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Viktor Franz Hess—Discoverer of Cosmic Radiation

      In 1911, Dr. Viktor Hess made 10 difficult balloon ascensions (to an altitude of up to 6 miles) to collect data on radiation. At that time, it was thought that small quantities of radioactive material were present everywhere—in the soil and in the air—and that this material produced background radiation. The balloon experiments were expected to show that radiation would be less at high altitudes because of the distance from the radioactivity in the soil. Much to everyone's surprise, Hess reported that although the level of radiation decreased to a height of approximately 150 meters above sea level, it was considerably increased at 5,000 meters. Radiation levels were the same at night as in the daytime, he noted, and consequently could not be the result of the direct rays of the sun. Hess concluded that the radiation at high altitudes had entered the atmosphere and must be of cosmic origin. “Cosmic rays” were so named by Robert A. Millikan, American physicist (1868–1953), in 1925.
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