Rossi, Bruno Benedetto

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(b. Venice, Italy, 13 April 1905; d. Cambridge, Massachusetts, 21 November 1993), physics, space science.

Rossi was a pioneer in the experimental investigation of cosmic rays and the properties of the new unstable particles produced in the interactions of cosmic rays with matter. In the early days of space research he initiated the research programs that led to direct measurements of the ionized gas in interplanetary space and to the beginning of extrasolar x-ray astronomy.

Rossi was born in Venice, the eldest of three sons of Lina Minerbi and Rino Rossi. In his autobiography (1990, p. 1) Rossi attributed to his father, an electrical engineer involved in the electrification of Venice, the influence that turned what may have been an “inborn tendency toward science … into a lifelong commitment.” After schooling in Venice, Rossi studied for two years at the University of Padua, and another two years at the University of Bologna from which he received the doctoral degree in physics. In 1928 he took up his first appointment as assistant at the University of Florence in the Institute of Physics located on the hill of Arcetri overlooking the city. After a year of searching for a suitable research topic that he could pursue within the limited resources of the Institute, Rossi found it in cosmic rays.

At that time, the only known “fundamental” particles were the negative electron, the positive proton, and the neutral photon. The highest energy particles available for experiments were emitted in the decay of radioactive elements with energies of no more than a few million electron volts. There was also the mysterious “Höhenstrahlung,” the “radiation from above” that had been discovered by Viktor Hess in 1912. In the 1920s Robert Millikan took up the study of the radiation from above, which he dubbed “cosmic rays.” From extensive measurements with ionization chambers of cosmic ray intensities under water and in the atmosphere, Millikan concluded that cosmic rays were ultrahigh energy gamma rays produced from the energy released in the formation of atoms by fusion of hydrogen in interstellar space.

To the limited extent that Rossi had thought about cosmic rays before 1928 he had accepted the conventional wisdom that they were gamma rays, though he doubted Millikan’s theory of their origin. He was apparently not yet aware of the recent discoveries of Jacob Clay and Dmitry Skobelzyn that contradicted Millikan’s ideas. However, in 1929 Rossi read the paper of Walther Bothe and Werner Kohlhörster describing an experiment that showed that cosmic rays contain charged particles capable of penetrating large thicknesses of dense matter. The experiment employed two inventions that would transform experimental cosmic-ray and particle physics, namely the “discharge counter,” recently invented by Hans Geiger and Wilhelm Müller, and the “coincidence method” invented by Bothe. In their experiment, Bothe and Kohlhörster found that two parallel counters surrounded by thick shielding of lead and iron and separated by several centimeters in a vertical plane were occasionally discharged in coincidence by the passage of a charged particle through the shield and the two counters. They detected such events by attaching the counters to separate fiber electrometers and photographing on a moving film the deflections of the fibers caused by discharges of the counters. They found that the rate of coincidences decreased by only a small fraction when a 4.1 centimeter thick gold brick was inserted between the two counters,

clear proof of the high energy and penetrating power of the detected particles.

In his autobiography (p. 10) Rossi wrote that the Bothe-Kohlhörster paper opened “a field of inquiry rich in mystery and promises. Working in a field of this kind had been my dream.” Rossi fabricated Geiger-Müller counters according to the published description and powered them with a bank of batteries. And he invented the electronic “coincidence circuit” to register the occurrence of coincident pulses from two or more counters. His novel circuit could be adapted to register coincidences among any number of counters, and it achieved a much shorter resolving time than the mechanical coincidence method employed by Bothe and Kohlhörster. The coincidence circuit was an essential device in all of Rossi’s cosmic-ray experiments, and was rapidly adopted by experimenters around the world.

During a brief visit to Bothe’s Berlin laboratory in the summer of 1930, Rossi learned about the mathematical theory of Carl Störmer regarding the motion of charged particles in a dipole magnetic field similar to that of Earth. Rossi perceived in the complex theory a simple and observable consequence that would permit an experimental determination of the sign of the electric charge carried by the primary cosmic rays that arrive from outer space. In a letter to the Physical Review he predicted a difference between the intensities of cosmic rays arriving from the east and the west depending on the sign of their charge. Back at Arcetri he attempted to measure the “east-west effect” with a cosmic-ray telescope consisting of two parallel counters mounted several centimeters apart on a swivel base and connected to his coincidence circuit. Failing to obtain a statistically certain difference between the coincidence rates with the telescope pointed east and west, Rossi began plans for an expedition to a mountain site at low geomagnetic latitude where, according to the theory, the east-west difference at ground level would be much larger.

In an introductory talk on cosmic rays at the Rome conference on nuclear physics in the fall of 1931, Rossi explained how recent discoveries had disproved Millikan’s theory about the nature and origin of cosmic rays. Both Arthur Compton and Millikan were in the audience. In his autobiography (p. 18) Rossi wrote that Millikan was not pleased by the demolition of his theory and that “from that moment on he refused to recognize my existence.” Rossi also wrote that years later Compton told him “his interest in cosmic rays was born from my presentation.”

In a series of coincidence experiments with various arrangements of counters and absorbers, Rossi demonstrated that the local cosmic rays contain a component that is rapidly attenuated in dense matter by interactions that produce showers of secondary particles. His report on this phenomenon was so astonishing at the time that it was refused for publication by the first journal to which he submitted it, and accepted by another only after Werner Heisenberg vouched for Rossi’s reliability. It was confirmed the following year by Patrick Blackett and Giuseppe Occhialini who published photographs of particle showers in a cloud chamber whose expansions were triggered by the signal from a Rossi coincidence circuit connected to an arrangement of Geiger-Müller counters. The idea for counter control of a cloud chamber had been brought to Blackett’s laboratory by Occhialini, who was Rossi’s first student at Arcetri. The showers were soon identified as cascades of photons, electrons, and positrons, and Rossi’s measurements of their growth and attenuation in matter provided an early test of the cascade shower theory based on the new quantum electrodynamics.

In another experiment, Rossi demonstrated that there are cosmic-ray particles that can penetrate more than one meter of lead with an energy loss by ionization in excess of 2 billion electron volts. In 1936–1937 experiments at the California Institute of Technology and at Harvard proved that these penetrating particles have a mass between the electron and the proton. They were initially called mesotrons, and are now called muons.

In the fall of 1932, having won a competition for a university position, Rossi was called to the University of Padua as professor of experimental physics. There, in addition to research and teaching, he was responsible for planning and supervising the construction of a new Physics Institute of which he became the director. At Padua Rossi finally found time to test his prediction of the east-west effect in an experiment that he carried out in the fall of 1933 near the town of Asmara in the Italian colony of Eritrea. He found the intensity of penetrating particles was about 20 percent greater from the west than the east, which proved that the primary cosmic rays that arrive from outer space are particles with positive charge. Essentially identical results were obtained in two experiments carried out a few months earlier at Mexico City by Thomas Johnson and by Luis Alvarez and Arthur Compton. After World War II, direct observations by various groups, using detectors carried to great altitudes by balloons, proved that the primary cosmic rays are indeed positive and that they consist of protons and the bare nuclei of helium and heavier elements.

During tests of his equipment at Asmara, Rossi discovered particle showers that are initiated high in the atmosphere by ultrahigh energy primary cosmic rays and are spread over wide areas when they arrive at ground level. He wrote about it in his autobiography (p. 36), and provided a translation of a portion of his report on the Eritrean expedition:

it seems that once in a while the recording equipment is struck by very extensive showers of particles, which cause coincidences between counters, even placed at large distances from one another. Unfortunately, I did not have the time to study this phenomenon more closely.

These particle showers came to be called “extensive air showers” to distinguish them from the particle showers produced locally in interactions of cosmic rays with condensed matter. Pierre Auger published the first detailed study of extensive air showers in 1937. Measurements of extensive air showers would eventually provide the means for determining the energies and arrival directions of the highest-energy primary cosmic rays. After the war they were a major topic of research by Rossi’s group at the Massachusetts Institute of Technology (MIT) and by others at laboratories around the world.

In April 1938 Rossi married Nora Lombroso, daughter of Silvia Forti and Ugo Lombroso, professor of physiology at the University of Genoa. The following September, Rossi was dismissed from his position as a result of the anti-Jewish decrees of the fascist government. The Rossis managed to obtain passports and left Italy in October. They were welcomed, first by Niels Bohr at his institute in Copenhagen, and then at Manchester University by Patrick Blackett, who had obtained for Rossi a fellowship from the Society for the Preservation of Science and Culture. During this sojourn Rossi began to focus his attention on the experimental problem of testing the stability of mesotrons.

In the spring of 1939 Rossi accepted an invitation from Arthur Compton to attend a July conference on cosmic rays at the University of Chicago. The Rossis arrived in New York in June. After a brief visit with the Fermis, already established at Columbia University, they drove with Hans Bethe to Chicago. The expected instability of mesotrons was a principal topic of the conference. Afterward, during a visit to the Comptons’ summer cottage, Rossi presented to Arthur Compton his idea for a definitive experiment on mesotron decay based on a comparison between the attenuation of the intensity of the penetrating component of cosmic rays in air and in an equivalent amount of graphite. With Compton’s encouragement and support, Rossi prepared and carried out the experiment on Mount Evans in Colorado before the winter set in. The experiment proved that mesotrons decay in flight with a lifetime of the order of microseconds.

In the fall of 1940, on the recommendation of Bethe, a member of the physics faculty at Cornell University, Rossi was appointed associate professor of physics at Cornell. There, with a graduate student, Norris Nereson, he made a precise measurement of the mean life of mesotrons at rest, the first such measurement of an unstable subatomic particle. For that work they invented an electronic time-to-amplitude converter of which various versions have since been widely used in nuclear and particle physics.

During World War II Rossi worked first as a part-time consultant on instrumentation for radar development at the Radiation Laboratory of the Massachusetts Institute of Technology. Then, in the late spring of 1943, he joined the atomic bomb project at Los Alamos, New Mexico, where he was responsible, with Hans Staub, for development of detectors for nuclear experiments. Among their inventions was the “fast ionization chamber.” Rossi used it in various experiments including a series of tests that validated the implosion method for detonating the plutonium bomb and in a measurement of the exponential growth of the chain reaction in the test of the first plutonium bomb at Trinity. Rossi remained at Los Alamos for a year after the end of the war to complete with Staub a treatise on the instruments their group had developed.

In the fall of 1946 Rossi came to MIT where he established the Cosmic Ray Group to carry out research on the greatly increased scale made possible by the new availability of government support and the recent advances in technology. In these new circumstances, as professor of physics responsible for teaching and for leading a large group of students and senior coworkers, Rossi changed his mode of operation. In his autobiography he wrote:

[Previously], working alone or, at most, with the help of a few students, I would build the instruments needed for my experiments, I would take them to the place where they had to be used, I would make the measurements and analyze the results. Now I had the responsibility of an entire group, and what mattered was no longer my own work but the work of the group. In the first place my task was to identify the most promising research programs among those that were within our reach. I had then to help where help was needed in the planning of the instrumentation and evaluation of the experimental results, all of this without discouraging the individual initiative of the researchers. (p. 101)

Under Rossi’s leadership, the Cosmic Ray Group carried out a wide variety of investigations aimed at determining the properties of the primary cosmic rays, elucidating the processes involved in their propagation through the atmosphere, and measuring the unstable particles generated in the interactions of cosmic rays with matter. Among these investigations was an experiment carried to a high altitude by a cluster of balloons that placed an upper limit of 1 percent on the proportion of electrons and photons in the primary cosmic rays. Experiments with a multiplate cloud chamber yielded early evidence for the neutral pion and its decay into a pair of gamma rays, measurements of the properties of kaons and hyperons, and the first direct observation of the annihilation of an antiproton. Studies of extensive air showers determined the energy spectrum of the primary cosmic rays in the range from 1015 to 1020 electron volts.

In 1958, when cosmic-ray phenomena were well understood and experiments at particle accelerators had come to dominate particle physics, Rossi’s interests turned to the new opportunities for exploratory investigations made possible by the availability of space vehicles. At MIT he initiated a program aimed at measuring the properties of the ionized gas (plasma) in interplanetary space that would eventually extend to the farthest reaches of the solar system. In 1961, an MIT “plasma probe,” carried on the spacecraft Explorer 10, yielded the first direct measurement of the density, speed and direction of the solar wind at the boundary of Earth’s magnetosphere.

In the same period, Rossi inspired the program in x-ray astronomy at American Science and Engineering, Inc., (AS&E) a small research and development company founded in 1958 by his former student, Martin Annis, and for which Rossi served as scientific consultant and chairman of the board. Riccardo Giacconi, a former student of Occhialini at the University of Milan, had recently been hired by Annis to develop a program in space science at the new company. At Rossi’s suggestion, Giacconi began a study of the possibilities for x-ray astronomy that led to a series of AS&E rocket experiments, sponsored by the U.S. Air Force. After two failures, the third experiment, launched in June 1962, discovered the first extrasolar source of x-rays that came to be called Sco X-1. AS&E took the lead for many years in developing x-ray astronomy with the support of the National Aeronautics and Space Administration. This new field of astronomy proved to be an essential source of information about neutron stars, black holes, and the high-energy processes in the universe.

Rossi was an inspiring teacher, speaker, and writer. In 1965 he was appointed Institute Professor, the highest honor given to members of the MIT faculty. Among his many other honors were memberships in the National Academy of Sciences, the American Academy of Arts and Sciences, and the Academia Nazionale dei Lincei of Rome. His awards included the Gold Medal of the Italian Physical Society, the U.S. National Medal of Science and honorary doctorates from the universities of Durham, Palermo, and Chicago. The 1987 Wolf Prize in Physics for the development of x-ray astronomy was awarded jointly to Rossi, Riccardo Giacconi, and Herbert Friedman, the latter having pioneered the study of solar x-rays at the U.S. Naval Research Laboratory in the 1950s. After his official retirement in 1970, Rossi remained active for many years as an advisor and commentator on interplanetary plasma research and x-ray astronomy, and in the preparation of his scientific autobiography.

Bruno Rossi died at his home in Cambridge, Massachusetts, on 21 November 1993. He was survived by his wife; three children, Florence, Frank, and Linda; and three grandchildren. His ashes lie under a monument in the graveyard of the church of San Miniato al Monte, overlooking the city of Florence and across a valley from the hill of Arcetri, where he began his scientific career.



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George W. Clark

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Rossi, Bruno Benedetto

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