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Cosmic Ray Observations in Eritrea

Research notes of Bruno Rossi, 1933


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Research on Cosmic Ray Physics was still in its early days when Bruno B. Rossi (1905-1993), an Italian physicist, first read a paper by Bothe and Köhlhorster (1929) indicating that the so-called radiation from above (Höhenstrahlung), discovered by Victor Hess in 1912, was most likely caused by high-energy particles. Intrigued by those results, Rossi improved on Bothe’s experimental technique by inventing the triode multiple-coincidence circuit, which increased the time resolution of the measurements by up to 100 times. With this technique, he was soon able to provide further evidence for the corpuscular nature of cosmic rays.

Already in 1927 some observational evidence suggested that the intensity of cosmic rays was affected by latitude, indicating that cosmic rays may consist of charged particles. Building upon the theory developed by Carl Störmer, Rossi predicted that there should be an asymmetry in a spatial distribution of cosmic rays, dependent on the sign of their electrical charge. This effect, most pronounced close to the geomagnetic equator, came to be known as the East-West effect. Observations conducted in the fall of 1930 near the city of Florence, where he was an assistant at the university, were inconclusive. Determined to prove the presence of the effect, Rossi began planning an expedition to Eritrea, an Italian colony in Africa. However, because he was called to build a new Institute of Physics in Padua in 1932, the expedition was delayed until the fall of 1933. The lower geomagnetic latitude and higher elevation of the selected expedition site, in the vicinity of Asmara, provided better conditions for successful measurements. The resulting observations by Rossi and his collaborators, Benedetti and Ranzi, confirmed the presence of the East-West effect, corroborating results obtained several months earlier in two separate experiments in Mexico (one conducted by Johnson, the other by Alvarez and Compton). Rossi’s measurements proved that most of the primary cosmic rays consisted of positively charged particles. During the same expedition, while testing instrumentation, Rossi and his collaborators discovered the occurrence of extensive showers of particles produced by interactions of cosmic rays in the atmosphere, a phenomenon subsequently studied by Pierre Auger, whose name became associated with its discovery. The drawing and data reproduced here are taken from one of several research notebooks documenting Rossi’s work in Eritrea in October and November 1933.

Bruno Rossi was dismissed from his post as the director of the Physics Institute at the University of Padua in September 1938 because of his Jewish origins. Soon thereafter he and his young wife, Nora Lombroso, embarked on a journey that led them via Copenhagen, Manchester, Chicago, and Ithaca, N.Y., to Los Alamos, N.M., where he joined the team of scientists working on the atomic bomb. Rossi, together with Hans Staub, led the group for development of detectors and electronics. After the war, Rossi organized the Cosmic Ray Group in the Laboratory for Nuclear Science at MIT. In the 1960s he became a driving force behind the development of the new fields of physical research, space physics and X-ray Astronomy.

The research notebooks, correspondence, photographs, and other materials documenting the scientific life of Bruno Rossi can be found in the papers of Bruno B. Rossi (MC166) at the MIT Institute Archives and Special Collections. The collection is available for use in the Archives reading room in building 14N-118.


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