS)." Since they all travel at close to the speed limit, the shower particles (mostly electrons, photons, and muons) move together in a more or less flat and thin disk hundreds of meters in diameter and perpendicular to the arrival direction of the primary particle. Thus the arrival of such a rare cosmic ray can be detected over a wide area on the ground.
Rossi suggested that if we put several large-area scintillation detectors in an array spread over an area tens or even hundreds of meters in diameter, and measured at each detector the relative arrival times and densities of particles in an EAS, then we could figure out its arrival direction, where its core struck, and its size. From the size we could estimate the energy of the primary particle.