Bruno Rossi
Bruno Benedetto Rossi was an Italian-American experimental physicist. He made major contributions to particle physics and the study of cosmic rays. A 1927 graduate of the University of Bologna, he became interested in cosmic rays. To study them, he invented an improved electronic coincidence circuit, and travelled to Eritrea to conduct experiments that showed that cosmic ray intensity from the West was significantly larger than that from the East.
Forced to emigrate in October 1938 due to the Italian racial laws, Rossi moved to Denmark, where he worked with Niels Bohr. He then moved to Britain, where he worked with Patrick Blackett at the University of Manchester. Finally, he went to the United States, where he worked with Enrico Fermi at the University of Chicago, and later at Cornell University. Rossi stayed in the United States and became an American citizen.
During World War II, Rossi worked on radar at the MIT Radiation Laboratory, and he played a pivotal role in the Manhattan Project, heading the group at the Los Alamos Laboratory that carried out the RaLa Experiments. After the war, he was recruited by Jerrold Zacharias at MIT, where Rossi continued his pre-war research into cosmic rays.
In the 1960s, he pioneered X-ray astronomy and space plasma physics. His instrumentation on Explorer 10 detected the magnetopause, and he initiated the rocket experiments that discovered Scorpius X-1, the first extra-solar source of X-rays.
Italy
Rossi was born to a Jewish family in Venice, Italy. He was the eldest of three sons of Rino Rossi and Lina Minerbi. His father was an electrical engineer who participated in the electrification of Venice. Rossi was tutored at home until the age of fourteen, after which he attended the Ginnasio and the Liceo in Venice. After beginning his university studies at the University of Padua, he undertook advanced work at the University of Bologna, where he received a Laurea in Physics in 1927. His thesis advisor was Quirino Majorana, who was a well-known experimentalist and an uncle of the physicist Ettore Majorana.Florence
In 1928, Rossi began his career at the University of Florence, as assistant to Antonio Garbasso, who had founded the University's Physics Institute in 1920. It was located in Arcetri, on a hill overlooking the city. When Rossi arrived, Garbasso was Podestà of Florence, appointed by Benito Mussolini's fascist government of Italy. However, he brought to the Institute a group of brilliant physicists which included Enrico Fermi and Franco Rasetti before they moved to Rome, as well as, Enrico Persico, and Giulio Racah. In 1929, Rossi's first graduate student, Giuseppe Occhialini, was awarded the doctoral degree.In search of pioneering research, Rossi turned his attention to cosmic rays, which had been discovered by Victor Hess in manned balloon flights in 1911 and 1912. In 1929, Rossi read the paper of Walther Bothe and Werner Kolhörster, which described their discovery of charged cosmic ray particles that penetrated of gold. This was astonishing, for the most penetrating charged particles known at the time were electrons from radioactive decay, which could penetrate less than a millimetre of gold. In Rossi's words, it
Rossi coincidence circuit
In 1954, Bothe was awarded the Nobel Prize in Physics "for the coincidence method and his discoveries made therewith" for a method of assessing coincident events he implemented prior to 1924. However, his implementation of this method was very cumbersome, for it involved visual correlation of photographed pulses. Within a few weeks of reading his paper with Kolhörster, Rossi invented an improved electronic coincidence circuit, which made use of triode vacuum tubes. The Rossi coincidence circuit has two major advantages: it offers very precise temporal resolution and it can detect coincidences among any number of pulse sources. These features make it possible to identify interesting events that produce coincident pulses in several counters. These rare events stand out even in the presence of high rates of unrelated background pulses in the individual counters. The circuit not only provided the basis for electronic instrumentation in nuclear and particle physics, but also implemented the first electronic AND circuit, which is a fundamental element of the digital logic that is ubiquitous in modern electronics.At the time, an improved tubular version of the original Geiger counter, invented by Hans Geiger in 1908, had just been developed by his student Walther Müller. These Geiger–Müller tubes made possible Bothe's investigations. With Occhialini's help in the construction of GM tubes, and with the aid of a practical coincidence circuit, Rossi confirmed and extended the results of Bothe, who invited him to visit Berlin in the summer of 1930. Here, with financial support arranged by Garbasso, Rossi collaborated on further investigations of cosmic ray penetration. He also studied Carl Størmer's mathematical description of the trajectories of charged particles in the Earth's magnetic field. On the basis of these studies, he realised that the intensity of cosmic rays coming from eastward directions might be different from that of westward ones. From Berlin, he submitted the first paper suggesting that observations of this east–west effect could not only confirm that cosmic rays are charged particles, but also determine the sign of their charge.
Rome conference
In the autumn of 1931, Fermi and Orso Mario Corbino organised in Rome an international conference on nuclear physics, which was sponsored by the Royal Academy of Italy. Fermi invited Rossi to give an introductory talk on cosmic rays. In the audience were Robert Millikan and Arthur Compton, both of whom had won the Nobel prize in physics, in 1923 and 1927, respectively. During the 1920s, Millikan, who is famous for his oil drop experiment, made extensive measurements of the mysterious radiation discovered by Hess. He coined the name "cosmic rays" and proposed that they were photons created by the fusion of hydrogen in interstellar space. He was not pleased by the presentation of evidence that most observed cosmic rays are energetic charged particles. Later, Rossi wrote:Compton, who is famous for the Compton effect, had a more positive reaction, for he told Rossi later that the talk had motivated him to begin his own research on cosmic rays.
Rossi curve
Immediately after the Rome conference, Rossi carried out two experiments that led to a significant advance in the understanding of cosmic rays. Both involved triple coincidences of pulses from three Geiger counters, but in the first, the counters were aligned and separated by blocks of lead, while in the second, they were placed in a triangular configuration such that all three could not be traversed by a single particle travelling in a straight line. Results from the first configuration demonstrated the existence of cosmic-ray particles capable of penetrating of lead.With the second configuration enclosed in a lead box, the results showed that some cosmic rays interact in lead to produce multiple secondary particles. In an extension of the second experiment, he measured the rate of triple coincidences as a function of the amount of lead above the counters. A plot of this rate against thickness, which came to be known as the Rossi curve, showed a rapid rise as the lead layer was increased, followed by a slow decline. These experiments showed that ground-level cosmic rays consist of two components: a "soft" component, which is capable of prolific generation of multiple particle events, and a "hard" component, which is capable of traversing great thicknesses of lead. At the time, the physical nature of both was a mystery, for they did not yet fit into the growing body of knowledge about nuclear and particle physics.
Late in 1931, Rossi arranged for Occhialini to work in the Cavendish Laboratory at the University of Cambridge with Patrick Blackett, whom he had met in Berlin. With the aid of the new technique of electronic coincidence, Occhialini helped Blackett develop the first counter-controlled cloud chamber, with which they confirmed Carl Anderson's discovery of the positron and deduced that the positive electrons are produced in association with negative ones by pair production. Up to 23 positive and negative electrons were observed in some events, which were clearly related to the showers of Rossi's soft component.
Padua
In 1932, Rossi won a competition for an academic position in an Italian university and was appointed professor of experimental physics at the University of Padua. Soon after Rossi arrived, the rector asked him to oversee the design and construction of Padua's new Physics Institute. Although this task diverted his attention from research and teaching, he complied willingly, and the institute opened in 1937.East-West effect
In spite of this distraction, Rossi was able to complete, in 1933, an experiment on the east–west effect that he had begun before leaving Arcetri. Because this effect is more prominent near the equator, he organised an expedition to Asmara in Eritrea, which was then an Italian colony on the Red Sea at a latitude of 15° N. With Sergio De Benedetti, he set up a "cosmic ray telescope", which consisted of two separated GM counters in coincidence, whose axis of maximum sensitivity could be pointed in any direction. It soon became apparent that cosmic ray intensity from the West was significantly larger than that from the East. This meant that there was a larger influx of positive primary particles than of negative ones. At the time, this result was surprising because most investigators held the preconceived notion that the primaries would be negative electrons.Just as Rossi left Eritrea, he received news of two observations of a similar east–west effect. These were published in the Physical Review. One was by Thomas H. Johnson, and the other was by Compton and his student, Luis Alvarez, who reported observations at Mexico City, where the latitude is 19° N. Because others had carried out the first experimental exploitation of his important idea of 1930, Rossi was disappointed, but published his results immediately after returning to Padua. Later, with Frederick C. Chromey, Alvarez and Rossi patented a "Vertical Determination Device", which made use of cosmic ray telescopes.
In Eritrea, Rossi discovered another phenomenon that would become a principal theme of his postwar cosmic ray research: extensive cosmic ray air showers. The discovery occurred during tests to determine the rate of accidental coincidences between the Geiger counters of his detector. To ensure that no single particle could trigger the counters, he spread them out in a horizontal plane. In this configuration, the frequency of coincidences was greater than that calculated on the basis of the individual rates and the resolving time of the coincidence circuit. Rossi concluded that:
In 1937, Rossi became acquainted with Nora Lombroso, the daughter of Ugo Lombroso, a professor of physiology at the University of Palermo, and Silvia Forti. Her grandfather was the renowned physician and criminologist Cesare Lombroso, and her aunts, Gina Lombroso and Paola Lombroso Carrara, were well-known Italian writers and educators. In April 1938, Bruno and Nora married and set up a household in Padua.
Although Rossi avoided politics, some of Rossi's associates were active opponents of the fascist state. For example, he mentored Eugenio Curiel, who became a member of the communist party, while completing a degree at Padua. Later, in 1943, Curiel joined the resistance in Milan, and in 1945, was assassinated by soldiers of the Republic of Salò, a German puppet state. Similarly, Ettore Pancini, who received his laurea under Rossi in 1938, spent the war years alternating between cosmic ray research and active participation in the Italian resistance movements of Padua and Venice.
Because of these associations, and because both Rossis were Jewish, they became apprehensive as Italy's antisemitism grew under the influence of Nazi Germany. Eventually, as a result of anti-Jewish laws resulting from the Manifesto of Race, Rossi was dismissed from his professorship. In his words: