Bruno Rossi

Bruno Benedetto Rossi (13 April 1905 – 21 November 1993) was an Italian experimental physicist.

He made major contributions to particle physics and the study of cosmic rays.

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 (GM tubes or counters) made possible Bothe's investigations.

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.

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.

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.

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 Gilberto Bernardini, Enrico Persico, and Giulio Racah.

In 1929, Rossi's first graduate student, Giuseppe Occhialini, was awarded the doctoral degree.

In 1929, Rossi read the paper of Walther Bothe and Werner Kolhörster, which described their discovery of charged cosmic ray particles that penetrated 4.1 cm 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"came like a flash of light revealing the existence of an unsuspected world, full of mysteries, which no one had yet begun to explore. It soon became my overwhelming ambition to participate in the exploration."

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.

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 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.

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.

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.