In the early 1900s, following Becquerel’s discovery of radioactivity, studying radiation was high priority for many physicists. Portable electrometers were used to measure the ionization of air in a variety of terrestrial environments, from fields and lakes to caves and mountains. With the idea that ionization should decrease with altitude, balloon flights were undertaken to count the number of ions per cm3 of air in function of altitude. First results indeed indicated a decrease up to 1300 m, but a subsequent ascentin 1909 to 4500 m by Albert Gockel, professor of physics at University of Fribourg, concluded that ionization does not decrease and possibly increases with altitude. Gockel, however, who later would coin the term “cosmic radiation”, was unable to obtain the hydrogen needed to reach higher altitudes. And so it fell to Austrian physicist Victor Hess to settle the case.

At the 42nd international balloon festival in Château-d’Oex, 25 January to 2 February, 2020, one balloon flight was special. Manned by two students from Fribourg, the colloquium speaker, and a prize-winning pilot, who won the international Gordon-Bennett cup 2019, a balloon ascended to 4000 m altitude, commemorating Albert Gockel’s pioneering flight measuring cosmic rays.
Using 21st century technology, the Cosmic Hunter from CAEN, a muon telescope counting coincidences in two scintillating-fibre tiles of 15 × 15 cm2, separated by 15 cm, verified that the flux of cosmic rays increases with altitude.

This colloquium tells the story of how this commemorative flight became possible, recalls the early days of cosmic-ray research and shows how studying these has contributed and still is contributing to particle physics, to astrophysics and to cosmology.