Introduction to Cosmic Rays
 Cosmic rays are caused by protons from outer space. When a proton (shown in yellow) hits the air in the earth's upper atmosphere it produces many particles. Most of these decay or are absorbed in the atmosphere. One type of particle, called muons (shown in red), lives long enough that some reach the earth's surface. |
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| The apparatus (shown at right) counts muons that pass through one of the three pairs of black-wrapped scintillation counters (sensors). The electronics panels record a count when both of the sensor panels in a pair register the passage of a muon. |
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A typical count of muons showed:
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| Fewer muons are counted from the horizontal direction than from the vertical. In the horizontal direction, the muons must travel further to reach the surface of our planet, so more of them decay. |
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