The son of an electrical engineer, Rossi began his college studies at the University of Padua and received his doctorate in physics from the University of Bologna in 1927. He began his scientific career at the University of Florence and then became the chair in physics at the University of Padua, serving in that post from 1932 to 1938. Early in his career, Rossi’s experimental investigations of cosmic rays and their interaction with matter helped establish the foundation of modern high-energy particle physics. He carefully measured the nuclear particles associated with such cosmic ray showers and helped turn Earth’s atmosphere into one giant nuclear physics laboratory. Rossi began his detailed study of cosmic rays in 1929. That year, to support his cosmic ray experiments, he invented the first electronic circuit for recording the simultaneous occurrence of three or more electrical pulses. This circuit is now widely known as the Rossi coincidence circuit. It has become one of the fundamental electronic devices used in high-energy nuclear physics research and also was the first electronic AND circuit—a basic element in modern digital computers. While at the University of Florence, Rossi demonstrated in 1930 that cosmic rays were extremely energetic, positively charged nuclear particles that could pass through lead shield more than 1 meter thick. Through years of research, Rossi helped remove much of the mystery surrounding the Höhenstrahlung (“radiation from above”) first detected by Victor Hess in 1911–1912.
Rossi suddenly lost his position at the University of Padua in 1938. That year, Italy’s fascist leaders decided to scour the major Italian universities and purge any “dangerous intellectuals” who might challenge the nation’s totalitarian government and/or its alliance with Nazi Germany. Like many other brilliant European physicists in the 1930s, Rossi became a political refugee from fascism. So, with his new bride, he departed Italy in 1938 for the United States. The refugee couple arrived in the United States in 1939, after short stays in Denmark and the United Kingdom. Rossi eventually joined the faculty of Cornell University in 1940 and remained there as an associate professor until 1943. In spring 1943, Rossi’s official status as “enemy alien” was changed to “cleared to top secret,” and Rossi was soon able to join the many other refugee nuclear physicists at the Los Alamos National Laboratory in New Mexico. Rossi collaborated with other scientists from Europe in the development of the world’s first atomic bomb under the top-secret Manhattan Project.
Rossi used all his skills in radiation detection instrumentation to provide his colleague Enrico Fermi (1901–1954) an ultrafast measurement of the exponential growth of the chain reaction in the world’s first plutonium bomb (called Trinity) as this device was tested near Alamogordo, New Mexico, on July 16, 1945. On a 1-microsecond oscilloscope trace, Rossi’s instrument captured the rising intensity of gamma rays from the implosion bomb’s supercritical chain reaction—marking the precise moment in world history before and after the age of nuclear weapons. Following World War II and the successful completion of the Manhattan Project, Rossi left Los Alamos in 1946 and became a professor of physics at the Massachusetts Institute of Technology (MIT). In 1966, he became an institute professor—an academic rank at MIT reserved for scholars of great distinction. Upon retirement in 1971, the university honored his great accomplishments by bestowing upon him the distinguished academic rank of institute professor emeritus.
By the mid-1950s, large particle accelerators had replaced cosmic rays in much of the contemporary nuclear particle physics research. So Rossi used the arrival of the space age in late 1957 to become a pioneer in two new fields within observational astrophysics: space plasma physics and X-ray astronomy. In 1958, he focused his attention on the potential value of direct measurements of ionized interplanetary gases by space probes and Earth-orbiting satellites. He and his colleagues constructed a detector (called the MIT plasma cup) that flew into space on board NASA’s Explorer X satellite in 1961. This instrument discovered the magnetopause—the outermost boundary of the magnetosphere beyond which Earth’s magnetic field loses its dominance.
In 1962, Rossi collaborated with other scientists and launched a sounding rocket from White Sands, New Mexico. The rocket carried an early grazing-incidence X-ray mirror as its payload. To his great surprise, the instrument detected the first X-ray source from beyond the solar system— from Scorpius X-1, the brightest and most persistent X-ray source in the sky. Rossi’s fortuitous discovery of this intense cosmic X-ray source marks the beginning of extrasolar (cosmic) X-ray astronomy.
Walter Bruno Henning
