Why are primary cosmic rays not directly detected at Earth’s surface?

Primary cosmic rays are not directly detected at Earth’s surface because they collide with atmospheric nuclei upon entering the upper atmosphere. These collisions trigger a cascade of interactions that transform the original primary particles into "secondaries"—a collection of fragments and short-lived particles, such as muons and neutrons, which are what researchers observe at sea level.1

For very high-energy primaries, these collisions produce extensive air showers (EASs) that can spread across large areas, allowing scientists to infer the presence of the original particle by detecting the resulting shower of secondary particles at ground level. Because of this atmospheric transformation, direct study of primary cosmic rays requires instruments placed above the atmosphere, typically using high-altitude balloons, rockets, or satellites.213