Photograph from Occhialini and Blacket’s paper showing tracks of radiation (Image: Blackett, P.M.S., & Occhialini, G.P.S., Royal Society of London Proceedings Series A 139 (1933) 699)

In 1932 Carl Anderson, a young professor at the California Institute of Technology in the US, was studying showers of cosmic particles in a cloud chamber and saw a track left by "something positively charged, and with the same mass as an electron". After nearly a year of effort and observation, he decided the tracks were actually antielectrons, each produced alongside an electron from the impact of cosmic rays in the cloud chamber. He called the antielectron a "positron", for its positive charge and published his results in the journal Science, in a paper entitled The apparent existence of easily deflectable positives (1932).

The discovery was confirmed soon after by Occhialini and Blacket, who in 1934 published Some photographs of the tracks of penetrating radiation in the journal Proceedings of the Royal Society A. Anderson's observations proved the existence of the antiparticles predicted by Dirac. For discovering the positron, Anderson shared the 1936 Nobel prize in physics with Victor Hess.

For years to come, cosmic rays remained the only source of high-energy particles. The next antiparticle physicists were looking for was the antiproton. Much heavier than the positron, the antiproton is the antipartner of the proton. It would not be confirmed experimentally for another 22 years.

Bothe and Kolhorster’s experiment (Image: L. Bonolis, American Journal of Physics, 79 (2011), 1133. Reproduced under Creative Commons license)

In 1929 Hans Geiger and Walter Müller developed a gas filled ionization detector – a tube that registers individual charged particles. This Geiger-Müller counter was ideal for studying high-energy cosmic rays. Two such tubes placed one above the other could register 'coincidences' – when an incoming particles passes through both tubes – and thus define the path of a cosmic ray. Walther Bothe and Werner Kolhörster connected two Geiger counters to electrometers and immediately observed these ‘coincidences’.

A gamma ray only fires a Geiger counter if it knocks an electron out of an atom. The observation of coincident signals suggests that a cosmic gamma ray had either produced two electrons or that a single electron had fired both counters. To test if it was an electron that had set off both counters Bothe and Kolhörster put gold 4 cm thick between the counters to absorb the electrons knocked off from the atoms. They found that the rays were not affected and concluded that cosmic rays consisted of electrically charged particles and not gamma rays. Interposing a 4 cm thick gold piece between the tubes only slightly reduced the coincidence rate proving that cosmic rays contain charged particles of much higher energy than the Crompton electrons that would be produced by gamma rays. 

Robert Millikan originally set about to disprove Hess and Kolhörster’s discovery. He and Ira Sprague reached a height of 1500 m in a balloon over Texas where they recorded a radiation intensity of approximately one quarter of Hess and Kohörster's measurement. The difference was caused by a geomagnetic difference between Texas and Central Europe but was blamed on turnover in the intensity curve at high altitude.

Millikan and Harvey Cameron reported on experiments on high-altitude lakes in 1926. They measured ionization rates at various depths in lakes at altitudes of 1500 m and 3600 m. The underwater rate of the lower lake corresponded to the rate obtained 2 m deeper in the higher lake. The pair concluded that particles shoot through space equally in all directions. This demonstrated that two metres of water absorbed about the same as two kilometers of air, and convinced Millikan that rays do come from above.

Millikan was convinced that penetrating radiation entering the atmosphere was electromagnetic and coined the term ‘cosmic rays’ in a paper where he argued that cosmic rays were the ‘birth cries of atoms’ in the galaxy.

Read more: "The Origin of the Cosmic Rays" – R.A. Millikan, G.H. Cameron, Physical Review Letters, 32 (1928) 533

Increase in ionization with height measured by Hess and Kolhörster (Image: Wikimedia Commons)

German physicist Werner Kolhörster took balloon measurements up to a height of 9300 m, confirming Hess’s results for greater heights. His results confirmed unambiguously that an unknown radiation with an extreme penetrating power was causing ionization. The intensity of the radiation was relatively constant, with no day-night or weather-dependent variations.

In 1913 Kolhörster made three balloon flights, reaching 6200 m on the third flight. In 1914 he reached an altitude of 9300 m where he found the ionization was nine times the value on the ground. Kolhörster’s final flight on 28 June 1914 was the same day as the assassination of Franz Ferdinand and the beginning of the First World War. Research on cosmic rays ceased during the war as scientists became involved in other duties and only resumed in the early 1920’s. 

Hess back from one of his balloon flights in 1912 (Image: Wikimedia Commons)

In 1911 and 1912 Austrian physicist Victor Hess made a series of ascents in a balloon to take measurements of radiation in the atmosphere. He was looking for the source of an ionizing radiation that registered on an electroscope – the prevailing theory was that the radiation came from the rocks of the Earth. In 1911 his balloon reached an altitude of around 1100 metres, but Hess found "no essential change" in the amount of radiation compared with ground level. Then, on 17 April 1912, Hess made an ascent to 5300 metres during a near-total eclipse of the Sun. Since ionization of the atmosphere did not decrease during the eclipse, he reasoned that the source of the radiation could not be the Sun – it had to be coming from further out in space. High in the atmosphere, Hess had discovered a natural source of high-energy particles: cosmic rays.

Hess shared the 1936 Nobel prize in physics for his discovery, and cosmic rays have proved useful in physics experiments – including several at CERN – since.

Read more: "A discovery of cosmic proportions" – CERN Courier

To measure ionizing radiation away from the earth’s surface, several researchers took to the air in balloon flights in the first decade of the 20^th century. One of these pioneers, Albert Gockel, measured the levels of ionizing radiation up to a height of 3000 metres. He concluded that the ionization did not decrease with height and consequently could not have a purely terrestrial origin. He also introduced the term “kosmische Strahlung” – cosmic radiation.

Later calculations by Schrödinger showed that the radioactivity came in part from above and in part from the Earth’s crust and that the decrease in the radioactivity from the Earth’s crust could be offset by the growth of radioactivity from extraterrestrial sources up to 3000 m. 

Cloud formed on ions due to α-Rays (Image: CTR Wilson Roy, Proceedings of the Royal Society A, Volume 85, Plate 9)

The cloud chamber was fundamental in the history of particle physics and cosmic rays. This device made it possible to record individual charged particles in the secondary particle showers that are initiated when cosmic rays strike particles in the upper atmosphere. Wilson won the 1927 Nobel Prize for his development of the cloud chamber, which he originally undertook to study atmospheric phenomena. In April 1911 he presented his first rough photographs of particle tracks at the Royal Society in London.

A cloud chamber is a box containing a supersaturated vapor. As charged particles pass through, they ionize the vapor, which condenses to form droplets on the ions. The tracks of the particles become visible as trails of droplets, which can be photographed. During the first half of the 20th century, experiments that looked at cosmic rays passing through cloud chambers revealed the existence of several fundamental particles, including the positron, the muon and the first strange particles.

Domenico Pacini making a measurement on 20 October 1910 (Image: Wikimedia Commons)

In 1911, Italian physicist Domenico Pacini took readings on a Wulf-style electroscope in various locations and noted a 30% reduction in radioactivity between ionization levels on a ship 300 m off shore from Livorno compared to measurements on land. This result suggested that a significant portion of the penetrating radiation must be independent of emission from the Earth’s crust. He published his paper Penetrating radiation at sea on the 2 April 1911.

Pacini also measured the levels of radiation in the deep sea of the Genova gulf. This experiment pioneered the technique of underwater measurement of radiation. He noted that there was 20% less radiation 3 metres below the water compared to on the surface, concluding that the ionizing radiation must come from the atmosphere.

Read more: "Domenico Pacini and the origin of cosmic rays" – CERN Courier

The original Wulf electroscope (Image: Wikimedia Commons)

In 1909 Theodor Wulf, a Jesuit priest, designed and built a more sensitive and more transportable electrometer than the gold leaf electroscopes. He measured the ionization of the air in various locations in Germany, Holland and Belgium, concluding that his results were consistent with the hypothesis that the penetrating radiation was caused by radioactive substances in the upper layers of the Earth’s crust.

Wulf then started measuring changes in radioactivity with height to understand the origin of the radiation. The hypothesis was simple: if the radioactivity was coming from the Earth, it should decrease with height.

Wulf took his electroscope to the top of the Eiffel tower in 1909 and found that the intensity of radiation “decreases at nearly 300 m [altitude to] not even to half of its ground value”. This was too small a decrease to confirm his hypothesis.

However, unknown to Wulf, his results were due to the radioactive metal of the Eiffel tower. The search for the source of the mysterious ionizing radiation would continue. 

In studying electrical conduction through air in 1899, Julius Elster and Hans Geitel designed a key experiment where they found that surrounding a gold leaf electroscope with a thick metal box would decrease its spontaneous discharge. From this observation, they concluded that the discharge was due to highly penetrating ionizing agents outside of the container. In a similar experiment at about the same time, Charles Thomson Rees Wilson in Cambridge came to the same conclusion.

To test whether the ionizing radiation originated beyond the atmosphere, in 1901 Charles Thomson Rees Wilson took measurements of natural radioactivity using an electroscope inside an old railway tunnel in Scotland. If the radiation were coming from outer space, Wilson could have expected to measure a signification reduction in the tunnel compared to outside on the surface. But he saw no such reduction. Following Wilson’s observations, the scientific community largely dismissed the extra-terrestrial theory.

Since some of the radiation was found to be too penetrating and perhaps too abundant to originate from known sources, altitude-dependent studies were carried out to test the idea of an extraterrestrial source – although at first the results were contradictory.