Bruno Pontecorvo made groundbreaking contributions in several areas of experimental and theoretical physics, but his name remains largely unknown because he defected to the Soviet Union in 1950; it has been suggested that had he remained in the West, he might have shared a Nobel Prize for his work on neutrino theory. The story of his life and work is worthy of much greater appreciation, and Oxford University physicist and science writer Frank Close wrote this volume to fill this lacuna. With a combination of careful archival research, extensive interviews, and an engaging style, he has done a superb job.
Close reviews Pontecorvo's life largely chronologically. The first half of this book covers his life in Europe and America, and the second his Soviet years. Nine chapters within each half offer descriptions of what was occurring in physics in parallel with contemporaneous events in Pontecorvo's life. The “Half-Life” of the title reflects the fact that Pontecorvo's defection occurred at about the halfway point in his life of 80 years. The “or Spy” part of the title might better have been cast as “and Spy?” Pontecorvo was an outstanding physicist; whether or not he was a spy remains an open question. Close identifies Pontecorvo as a plausible suspect in two incidents that originated in Canada, but in the end there is no airtight evidence or confession. Pontecorvo always claimed that his defection was driven by idealism; either way, he paid an enormous personal and professional cost. After the defection, his wife, whose mental state had always been somewhat precarious, eventually descended into psychiatric-level depression, and one of his sons accused him of being an idiot for having left the West. By the time of the breakup of the Soviet Union, Pontecorvo had become bitterly disillusioned with communism, telling a British journalist that “I was a cretin.”
Pontecorvo was born in 1913 to a well-off family in Pisa. He entered the University of Pisa to study engineering, became attracted to physics, and after two years transferred to Rome to become the youngest of Enrico Fermi's “Via Panisperna boys.” He soon became involved in some of the key experiments that led to the discovery of slow-neutron induced radioactivity, and his name appeared on a patent for the technique. This was also about the time that Fermi was developing his theory of beta-decay; Pontecorvo must have learned of the conjectured existence of neutrinos during his Rome years. In 1936, he moved to Paris to work with Frédéric and Irène Joliot-Curie, and while there came under the influence of his cousin Emilio Sereni, an avowed communist who in postwar years would serve as a minister in the Italian government and who had extensive connections to the Soviet Union; Pontecorvo himself joined the Communist Party on the day of the Hitler-Stalin pact in August, 1939. It was also in Paris that he met his wife, Marianne Nordblom, who he married in early 1940.
With the German invasion of France, some members of the Joliot-Curie group left for England with 40 gallons of heavy water, but British security regarded Pontecorvo as undesirable and refused him entry. But fate intervened for him from across the Atlantic in the person of Emilio Segré, who was by then settled in Berkeley. Segré recommended Pontecorvo for a job with an oil-prospecting firm in Oklahoma, which allowed him to secure a visa. The tale of the family's escape from France is both comic and harrowing, with Marianne suffering a miscarriage just before they departed for New York in August 1940.
The family made its way to Tulsa, where Pontecorvo took up his job with Well Surveys, a firm that was pioneering searching for oil deposits by means of neutron irradiation. Oil-bearing shales contain deposits of uranium and thorium, which can be induced to emit detectable gamma radiation upon neutron bombardment; Pontecorvo contributed several experimental and analytic techniques to this work. He also accrued extensive knowledge of how various materials respond to neutron bombardment, information that would be of value when he became involved in reactor development.
In late 1942, the Manhattan Engineer District established a reactor project in Canada; the French heavy water made its way there. Pontecorvo was interviewed for a position with the project, and was given a clear endorsement by British security—a marked contrast to his treatment in 1940. In the meantime, however, an event occurred that was to have ramifications years later. In September 1942, the Pontecorvo house was visited by FBI agents in view of his status as an enemy alien. Pontecorvo was away at the time, but the agents spoke with Marianne, and noted the presence of a considerable amount of communist literature in the house. This information was passed on to British security officials in Washington in early 1943, just as Pontecorvo was getting briefed on the reactor project. This report was apparently “lost,” but would resurface later to great embarrassment for the British. British security was notoriously infiltrated by communist sympathizers and double-agents, so it is anybody's guess as to whether the report simply went astray in bureaucratic machinery or was deliberately sidelined.
In Canada, Pontecorvo's knowledge of neutron physics made him a valued member of the reactor group. This group developed two heavy-water reactors, which were built at Chalk River, Ontario: The Zero Energy Experimental Pile, and the later high-neutron-flux NRX reactor. Close identifies Pontecorvo as a prime suspect in two incidents during his time at Chalk River. Blueprints for the NRX reactor somehow made their way to the USSR, as did a sample of uranium. The evidence is circumstantial, however, based on the fact that Pontecorvo made occasional trips to the Canada/US border, ostensibly to deal with citizenship issues, as did courier Lona Cohen, who would meet a Canadian-based contact there. Cohen, who was American-born, was recruited as a Soviet agent by her husband, Morris (also American-born); she took over his network of sources when he was drafted. Her contacts included physicist Ted Hall at Los Alamos, and a Canadian contact whom she never identified.
Pontecorvo remained at Chalk River until early 1949, and it was during this period that he turned to neutrino physics. He generated several seminal ideas, including searching for reactor-produced neutrinos using carbon tetrachloride as a capturing agent, and that the Sun should be a copious source of neutrinos. Close explains how these ideas actually had their genesis with others (Jules Guéron and Maurice Pryce), but Pontecorvo came to be associated with them when he discussed them in a publicly-available 1946 report; the solar idea stimulated Ray Davis's 40-year quest to detect solar neutrinos. As for reactors, they proved to produce anti-neutrinos, as verified by Fred Reines and Clyde Cowan in 1956. Pontecorvo also turned to the analysis of cosmic rays, which led to his proposing that muons are heavier versions of electrons; he also suggested that a diverse variety of phenomena could be due to a universal weak force, thereby helping to build what became the Standard Model of particle physics.
In early 1949, Pontecorvo took a position at the Harwell laboratory of the British Atomic Energy Research Establishment; Harwell was also where Klaus Fuchs had landed after the war. Pontecorvo seemed to be well-settled at Harwell, but later that year his old colleague Emilio Segré triggered a series of events that may have led to Pontecorvo's defection. The Atomic Energy Commission was investigating the backgrounds of the Via Panisperna group as a consequence of patent litigation involving their claim for the discovery of neutron activation, and Segré related Pontecorvo's communist connections to an AEC official; Close suggests that Segré may have felt compromised. This information was passed to the FBI, which also still had a record of the visit to his house in 1942. The Bureau began checking into Pontecorvo more thoroughly, and passed on their concerns to British security. A few months later, the British, having just dealt with Fuchs, turned their attention to Pontecorvo, who they interviewed in early April, 1950.
In the summer of 1950, Pontecorvo and his family set out on a European vacation. They left England in late July, and everything seems to have been normal until around August 23, when he sent a telegram to his parents to announce that he had to cancel a planned visit. It is apparently at this time that he decided to defect; he might have heard from his cousin Emilio Sereni that the FBI was interested in him. Suddenly armed with a wad of American dollars, the family flew from Rome to Stockholm and then on to Helsinki, from where they were spirited into Russia by car. They were taken to Moscow, where they were lodged in an apartment while they became oriented to Soviet society and given language lessons. After a couple months, they were moved to Dubna, where the Soviets were building a powerful accelerator. The family was provided with a house that was beautiful by Soviet standards, but their existence was a gilded cage one; their every move was monitored, and social contacts were minimal.
The Pontecorvos were held incommunicado for their first five years in Russia; not until early 1955 was Bruno's name allowed to surface, in an Isvestia article promoting a nuclear disarmament campaign. In the meantime, he had been developing various ideas in particle physics, but, because he was isolated, he would receive no immediate credit for this work. These ideas included the notion that decaying muons might produce more than one type of neutrino, that neutrinos might change their identity in mid-flight, and that the process of formation of strange particles is not the reverse of their decay. This latter process is now known as “associated production,” and led to the later concept of strangeness, which inspired Murray Gell-Mann's “eightfold way” particle-classification scheme. Associated production was detected at Brookhaven in 1954, and in most texts is attributed to Abraham Pais.
In 1959, Pontecorvo was allowed to travel to Kiev for a conference, at which he proposed an experiment that would create muon-antineutrinos, rather than Reines and Cowan's electron-antineutrinos. Pontecorvo published his idea in a Russian journal, but two years passed before the paper became available in English, by which time he had been scooped by the Brookhaven team of Melvin Schwartz, Jack Steinberger, and Leon Lederman. This group snared the 1988 Nobel Prize for physics, and did acknowledge Pontecorvo's ideas. Pontecorvo's first trip outside the Soviet Union did not occur until 1978, when he was allowed to visit Rome as a delegate to a 70th birthday celebration for his old friend Edoardo Amaldi.
After 70 years, the question of any wartime espionage on Pontecorvo's part is now moot; his legacy lies in particle physics. This book will help bring that legacy the recognition it deserves, and the book itself deserves a place on the shelf of anybody interested in twentieth-century physics and its connections to the Cold War.
Cameron Reed is the Charles A. Dana Professor of Physics at Alma College. He served as the editor of the American Physical Society's “Physics & Society” newsletter from 2009–2013, and is currently Secretary-Treasurer of the APS's Forum on the History of Physics. His interests lie in the physics and history of nuclear weapons; his text “The History and Science of the Manhattan Project” was published by Springer in late 2013.