The fourth of eight children of a wealthy Jewish-Italian family, Pontecorvo studied physics at the Sapienza University, under Fermi, becoming the youngest of his Via Panisperna boys.

Influenced by his cousin, Emilio Sereni, he joined the Italian Communist Party, whose leader were in Paris as refugees, and as did his sisters Giuliana and Laura and brother Gillo.

The Italian Fascist regime's 1938 racial laws against Jews caused his family members to leave Italy for Britain, France and the United States.

When the German Army closed in on Paris during the Second World War, Pontecorvo, his brother Gillo, cousin Emilio Sereni and Salvador Luria fled the city on bicycles.

His grandfather on the maternal side, Arrigo Maroni (1852–1924), born in Mantua, was director of the Fatebenefratelli Hospital in Milan;[4] his mother's cousin was a notable zoologist Elisa Gurrieri-Norsa (1868-1939).

[10] In 1934, Pontecorvo contributed to Fermi's famous experiment showing the properties of slow neutrons that led the way to the discovery of nuclear fission.

[12] An Italian patent was granted for the process in October 1935, in the name of Fermi, Pontecorvo, Edoardo Amaldi, Franco Rasetti and Emilio Segrè.

[13] In February 1936, Pontecorvo left Italy and moved to Paris to work in the laboratory of Irène and Frédéric Joliot-Curie at the Collège de France on a one-year scholarship to study the effects of collisions of neutrons with protons and on the electromagnetic transitions among isomers.

[16] Whether because of his relationship with Marianne, his interesting work on isomers, or the deteriorating political situation in Italy, he turned down an opportunity in 1937 to apply for a tenured position at the University of Rome to stay in Paris.

Travelling back to Paris alone, he dined with Manne Siegbahn and met with Niels Bohr and Lise Meitner on 12 October 1938.

Although the British offered refuge to French nuclear scientists, including Hans von Halban and Lew Kowarski, they regarded Pontecorvo as an "undesirable".

[26] Fortunately, Segrè had been given an offer of employment in Tulsa, Oklahoma, by two European expatriates who were looking for an expert on neutron physics.

On 13 June, just a day before the Germans entered Paris, Pontecorvo, his brother Gillo, cousin Emilio Sereni and Salvador Luria set out for Toulouse on bicycles.

Marianne had a miscarriage, and was hardly fit to travel, but nonetheless boarded the liner Quanza on 9 August 1940 on its voyage bringing refugees to the United States.

[29] In Tulsa, Pontecorvo went to work for two European migrants, Jakov "Jake" Neufeld and Serge Alexandrovich Scherbatskoy, who had founded a company called Well Surveys with funds provided by Standard Oil.

Inspired by the work done in Italy and France, they reasoned that neutrons, being without electrical charge, might be able to detect different elements beneath the surface by inducing radioactivity on the rocks.

[34] The meeting with Fermi yielded no supplies, but it did result in Pontecorvo receiving an offer from von Halban and Placzek to join the Tube Alloys team at the Montreal Laboratory in Canada.

[34] There was some concern from Sir Edward Appleton over his appointment, not because of Pontecorvo's political beliefs, but because he was not a British national, and there were already a large number of foreign scientists working on Tube Alloys.

[37] In August 1943, Churchill and Roosevelt negotiated the Quebec Agreement, which resulted in a resumption of cooperation between the United States and Great Britain, merging Tube Alloys into the Manhattan Project.

With an eye on a post-war nuclear program, he had Pontecorvo and Alan Nunn May "debrief" Manhattan Project scientists who visited Canada, in practice spying for Britain.

[50] Physicists were in great demand after the war ended in August 1945, and Pontecorvo received attractive and lucrative offers from several universities in the United States.

[47] He acquired the nickname "Ramon Novarro" after the actor of that name following an adventure in which he made a trip to Boston with two women, which culminated in Marianne clearing out the bank account and departing for Banff with the children; but they were reconciled.

While Arnold had no evidence that Pontecorvo was a Soviet spy, he did feel that he was a security risk, and recommended that he be moved to a position where he did not have access to Top Secret material.

[55] On 1 September 1950, in the middle of a holiday in Italy, Pontecorvo abruptly flew from Rome to Stockholm with his wife and three sons without informing friends or relatives.

His abrupt disappearance caused much concern to many of the western intelligence services, especially those of Britain, Canada and the United States, that were worried about the escape of atomic secrets to the Soviet Union, and of Finland and Sweden, through which Pontecorvo and Marianne had been allowed to travel without valid passports and visas.

[69] In 1955, he appeared in public at a press conference where he explained to the world the motivations of his choice to leave the West and work in the Soviet Union.

[70] Pontecorvo was not permitted to leave the Soviet Union for many years; his first trip abroad being in 1978 when he travelled to Italy for celebrations of Amaldi's 70th birthday.

[73][74] Initially neutrinos were thought to be undetectable, but in 1945 Pontecorvo noted that a neutrino striking a chlorine nucleus could transform it into unstable argon-37 that emits, with a 34 days half-life, after a K-capture reaction, a 2.8 keV Auger electron allowing its direct detection:[75][76][77] Pontecorvo's 1945 paper credits the idea using carbon tetrachloride (CCl4) to the French physicist Jules Guéron.

[88] In 1988 Jack Steinberger, Leon M. Lederman and Melvin Schwartz were awarded the Nobel Prize in Physics for the discovery of the muon neutrino.

The prize, awarded annually to an individual scientist, recognises "the most significant investigations in elementary particle physics", as acknowledged by the international scientific community.