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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