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Bruno Rossi, cosmic rays and the invention of the coincidence loop

On April 13, 1905 was born in Venice Bruno Benedetto Rossi, a true authority of physics and astrophysics of the twentieth century, known mainly for his pioneering studies on cosmic rays.

After attending the classical high school, he enrolled at the University of Padua and then moved to Bologna where he graduated in 1927 under the guidance of the famous experimental physicist Quirino Majorana, uncle of the later famous Ettore. At that time Rita Brunetti, who had been an assistant for several years in Florence, also arrived, and in order to make up for the lack of instrumentation at the University of Ferrara, had transferred her experimental research to Bologna. Brunetti had a great influence on Rossi and it was she who introduced him to the Arcetri group.
Rossi arrived in Florence in March 1928, when he was not even 23 years old, to be the assistant of Antonio Garbasso who had founded the Institute of Physics there. The Arcetri group, which had many similarities with Fermi’s group in Rome as well as continuous contact with Persico, was composed, among others, of Enrico Persico, Giuseppe Occhialini, Gilberto Bernardini, Daria Bocciarelli and Giulio Racah.

Here research interests did not immediately become clear to Rossi, but in 1929 Walther Bothe and Werner Kolhörster performed an experiment on the nature of extraterrestrial radiation. They proved that cosmic radiation had a penetrating power far superior to that of gamma rays produced by radioactive decay in atomic nuclei; for the realization of the experiment was essential the use of Geiger-Müller counters (instruments based on the ionizing power of charged particles that allowed their detection by electronic circuits). This result appeared as “a flash of light that reveals the existence of an unexpected world, full of mysteries and that no one had yet begun to explore”. It opened a heated debate on the nature of cosmic rays: were they electromagnetic waves with high energy or were they corpuscles, as claimed by Robert Millikan?

In a few weeks Rossi, who had found in front of him “a field of investigation full of mystery and promise”, invented an instrument that would become of crucial importance both for the study of cosmic rays and for investigations in nuclear physics, the coincidence circuit. The circuit, consisting of triodes and Geiger-Müller counter, allowed the automatic recording of coincident pulses between different counters with resolution times more accurate than those of Bothe and was used by Rossi to highlight the extraordinary power of penetration of cosmic radiation particles through layers of lead of more than one meter; he also discovered that cosmic radiation produces in matter groups of particles that will become known as “cosmic rays swarms”.

Rossi’s first results began to circulate in the scientific world, bringing the appreciation of many physicists, including Enrico Fermi who invited him to give the introductory speech at the International Conference on Nuclear Physics held in Rome in October 1931.

In 1932 he arrived second in the competition for a total of three chairs where, in the words of Fermi, the winning trio was a compromise and did not shine for excessive logic. This comment was moved, probably, by the failure to assign a place to Emilio Segré (who won the competition in 1935 going to Palermo) and Fermi himself in a letter said that to get among the winners at least Bruno Rossi had to endure a long struggle with the commission, of which he was part, which seemed to look more to the seniority of the candidate than to the scientific merits.

Rossi arrived in Padua, where he became a full professor in 1936. In 1938 with the promulgation of racial laws, he and his wife, Nora Lombroso, both Jews had to leave Italy. Rossi himself said:

“I was no longer a citizen of my country and in Italy my activity as a teacher and scientist was over.”

The way in which Bruno and his wife left Italy is quite articulate, since their documents were not regular but, again in Rossi’s words, “in Italy, as I could see once more, you can always find something that, in case of need is ready to help and is able to do so”. So the then vice-president of the Accademia d’Italia, Giancarlo Vallauri, helped him to obtain new documents accompanied by a small sum of money.

Rossi’s first stop was in Copenhagen, where Bruno and his wife were guests of Niels Bohr. After two months of stay in Denmark he arrived in England. Here Patrick Blackett (future Nobel Prize winner for physics in 1948), who in 1931 had collaborated with Occhialini studying cosmic rays, set in motion the English bureaucratic machine to try to get Rossi to work at the Physical Laboratories of the University of Manchester. In this regard he said:

“It will be of great importance for the cosmic rays research of this country if Professor Rossi will be given a working position. I would be extremely pleased if it were possible to have Professor Rossi stay here permanently, but it may prove difficult.”

In addition, Bohr’s word was added:

“A large part of the problems which are at present assiduously investigated in the laboratories of various countries flow directly from the results and investigations of Professor Rossi.”

After one year, Rossi arrived in the United States obtaining a researcher position at the University of Chicago but after only one year he moved, on recommendation of Hans Bethe, to Cornell University where he will realize another fundamental instrument, the Time-to-Amplitude Converter (TAC).

In 1943 he was recruited by Hans Bethe at Los Alamos to develop the project of the atomic bomb, accepting only after a long reflection. He recalled that period by saying:

“The days that followed this invitation were among the hardest of my life. I could easily imagine what was being done at Los Alamos and I shied away from the idea of participating in the development of a device as frightening as the atomic bomb. On the other hand I was terribly worried, as were many others, about the danger that in Germany, where fission had been discovered, they were close to making the bomb. Having resigned myself to the fact that neither by accepting nor refusing the request from Los Alamos could I avoid a heavy responsibility, I saw that the choice could only be based on the need to combat the immediate danger. I remember clearly with what spirit I decided to go to Los Alamos. I hoped that our work would prove the impossibility of making the bomb, but I had also concluded that if, on the other hand, it turned out to be possible, it was necessary to avoid at all costs that Hitler had the bomb before us.”

At the end of the war he moved to the prestigious Massachusetts Institute of Technology (MIT) where he established himself as one of the major international authorities in the field of cosmic ray physics and astronomical and space research.

In 1974 he returned to Italy, precisely in Palermo, where he was given the chair of general physics at the University of Palermo. He died on November 21, 1993.