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  • cosmic rays

One hundred years ago, using balloon flights up to 5 kilometers altitude, Victor Hess demonstrated that the intensity of penetrating ionizing radiation increased with altitude, indicating that Earth is exposed to high-energy radiation from space [Hess, 1912]. Since that observation, these “cosmic rays” have enabled discoveries basic to elementary particle physics and astrophysics. This discovery earned Hess the 1936 Nobel Prize in Physics, shared with Carl Anderson, who discovered the positron among the secondary cosmic rays near the ground [Anderson, 1933]. Then, the only known ionizing radiation with range in air more than about 30 centimeters was the g ray (electromagnetic radiation with energy above about 100 kiloelectron volts), so the radiation from space was assumed to be g rays and was called “cosmic rays.” That name has stuck, although the “cosmic rays” studied today are not actually rays but particles. Indeed, g rays do impinge on Earth, and g ray astronomy is a burgeoning area of astrophysics, but the term “cosmic rays” continues to apply to the charged particles that make up the bulk of the incident ionizing radiation.