GUANO

Grenfell

Upper

Atmosphere

Near space

Observatory

Grenfell Campus, Memorial University of Newfoundland,
Corner Brook, Newfoundland, Canada

 
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Physics@Grenfell

What are Cosmic Rays?

Earth is constantly pelted by cosmic rays. Described as “a thin rain of charged particles”, cosmic rays range from low-energy particles originating in the Sun to extremely high energy particles that originate outside our own Milky Way galaxy in stellar explosions known as supernovae. Some of these particles can have far higher energies than we can ever hope to duplicate in accelerators on Earth, and their study may shed light on the fundamental structure of matter, and on the structure and origin of the Universe.

When a cosmic ray enters Earth’s atmosphere, it eventually collides with atoms in the stratosphere. This collision begins a chain reaction in which the broken bits of the atom move on to break apart other atoms, and so on. The result is a shower of secondary particles, similar to that shown in the figure at right. Most of these particles have low energy and they decay or are absorbed in the air before they reach the surface. The only particles that arrive at the ground are either very energetic or relatively stable. One such particle is the muon, which is a high-energy, heavier version of the electron. At sea level about six hundred particles from cosmic ray air showers bombard your body every minute.

The extraterrestrial nature of cosmic rays was first discovered by Victor Hess, who starting in 1911 made a series of balloon flights and noticed that the intensity of ionizing radiation at a height of 5 km was significantly greater than at sea level. This meant the radiation was entering the atmosphere from outer space. The intensity of particles increases with height because the lower atmosphere filters out the lower energy secondary particles. At an altitude of about 25 km the flux begins to decrease as fewer of the unfiltered secondary and more of the primary particles begin to be detected.