how cosmic rays were discovered on a balloon ride

The Mechanism

*Victor Franz Hess* (born Schloss Waldstein, Styria, *24 June 1883*; died Mount Vernon, New York, *17 December 1964*, aged 81) was, in 1912, a *29-year-old assistant* at the *Institute for Radium Research* of the Imperial Academy of Sciences in Vienna. The problem that had been preoccupying European atmospheric physics since the late 1890s was the *air-ionisation rate*: gold-leaf electroscopes left to themselves at room temperature consistently lost charge at a slow but reproducible rate, suggesting that the air around them contained ions produced by some source of *penetrating ionising radiation*. The natural hypothesis — championed by *Julius Elster* and *Hans Geitel* in Wolfenbüttel, and later by *Charles Thomson Rees Wilson* at the Cavendish — was that the radiation was *terrestrial*: a fraction of the natural radioactivity (gamma rays from uranium, thorium, and radium in soils, rocks, and the granite walls of laboratory buildings) penetrating the air. If that hypothesis were correct, then the ionisation rate should *decrease with altitude*, since climbing higher would put more air, and more distance, between the electroscope and the radioactive rocks below. In *1909-10*, *Theodor Wulf* (a Jesuit physicist) had climbed the *Eiffel Tower* in Paris with a portable electrometer of his own design and measured ionisation rates at 300 metres — and found *less* of a decrease than the terrestrial hypothesis predicted, but the result was inconclusive. In *1910*, *Domenico Pacini* of the *Italian Office of Meteorology* had lowered his electrometer to a depth of three metres *underwater* in the Bay of Genoa and observed a *decrease* in ionisation rate, suggesting that the radiation was *not* coming from below — but again the result was insufficient to overturn the consensus. Hess proposed a different test: take an electrometer in a *manned balloon* up through *several kilometres* of atmosphere and measure the ionisation rate at each altitude directly. Through the spring and summer of 1911 and 1912 he made *seven* manned balloon ascents, one of them during the *total solar eclipse of 17 April 1912* (to test whether the radiation might be coming from the sun — it was not). The decisive ascent was the *sixth flight, 7 August 1912*. Hess took off at *0612* from a field outside the town of *Aussig* (modern *Ústí nad Labem*, in northern Bohemia, then Austria-Hungary, now the Czech Republic) in a hydrogen-filled balloon named *Böhmen*, supplied by the *German Aeronautical Association*. His two companions were the meteorologist *Captain W. Hoffory* (navigation) and the technician *Paul Wolf* (altitude and temperature observations). The basket carried *three* improved gold-leaf electrometers — two of Wulf's design and one of Hess's own — wrapped in *2 mm of zinc shielding* to suppress contributions from the radium content of the balloon fabric itself, and pre-calibrated against radium-bromide standards on the ground. The balloon rose for *two and a half hours*, drifting north-north-west across Saxony at a peak altitude of *5,350 metres* (about 17,500 ft), then descended slowly and landed at *0815*, six hours after takeoff, in a field near the village of *Pieskow*, in Brandenburg, *about 50 km east of Berlin*. The data Hess read off in flight, between altitude readings: the ionisation rate, *constant* up to about *1,000 m*, then *steadily increasing* to *4,000 m*, then *increasing more rapidly* above 4,000 m, until at peak altitude the rate was *approximately four times* the sea-level value. The result was the *opposite* of every plausible terrestrial-radiation prediction. Hess concluded that *"a radiation of very high penetrating power enters our atmosphere from above"* — and that, because the ionisation rate did not vary with day or night, or with the direction of the sun, the radiation could not be solar in origin and must come from *outside the solar system*. He published the result in *Physikalische Zeitschrift*, *vol 13, pages 1084-1091*, on *24 November 1912*, under the title *"Über Beobachtungen der durchdringenden Strahlung bei sieben Freiballonfahrten"* — *"On observations of the penetrating radiation in seven free balloon ascents."* The conclusion was confirmed independently by *Werner Kolhörster* of Berlin, who made parallel balloon ascents in 1913-14 to altitudes of *9,300 metres* and measured an ionisation rate *ten times* the sea-level value at peak height. The radiation was renamed *Höhenstrahlung* ("high-altitude radiation") in the German literature and *cosmic rays* (the term coined by *Robert Millikan* in 1925) in English. Hess shared the *1936 Nobel Prize in Physics* with *Carl D. Anderson* — Anderson for the *positron* discovery, made in 1932 in a cosmic-ray cloud-chamber experiment that depended entirely on Hess's earlier finding. Cosmic rays are now understood to be predominantly *high-energy protons and atomic nuclei* originating from *supernova remnants* and *active galactic nuclei*, with energies ranging from a few GeV up to *3 × 10²⁰ eV* (the highest-energy single particles ever observed — the *Oh-My-God particle* detected by the Fly's Eye experiment in Utah in 1991, carrying the kinetic energy of a fast-pitched baseball in a single subatomic particle). The atmospheric *secondary showers* produced by primary cosmic-ray collisions — first observed photographically by *Pierre Auger* in 1938 and now mapped continuously by ground-based observatories such as the Pierre Auger Observatory in Argentina — are the descendants of the radiation Hess detected in 1912. Hess, who was Jewish on his wife's side, was dismissed from his chair at the University of Innsbruck in 1938 by the Nazi authorities; he emigrated to the United States in September 1938 and took a chair at Fordham University in New York, where he taught until his retirement in 1956.