Bruno Pontecorvo | |
|---|---|
 Pontecorvo in 1955 | |
| Born | (1913-08-22)22 August 1913 |
| Died | 24 September 1993(1993-09-24) (aged 80) |
| Citizenship | Italy, Britain, Soviet Union, Russia |
| Alma mater | University of Rome La Sapienza |
| Known for | |
| Relatives | Gillo Pontecorvo (brother) Guido Pontecorvo (brother) Marco Pontecorvo (nephew) |
| Awards | Stalin Prize (1953) |
| Scientific career | |
| Fields | Nuclear physics |
| Institutions | Collège de France Well Surveys Montreal Laboratory Chalk River Laboratories Atomic Energy Research Establishment Joint Institute for Nuclear Research |
| Academic advisors | Enrico Fermi |
Bruno Pontecorvo (Italian:[ponteˈkɔrvo];Russian:Бру́но Макси́мович Понтеко́рво,Bruno Maksimovich Pontecorvo; 22 August 1913 – 24 September 1993) was anItalian–Russiannuclear physicist, an early assistant ofEnrico Fermi and the author of numerous studies inhigh energy physics, especially onneutrinos. A convincedcommunist, hedefected to the Soviet Union in 1950, where he continued his research on the decay of themuon and on neutrinos. The prestigiousPontecorvo Prize was instituted in his memory in 1995.
The fourth of eight children of a wealthy Jewish-Italian family, Pontecorvo studied physics at theSapienza University, under Fermi, becoming the youngest of hisVia Panisperna boys. In 1934 he participated in Fermi's famous experiment showing the properties of slowneutrons that led the way to the discovery ofnuclear fission. He moved to Paris in 1936, where he conducted research underIrène andFrédéric Joliot-Curie. Influenced by his cousin,Emilio Sereni, he joined theItalian Communist Party, whose leaders were in Paris as refugees, and as did his sisters Giuliana and Laura and brotherGillo. The ItalianFascist regime's1938 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 theSecond World War, Pontecorvo, his brother Gillo, cousin Emilio Sereni andSalvador Luria fled the city on bicycles. He eventually made his way to Tulsa, Oklahoma, where he applied his knowledge of nuclear physics to prospecting for oil and minerals. In 1943, he joined the BritishTube Alloys team at theMontreal Laboratory in Canada. This became part of theManhattan Project to develop the firstatomic bombs. AtChalk River Laboratories, he worked on the design of thenuclear reactorZEEP, the first reactor outside of the United States, which wentcritical in 1945, followed by theNRX reactor in 1947. He also looked intocosmic rays, the decay ofmuons, and what would become his speciality, neutrinos. He moved to Britain in 1949, where he worked for theAtomic Energy Research Establishment at Harwell.
After his defection to the Soviet Union in 1950, he worked at theJoint Institute for Nuclear Research (JINR) inDubna. He had proposed using chlorine to detect neutrinos. In a 1959 paper, he argued that theelectron neutrino (ν
e) and themuon neutrino (ν
μ) were different particles.Solar neutrinos were detected by theHomestake experiment, but only between one third and one half of the predicted number were found. In response to thissolar neutrino problem, he proposed a phenomenon known asneutrino oscillation, whereby electron neutrinos became muon neutrinos. The existence of the oscillations was finally established by theSuper-Kamiokande experiment in 1998. He also predicted in 1958 thatsupernovae would produce intense bursts of neutrinos, which was confirmed in 1987 whenSupernova SN1987A was detected byneutrino detectors.
Pontecorvo was born on 22 August 1913 inMarina di Pisa, the fourth of eight children of Massimo Pontecorvo and his wife Marianée Maroni. His older brotherGuido, who was born in 1907, became ageneticist. Another older brother, Paolo, who was born in 1909, became an engineer who worked on radar during World War II. His older sister Giuliana was born in 1911. His younger brotherGillo was born in 1919, and is best known as the director ofThe Battle of Algiers. He also had two younger sisters: Laura, who was born in 1921, and Anna, who was born in 1924, and a younger brother, Giovanni, who was born in 1926. His family was a wealthy family; Massimo owned three textile factories employing over 1,000 people. The family was Jewish and non-observant,[1][2][3] fromRome on his father's side and fromMantua - on his mother's. His grandfather on the maternal side, Arrigo Maroni (1852–1924), born in Mantua, was director of theFatebenefratelli Hospital inMilan;[4] his mother's cousin was a notable zoologist, Elisa Gurrieri-Norsa (1868–1939).[5][6][7]
He entered theUniversity of Pisa intending to study engineering, but after two years, he decided to switch to physics in 1931. On the advice of his brother Guido, he decided to study at theUniversity of RomeLa Sapienza, where Enrico Fermi had gathered together a group of promising young scientists known as the Via Panisperna boys after the name of the street where the Institute of Physics of Rome University was then situated. At the age of 18, he was admitted to the third year of Physics.[8] Fermi described Pontecorvo as "scientifically one of the brightest men with whom I have come in contact in my scientific career".[9] As their youngest member, the group nicknamed himCucciolo, which means "puppy".[10]
In 1934, Pontecorvo contributed to Fermi's famous experiment showing the properties of slow neutrons that led the way to the discovery ofnuclear fission.[11] Pontecorvo's name was included on the Via Panisperna boys' patent "To increase the production of artificial radioactivity with neutron bombardment". He was made a temporary assistant at the Royal Institute of Physics on 1 November 1934 and the University of Rome, and on 7 November, he was listed as co-author, along with Fermi and Rasetti, of a landmark paper onslow neutrons that reported that hydrogen slowed neutrons more than heavy elements, and that slow neutrons were more easily absorbed.[12] An Italian patent was granted for the process in October 1935, in the name of Fermi, Pontecorvo,Edoardo Amaldi,Franco Rasetti andEmilio Segrè. A US patent was granted on 2 July 1940.[13]
In February 1936, Pontecorvo left Italy and moved toParis to work in the laboratory ofIrène andFrédéric Joliot-Curie at theCollège de France on a one-year scholarship to study the effects of collisions of neutrons with protons and on the electromagnetic transitions amongisomers. During this period, influenced by his cousin,Emilio Sereni, he adopted the ideals of communism to which he remained loyal for the rest of his life.[14][15] He formed a relationship with Helene Marianne Nordblom, a Swedish woman working in Paris as a nanny.[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.[15][17]
Marianne moved in with Pontecorvo at theHôtel des Grands Hommes on thePlace du Panthéon on 4 January 1938. Their son, Gil, was born on 30 July. Her visa expired, and she had to return to Sweden in September. Pontecorvo accompanied her, leaving Gil behind in a residential nursery in Paris. Travelling back to Paris alone, he dined withManne Siegbahn and met withNiels Bohr andLise Meitner on 12 October 1938.[18] Pontecorvo was now unable to return to Italy because of theFascist regime'sracial laws against the Jews. This caused the breakup of the Via Panisperna boys, with Fermi moving to the United States.[19] Pontecorvo's family also dispersed.[20] Guido moved to Britain in 1938,[21] followed by Giovanni, Laura and Anna in 1939,[20] while Gillo joined Pontecorvo in Paris.[22]
Working in collaboration with the French physicist André Lazard at Joliot-Curie's laboratory atIvry-sur-Seine, Pontecorvo discovered what Frédéric Joliot-Curie called "nuclear phosphorescence"; the emission ofX-rays when neutrons andprotons were excited and returned to their ground state.[23][24] He also discovered that some isomers do not change into other elements on decaying radioactively. This expanded the scope for their use in medical applications. For this ground-breaking research, Pontecorvo received a Curie-Carnegie scholarship and funding for his work from theFrench National Centre for Scientific Research.[23]
In June 1939, Pontecorvo applied for a visa to visit Sweden, but his application was rejected. On 23 August came the news of theMolotov–Ribbentrop Pact. He joined theFrench Communist Party the next day as an affirmation of his personal faith in the Soviet Union. Marianne rejoined him in Paris on 6 September 1939, three days after theBritish and French declaration of war on Germany in response to the Germaninvasion of Poland that started the Second World War in Europe. They were married on 9 January 1940.[25]
As the Germans closed in on the city in May 1940, they decided to leave. Although the British offered refuge to French nuclear scientists, includingHans von Halban andLew 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. Segrè had turned down the offer — he already had a good job at the University of California — but recommended Pontecorvo.[27]
On 2 June 1940, he saw Marianne and Gil off with their chattels on a train to Toulouse, where his sister Giuliana lived with her husband, Duccio Tabet. On 13 June, just a day before the Germans entered Paris, Pontecorvo, his brother Gillo, cousin Emilio Sereni andSalvador Luria set out for Toulouse on bicycles. It took them ten days to reach Toulouse. Luria went to Marseilles, from whence he eventually made his way to the United States.[27] Pontecorvo, Marianne, Gil, Giuliana and Tabet boarded a train that took them to Lisbon via Madrid on 19 July 1940. Both women were pregnant. Marianne had a miscarriage and was hardly fit to travel, but nonetheless boarded the linerQuanza on 9 August 1940 on its voyage bringing refugees to the United States. Both women were seasick. On 19 August 1940, the ship reached New York City, where they stayed with his brother Paolo.[28] While there, he visited Fermi at his new home in Leonia, New Jersey.[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 byStandard Oil.[30] Their idea was to apply nuclear physics to searching for minerals. Agamma ray device had been successful at analysing rock outcroppings. 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 byinducing radioactivity on the rocks. In Pontecorvo, they had the expert they needed.[31]
Pontecorvo created a neutron source usingradium andberyllium, as the Via Panisperna boys had, withparaffin wax as a neutron moderator, and measured the absorption of different minerals using methods developed by Fermi and Amaldi. By June 1941, he had a device that could differentiateshale,limestone andsandstone, and map the transitions between them. The technique may be considered the first practical application of the discovery of slow neutrons, and would still be in use decades later forwell logging.[31][32] He filed four patents relating to his instrumentation.[33]
By late 1941, Pontecorvo was having difficulty securing the radioisotopes that he needed. Unbeknown to him, the Manhattan Project, the wartime effort to buildatomic bombs, was cornering the market. In an attempt to obtain them, he met with Fermi, von Halban andGeorge Placzek in New York in April 1942. He was unable to secure the supplies he wanted, but Fermi showed an unexpected keen interest in the Wells Surveys work.[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 SirEdward 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. Appleton was ultimately persuaded due to Pontecorvo's reputation and the fact that good physicists were in short supply.[35] Pontecorvo was officially appointed to Tube Alloys on 15 January 1943, and arrived in Montreal with his family on 7 February 1943.[36] The Montreal team designed anuclear reactor usingheavy water as a neutron moderator, but lacked the quantity of heavy water needed.[37] In August 1943, Churchill and Roosevelt negotiated theQuebec Agreement, which resulted in a resumption of cooperation between the United States and Great Britain, merging Tube Alloys into the Manhattan Project.[38]
John Cockcroft became director of the Montreal Laboratory in 1944. For safety reasons, he decided to build the reactor at the remote Chalk River Laboratories. With an eye on a post-war nuclear program, he had Pontecorvo andAlan Nunn May "debrief" Manhattan Project scientists who visited Canada, in practice spying for Britain. Unfortunately, Nunn May was also a Soviet spy.[39] Pontecorvo's second son was born on 20 March 1944, and was named Tito after theYugoslaviancommunist leader.[40] A third son, Antonio, was born in July 1945.[41] With heavy water supplied by the United States, the reactor at Chalk River, known as ZEEP, went critical on 5 September 1945.[42] In addition to reactor design, Pontecorvo also looked intocosmic rays, the decay of muons, and what would become his obsession, neutrinos.[43][44][45][46] He wrote 25 papers related to reactor design, although only two were published.[47][48][49] He also did some prospecting with his old firm, searching for uranium deposits nearPort Radium in theNorthwest Territories.[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.[51] Instead, on 21 February 1946, he accepted an offer from Cockcroft to join the BritishAtomic Energy Research Establishment (AERE). For the time being, he remained at Chalk River for the commissioning of the new NRX reactor in 1947. He was one of four physicists present in the control room when the NRX was started up on the night of 21–22 July 1947. At the time, it had five times theneutron flux of any other reactor, and was the most powerful research reactor in the world.[47] He acquired the nickname "Ramon Novarro" after theactor 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. Although he had previously taken steps to become a United States citizen, he instead became aBritish subject on 7 February 1948. He finally departed Chalk River for the United Kingdom on 24 January 1949.[52]
AtHarwell, Pontecorvo continued to be involved in reactor design projects. As a member of the Power Steering Committee (PSC), he was involved in discussions of the production and use offissile materials, and of the materials used in the construction of reactors.[53] In 1949, other Via Panisperna boys, particularly Emilio Segrè, began to press their claim over patents relating to the behaviour of slow neutrons, which were at the heart of nuclear reactor—and nuclear weapon—design. The AmericanFederal Bureau of Investigation (FBI), therefore, began looking into their backgrounds. It noted, from its 1943 Manhattan Project files, that Pontecorvo's siblings Giuliana, Laura and Gillo were communists, and that Pontecorvo and Marianne were probably communists too, and reported this toMI5 in Britain, andMI6'sKim Philby inWashington, D.C.[54]
In February 1950, Pontecorvo's Harwell colleagueKlaus Fuchs was arrested for espionage, and the AERE began to take security more seriously,[54] and Pontecorvo was interviewed by Henry Arnold, the security officer at AERE. 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.Herbert Skinner suggested to Pontecorvo that he apply for a newly created professorship at theUniversity of Liverpool, where Skinner held the Lyon Jones chair of experimental physics. In June 1950, Pontecorvo was offered the position.[55]
On 1 September 1950, in the middle of a holiday in Italy, Pontecorvo abruptly flew fromRome toStockholm with his wife and three sons without informing friends or relatives. On 2 September, he was helped by Soviet agents to enter the Soviet Union fromFinland. His abrupt disappearance caused much concern to many of the Western intelligence services, especially those of Britain, Canada and the United States, which 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.[56]
In 1952, Pontecorvo's potential role in the transfer of nuclear secrets to Russia was discussed in American newspapers.[57]
According toOleg Gordievsky, the highest-rankingKGB officer ever to defect,[58] andPavel Sudoplatov, the former deputy director of Foreign Intelligence for the Soviet Union,[59] Pontecorvo was a Soviet spy.[60][61] However, Sudoplatov misidentified Pontecorvo as the spy codenamed Mlad, who is now known to beTed Hall.[62] While Pontecorvo always denied working on nuclear weapons, in Canada, Britain or the Soviet Union,[63] he never confirmed or denied that he was a spy.[64] The actual evidence against him was flimsy.[65]Frank Close noted that the blueprints of the Canadian NRX reactor had made their way to the Soviet Union, andLona Cohen obtained a sample of uranium from the NRX after it started operation in 1947. Nunn May could not have been the culprit, so Pontecorvo is the prime suspect.[66] In the USSR, Pontecorvo was welcomed with honours and given privileges reserved only to the Sovietnomenklatura. He worked until his death in what is now theJoint Institute for Nuclear Research (JINR) inDubna, concentrating entirely on theoretical studies of high-energy particles and continuing his research on neutrinos and decay of muons. In recognition of his research, he was awarded theStalin Prize in 1953, membership in theCommunist Party of the Soviet Union in 1955 and theAcademy of Sciences of the Soviet Union in 1958, and twoOrders of Lenin.[67] He was awarded theLenin Prize in 1964 for his work on theweak interaction.[68] He began a lifelong affair with Rodam Amiredzhibi, the wife of poetMikhail Svetlov, in 1950.[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. As a result, the United Kingdom revoked his British citizenship on 24 May 1955.[70] Pontecorvo was not permitted to leave the Soviet Union for many years; his first trip abroad was in 1978 when he travelled to Italy for celebrations of Amaldi's 70th birthday. Thereafter, he made frequent trips to Italy and occasional visits to other countries. France refused to allow him to visit in 1982 and 1984, but relented in 1989.[71]
The scientific work of Pontecorvo is full of formidable intuitions, some of which have represented milestones in modern physics. Much of this involved the neutrino, asubatomic particle first proposed theoretically byWolfgang Pauli in 1930 in order to explain undetected energy that escaped duringbeta decay so that the law ofconservation of energy was not violated. Fermi named it the neutrino, Italian for "little neutral one",[72] and in 1934, proposed histheory of beta decay which explained that the electrons emitted from the nucleus were created by the decay of a neutron into a proton, an electron, and a neutrino.[73][74] Initially neutrinos were thought to be undetectable, but in 1945 Pontecorvo noted that a neutrino striking achlorine nucleus could transform it into unstableargon-37 that emits, with a 34 days half-life, after aK-capture reaction, a 2.8 keVAuger electron allowing its direct detection:[75][76][77]
Pontecorvo's 1945 paper credits the idea usingcarbon tetrachloride (CCl4) to the French physicistJules Guéron.[78] Experiments were conducted at Chalk River using the NRX as a neutrino source, but were unsuccessful, and were abandoned in 1949, after Pontecorvo had left.[47] The experiment was unsuccessful because, unknown at the time, nuclear reactors producedantineutrinos instead of neutrinos. In what is now known as theCowan–Reines neutrino experiment,Frederick Reines andClyde Cowan detected antineutrinos in 1955, for which Reines won theNobel Prize in Physics in 1995.[79]
The idea was taken up again in the search for solar neutrinos. Theoretically, the Sun produced neutrinos in the course ofnuclear fusion reactions. Pontecorvo creditedMaurice Pryce for this idea.[80] The most common, theproton–proton chain reaction in whichhydrogen is fused to formhelium produces neutrinos that are not energetic enough to interact with chlorine. However, the much less commonCNO cycle that producescarbon,nitrogen andoxygen does.[81] In the late 1960s,Ray Davis andJohn N. Bahcall detected solar neutrinos in theHomestake Experiment, for which Davis was awarded the Nobel Prize in Physics in 2002.[82] The experiment was the first to successfully detect and count solar neutrinos, but the number of neutrinos detected was between one third and one half of the predicted number. This became thesolar neutrino problem.[83] For a time, scientists contemplated the awful possibility that the Sun might have burned out.[84]
The problem had already been solved by Pontecorvo in 1968.[83] In 1959, a powerfulaccelerator (that was never built) was being designed, and he began considering experiments that could be performed with it. He contemplated a project investigating muons.[47]Julian Schwinger had hypothesised that particles experience the weak interaction through exchangingW bosons. The W boson would not be discovered until 1983, but a problem immediately surfaced.Gerald Feinberg pointed out that this implied that some interactions that had never been observed should occur, but conceded that this was only true if the neutrinos associated with electrons were the same as those associated with muons.[85]
In a 1959 paper, Pontecorvo listed 21 possible reactions involving neutrinos and noted that some of them could not occur unless the electron neutrino (ν
e) and the muon neutrino (ν
μ) were one and the same. (Thus, an inability to find those reactions would be evidence that there were two types of neutrinos.) This paper introduced this notation for neutrinos, which we use today,[47][86] and listed the reasons why he felt that having two types of neutrinos was "attractive from the point of view of symmetry and the classification of particles".[87] The prediction that neutrinos associated with electrons are different from those associated with muons was confirmed in 1962.[88] In 1988Jack Steinberger,Leon M. Lederman andMelvin Schwartz were awarded the Nobel Prize in Physics for the discovery of the muon neutrino.[89]
Pontecorvo's solution to the solar neutrino problem involved an idea that he had first considered in 1957 and developed over the following decade.[90][91] This was the idea that neutrinos may convert into other types of neutrinos, a phenomenon known asneutrino oscillation. Somewhere between the Sun and the Earth, electron neutrinos might transform into muon neutrinos. An important point was that for this to happen, neutrinos could not have zero mass, and therefore could not travel at the speed of light. The existence of the oscillations was finally established by theSuper-Kamiokande experiment in 1998 and later confirmed by other experiments.[92]
This prediction was recognised by the 2015 Nobel Prize in physics, awarded toTakaaki Kajita andArthur B. McDonald "for the discovery of neutrino oscillations, which shows that neutrinos have mass".[93] Pontecorvo also predicted in 1958 thatsupernovae would produce intense bursts of neutrinos.[87] Few scientists were more excited whenSupernova SN1987A was detected by neutrino detectors.[94]
Pontecorvo died inDubna on 24 September 1993,[47] afflicted byParkinson's disease.[95] In accordance with his wishes, half of his ashes were buried in theProtestant Cemetery in Rome, and another half in Dubna in Russia.[96] In 1995, in recognition of his scientific merits, the prestigious Pontecorvo Prize has been instituted by the Joint Institute for Nuclear Research. The prize, awarded annually to an individual scientist, recognises "the most significant investigations in elementary particle physics", as acknowledged by the international scientific community.[97] In 2006, Moscow historical societyMoskultprog unveiled an artistic plaque celebrating Pontecorvo's Moscow house at 9Tverskaya Street.[98]
Nobel Prize lecture
