Bruno graduated in Physics, with honors, on November 10, 1933. With Emilio Segrè and Edoardo Amaldi he worked on problems of atomic spectroscopy and only in the summer of 1934 he took part to the Fermi's research on the radioactivity induced by neutron bombardment. On October 20, of that year, Fermi and his group discovered the effect of the slowing down of neutrons: neutrons passing through hydrogenated substances, such as paraffin and water, produced an extraordinary increase in the induced radioactivity.
On November 1, 1934, Bruno was appointed assistant in charge
at the Institute of Physics of the Regia Università of Rome. He remained there until March 19, 1936 when he was granted a fellowship
by the Ministero dell'Educazione Nazionale. The destination where to spend the 6 month fellowship was not an easy choice. As a result of the invasion of Ethiopia, heavy economic sanctions had been just imposed to Italy and the fascist government had identified England as the main responsible. For this reason the prestigious Cambridge Laboratories were excluded and Pontecorvo decided to go to Paris to the Institut du Radium and then to the Collège de France. Thanks to a letter of introduction by Enrico Fermi he got in touch with Frédéric and Irène Joliot-Curie, respectively son-in-law and daughter of Pierre and Marie Curie. In France, Pontecorvo continued successfully his research on slow neutrons.
Once the grant was expired, he was offered to remain in France and though at the University of Rome, in 1937, it was issued a call for a permanent job as assistent, he decided not to get back to Italy. Under the supervision of Joliot he performed important researches in the field of nuclear phosphorescence
and nuclear isomerism
for which he was awarded the Curie-Carnegie
In the Joliot-Curie laboratory Bruno learned new techniques of particle acceleration, saw new detectors and had the opportunity to follow new discoveries. He took advantage of the French skills in the construction of High Voltage generators to produce intense X-ray sources with which he studied the phenomenon of the nuclear isomerism. In light of the interesting results, the French scholarship was renewed every six months until December 1939, after the outbreak of the Second World War.
In Paris, Pontecorvo lived in a modest room at the Hôtel des Grands Hômmes in Place du Panthéon, a few steps from the Institut du Radium where he was working. It is the center of the Latin quarter, where La Sorbonne, the Ecole Polythéchnique, the Collège de France are located and around a multitude of private laboratories, shops and ateliers.
In the atmosphere of the Front Populaire and of the Spanish War, he took a keen interest in politics. A large part of his colleagues belonged to the left-wing. Irène Joliot was a member of the socialist government of Léon Blum, her husband Frédéric an active communist. Thanks to his cousin, Emilio Sereni, intellectual and leader of the PCI, who fled to France because persecuted by the fascist regime, Bruno established relations with the political emigrants
and in August 1939, in the presence of Luigi Longo, he joined the Italian Communist Party.
He became a close friend of the physicist Sergio De Benedetti, a close collaborator of Bruno Rossi in the study of cosmic rays.
De Benedetti was in Paris because of the racial laws promulgated in Italy in November 1938 and for which he lost his position as assistant professor at the Institute of Physics in Padova.
In his autobiography
De Benedetti writes about the deep friendship and great admirer for Pontecorvo: "I could not help but share the opinion about Bruno that I heard in the laboratory. He had an intelligence beyond the majority of our colleagues: he had a deep insight on scientific problems. He had gained everyone's sympathy for his vitality and great spontaneity. He worked only when he wanted, that is when he had an interesting problem to solve."
Bruno was fascinated by the new environment, decidedly more democratic and diverse than in Pisa and Rome:
"I was very impressed by the general promiscuity, the presence of so many black people, the large number of girls frequenting the University, their attitude so casual. But above all the workers impressed me. In Paris there were the workers, they recognized themselves physically and frequented the same places where students and intellectuals used to go. They ate close to us, quietly. I think I never saw a worker in Rome. [..] certainly I had never eaten at the same table with them. " [M. Mafai - Il lungo freddo]
A few months after his arrival, he met a beautiful Swedish student, Marianne Nordblom, with whom he established a very close relationship that would lead him to marriage and soon to the birth of their first child, Gil
, the name of the brother Bruno was very close. Also Gillo, the nickname of Gilberto Pontecorvo, was in France. He arrived in Paris in 1938, at twenty years old, with the excuse of participating in a tennis tournament, a very popular sport in the family and of which Bruno was a true champion, and he decided not to return back to Italy because of the racial laws. On June 13, 1940 while the Wermacht was on the outskirts of Paris, Bruno, with Gillo and his friends Lauria and De Benedetti, escaped by bicycle to the south of France after having sent by train his wife and son to Toulouse where lived his sister Giuliana and her husband Duccio Tabet.
With the family he crossed Spain by train and reached Lisbon where, on the 9th of August, he embarked towards the United States, the only country where he could continue to work. The Pontecorvo family arrived in New York aboard the Steamship Quanza on August 20, 1940. Two days later Bruno was 27 and was already famous as a nuclear physicist of great experience.
For months, with the help of Fermi and Joliot-Curie, he had tried to have a work contract, a necessary condition for entering the USA. After an unsuccessful contact with the Westinghouse, he was introduced by Emilio Segrè to Well Surveys Inc.
who engaged him as a research physicist. Well Surveys was one of the first American companies that applied nuclear physics to oil prospecting.
Settled in Tulsa, Oklahoma, he developed one of the most important practical application deriving from the discovery of the neutron: the neutron well logging technique
, still used today in oil field.
This technique, much more efficient than traditional methods, exploits the fact that different types of rock behave differently when they are bombarded by the neutrons emitted by a source carried deep into the exploration well.
In addition to being the first example of a practical application of the Fermi's discovery of the slow neutrons
(1934), this invention simplified the process of identifying and extracting not only oil but also Uranium.
Bruno received several attractive offers for work in the oil industry. However, his interest in fundamental research prevailed over economic considerations and early in 1943 he moved with the family to Montreal, Canada. There, in 1944, was born their second child Tito Nils
and, in 1945, Antonio
While Fermi and Segrè were recruited in the Manhattan project
, the secret atomic weapons development project, Pontecorvo accepted to join the Anglo-Canadian project Tube Alloys
whose staff included several distinguished scientists, refugees from various countries in Europe.
In that period, studies for the design of a heavy water and uranium nuclear reactor had started under the direction of Hans von Halban at the Montreal laboratory, but the decision to build the National Research Experimental Reactor
(NRX), a few km north of the Chalk River village, was taken only in 1944 when John Cockcroft replaced von Halban. Bruno moved from Montreal to Chalk River in December 1945. He was responsible for several physics aspects of the reactor. NRX, that was designed to produce fissile material for the atomic bomb, became operational in 1947 and Bruno was one of the four physicists allowed to enter the control room at the start up.
In addition to von Halban, some of the physicists that Pontecorvo had met in Paris participated to the project, as for example
Lew Kowarski who had worked with Francis Perrin and Frédéric Joliot-Curie on nuclear fission. The French group had discovered that heavy water was an excellent moderator for the neutrons produced in the fission process and through the Ministry of Armaments, in 1940, bought from the Norwegian hydroelectric company Norsk Hydro the only amount of heavy water existing in Europe, 185 kg. That heavy water was secretly transferred to Great Britain and later to Canada on 19 June 1940, to avoid the risk of falling into German hands.
During the Canadian period, Pontecorvo continued his studies in fundamental physics. He was always fascinated by the weak interactions and gave fundamental contributions to their understanding.
In 1947, immediately after the famous experiment of Conversi, Pancini, Piccioni (Phys Rev 71, 1947) and the interpretation given by Fermi, with Teller and Weisskopf, it was understood that the mesotron produced in cosmic rays (today known as muon) was not the strongly interecting Yukawa particle (the pion), but a different particle that interacted with the nucleus of the atmosphere in a much weaker way.
After having read the Fermi's article on "The capture of negative mesotrons in matter", Pontecorvo published the paper entitled "Nuclear capture of mesons and mesons decay"
on the Physical Review. In the article, Pontecorvo claimed that "the probability of capture of a bound negative meson is of the order of the probability of ordinary K-capture processes, when allowance is made for the difference in the disintegration energy and the difference in the volumes of the K-shell and of the meson orbit"
and suggested "the possibility of a fundamental analogy between β--processes and processes of emission and absorption of charged mesons"
It was the key intuition at the base of the "Universality of weak interactions"
Unfortunately the paper had not a large echo and 2 years later Giampietro Puppi introduced the concept of Universal weak interaction"
without any reference to the previous Pontecorvo ideas.
With Ted Hincks, Bruno started accurate experimental measures on cosmic rays
from which he was convinced that the observed electrons came from the decay of muons in flight and developed a new technique of proportional counters
, based on very large amplification in the gas.
Bruno studied also the problem of revealing the neutrino, the particle hypothesized in 1930 by Pauli who wrote in his diary: Today I have done something which no theoretical physicist should ever do in his life: I have predicted something which shall never be detected experimentally.
To detect such elusive particle, Pontecorvo proposed a radiochemical method based on the observation of the reaction 37
Cl + ν → 37
Ar + e-
and other similar processes. He suggested the sun and nuclear reactors as possible intensive sources of neutrinos and anti-neutrinos.
It was a brilliant idea that provided inspiration for Ray Davis who was the first person to look for neutrinos from the sun and discover the solar neutrino anomaly. For his pioneering experiment which used the Pontecorvo Cl-Ar method, Davis was awarded the Nobel Prize in 2002. He credited Pontecorvo with these words:he opened everyone's eyes with his original insights including his early discussion of the advantages of using chlorine as a neutrino detector
Bruno remained in Canada until January 1949, when he moved to England and joined the new Atomic Energy Research Establishment (A.E.R.E) at Harwell, the nuclear research facility in Oxfordshire directed by John Cockcroft. At that time Bruno had already acquired British citizenship. Cockcroft had returned to England in September 1946.
In september 1949 he attended the Basle-Como International Conference on Nuclear Physics, Quantum Electrodynamics and Cosmic Rays, jointly organized by the Swiss and the Italian Physical Societies.
Bruno's stay at Harwell lasted only a bit more than a year. Early in 1950 he received an offer of a Chair at the University of Liverpool, where a large synchrocyclotron was under construction. Skinner, the director of the Physics Department, was conviced that Pontecorvo was the ideal candidate to lead the experimental activities. After a short visit to Liverpool he decided to accept and to move there in autumn, after a summer holiday in Italy.
Bruno and his family left for Italy on 25 July 1950 by car. They spent most of their holidays at the sea near Rome, and only towards the end of August, after his thirty-seventh birthday, they moved to Rome.
On 29 August 1950 he bought five round-trip tickets for Stockholm at the Scandinavian Airlines Agency which he payed the day after with the considerable amount of six hundred US dollars in cash then, on 1 September, they flew to the Swedish capital but did not visit Marianne's mother.
Pontecorvo and his family vanished behind the iron curtain. Now we know that he went to work in a new laboratory of Experimental Physics in Dubna, 150 km north of Moscow, but nobody knew about him and his family for 5 long years.
In Soviet Union he continued his studies and reserches on the elementary particles physics and in particular on neutrinos. He was director of the Laboratory of Nuclear Problems in Dubna, professor of Particle Physics at the Moscow University, Member of the Academy of Sciences of the Soviet Union, foreign member of the Accademia dei Lincei and doctor honoris causa
of numerous universities.
Afflicted with Parkinson's disease for several years, he died in Dubna on September 24, 1993 at the age of eighty.
Samoil Bilenky, a collaborator and friend of Pontecorvo for a long time, remembers him with these words: "Bruno Pontecorvo was a true scientist in the best, classical sense of the word. His wide and profound knowledge, his love of physics, his ingenious intuition and his ability to understand complicated problems in a clear and simple way were gifts of God."