Radiation and Altitude

The discovery of radioactive rocks was unexpected but when physicists attempted to discover how far the new “penetrating rays” could reach, they were in for an even bigger surprise.

Henri Becquerel’s discovery that uranium salts emit penetrating radiation was one of science’s great “accidents”. Uranium was first identified in 1789 but it was a century later when professor Becquerel applied his expertise in luminous substances to suggest that ordinary sunlight could energise certain materials, such as uranium salts. According to a review written by Lawrence Badash (published in Isis, Vol. 57, No. 2, 1966, available at https://www.jstor.org/stable/227967) Becquerel reported his first findings to the French Academie des Sciences on 24th February 1896, noting that phosphorescent crystals of potassium uranium sulphate “emit rays which can penetrate thick black paper and expose a photographic plate wrapped therein”.

To obtain meaningful pictures, Becquerel was using the silhouettes of metal objects and he intended to continue his investigations the following week but the weather was against him. For reasons that he did not record, but possibly on account of the emulsion’s lack of stability and a desire to install a fresh plate in his equipment, Becquerel decided to develop the “unused” plate on 1st March 1896, expecting to find only the weakest image. “To the contrary,” he wrote, “the silhouettes appeared with great intensity.” He concluded that the penetrating rays responsible for this effect must originate from within the rock itself without any need for activation by sunlight and in recognition of this discovery Henri Becquerel was awarded the Nobel Prize for Physics in 1903.

The next question was to ask how far these penetrating rays could travel. The answer was found using two technological advances that were achieved within three years of each other more than a century earlier. In 1783 the Montgolfier brothers demonstrated their first hot-air balloon and in 1786 Abraham Bennet invented a new instrument that could detect and measure electric charge, the gold-leaf electroscope.

It was known that gold-leaf electroscopes suffered from self-discharge and this was initially thought to be due to imperfect designs but the apparent “flaw” was subsequently considered as a possible effect of Becquerel’s penetrating rays. If that were the case then a suitably refined gold-leaf electroscope might provide numerical measurements of radiation intensity.

Father Theodore Wulf, a German scientist and Jesuit priest had the idea that if radioactivity originates in rocks then it should decrease with altitude. In 1909, he took his own version of the electroscope to the top of the Eiffel tower (about 300 m above the ground) and found the intensity of radiation did indeed show a slight decrease but not sufficient to provide definite confirmation. According to a post on the CERN website (https://timeline.web.cern.ch/theodor-wulf-new-electrometer-and-eiffel-tower) Wulf’s inconclusive results were due radioactive constituents in the metal of the Eiffel tower’s structure.

Hot-air balloons entered the story in December 1909, when Alfred Gockel ascended to 4500 m above sea level courtesy of the Swiss aeroclub. He found that his electroscope’s rate of self-discharge did not decrease with height as it should if radioactivity was due to the Earth itself. Three years later, the Austrian physicist Viktor Hess undertook a systematic study with seven balloon flights carrying Wulf-style electroscopes at altitudes up to 5350 m. He found a reduction in the self-discharge rate for altitudes up to 1000 m followed by an increase in self-discharge rates beyond 3000 m. This was the first evidence of radioactivity coming from above the Earth. Viktor Hess had discovered cosmic rays and was duly awarded the 1936 Nobel Prize for Physics (which he shared with Carl Anderson, discoverer of the positron).