by Close, Frank
Today, the weak force is recognized as one of the four fundamental forces of nature, along with gravity, electromagnetism, and the strong nuclear force. The discovery of the universal weak force is one of the most significant scientific advances of the twentieth century.14 Bruno also realized that this universality held the key to understanding how the electron, muon, and neutrino are related. Having confirmed that the muon is a sibling of the electron, he later applied this same idea to the neutrino, which he saw as having two varieties: one appears to be a sibling of the electron; the other of the muon. Bruno’s pairing of these fundamental particles into distinct families was the seed for the modern Standard Model of particles and forces.
These insights regarding the weak force and the choreography of particles are of Nobel Prize quality. Bruno Pontecorvo would be involved in all of them.
BRUNO THE INSTRUMENT MAKER
Between Bruno’s 1947 paper on the muon’s genealogy, and his investigation of its decay, he traveled to Europe to see his parents, visit Harwell, and spend a few days with his brother Guido in Glasgow.
At this time, Bruno and his colleagues at Chalk River were developing a novel instrument, in preparation for the experiment on muon decay. Today known as the proportional counter, this instrument measures the energy of radiation. If radiation has enough energy, it can knock electrons from atoms in an inert gas, leaving positively charged ions in its wake; low energy radiation, however, has no such effect. The liberated electron and the positively charged ion are known as an “ion pair.” In a proportional counter, the radiation passes through a small chamber filled with an inert gas, creating ion pairs. In the process, the radiation loses energy until eventually it is unable to create further ions. Thus the number of ions is proportional to the energy of the initial radiation. These electrically charged ion pairs create a signal in a detector, whose magnitude reveals the energy of the original radiation. Thus a proportional counter is especially good at measuring the energy of radiation, a technique in which Bruno Pontecorvo became an expert.
Meanwhile, in the physics department of the University of Glasgow, Samuel Curran, an expert on Geiger counters, was collaborating with John Angus, a research student of nuclear physics. Their goal was to develop new ways of detecting beta particles and measuring their energy. Their interests were very similar to Bruno’s. During his visit to Guido, Bruno called on the Glasgow physics department and met with Curran.
There is no record of whether Bruno was already aware of Curran’s work, and was making the visit to compare notes, or whether he learned of the project only when they met. In any event, the Glasgow team apparently felt that they had given out more information than they had gained, as a dispute arose over the invention of their technique. The relevant university documents note that, during the course of the Glasgow team’s work, “Curran had described the method to a member of the staff at Chalk River Laboratory, Canada.”15
In the August 21, 1948, edition of the journal Nature, Curran and Angus included a brief description of their new technique, as the second paragraph in a letter on the beta decay of tritium. One week later, Bruno and two colleagues described a similar idea in Physical Review.16 The Canadian group had submitted their paper in June, two months before the Glasgow team’s letter appeared. The submission date of the letter is not known.
With implicit reference to Pontecorvo’s visit the previous fall, the University of Glasgow record includes this comment: “One week after Curran’s letter to ‘Nature’ a rival paper from the Chalk River group (including Pontecorvo) appeared.”17 Given the similar expertise of the teams, it is possible that the ideas were in the air and would have developed independently in any event. As to who influenced whom, this we must add to the unresolved mysteries of this tale.
Bruno also visited Harwell “for a few weeks” during the fall, before flying to Italy on December 8. He gave a talk in Rome on December 17, which inspired the Italian state oil company, Agip, to use his neutron well-logging technique; Bruno supplied circuit diagrams of the electronics and other information.18
As part of Harwell’s normal vetting process, the British security authorities were already building a profile of Bruno Pontecorvo. They noted that he traveled back to Canada via Paris, where he spent New Year’s Eve in Montmartre with his former colleague Jules Guéron. The MI5 watchers reported that the trip seemed to be made for purposes of “jollification” and that “no other scientific contact was made.”19
The British authorities, with traditional xenophobia, suspected the political allegiance of French scientists in general, and of Frédéric Joliot-Curie, a declared communist, in particular. Bruno spent just twenty-four hours in Paris, and then passed through England en route to Canada on January 6. The Harwell security officer reported to MI5 that Bruno was a “straightforward fellow with no political leanings.”20
IN CANADA, AND SUBSEQUENTLY AT HARWELL, BRUNO PRESENTED himself as apolitical and unsophisticated in world affairs. In 1950, after Bruno’s disappearance, his brother Gillo told MI5, “My parents as well as [our eldest brother] Guido told me that they had the impression that Bruno after the war was no longer interested in politics and was avoiding any connection with communism, both in the form of literature and in personal contacts. My parents attributed this to his position at Chalk River and Harwell.” Gillo, who was especially close to his brother, saw through the charade. When Bruno visited Italy in June 1946, Gillo perceived that Bruno’s professed lack of interest in politics “was phoney, because Bruno was so well informed of details of certain questions that were so much at the heart of communism.” For example, Gillo pointed out that Bruno had a deep understanding of the motives of Yugoslavian dictator Josip Tito. Gillo concluded, “He must have kept thoroughly up to date with communist literature.”21
An example of Bruno’s tact can be seen in his reaction to the news that the atomic bomb had been successfully tested in New Mexico on July 16, 1945. When the news came in, everyone on the Canadian team began to discuss global politics, in an attempt to understand the implications of the test. The big question was whether the bomb would be used on Japan. In the memory of one of those present, Bruno alone was “certain of the answer.” He insisted that the Americans would have to use the bomb “for political reasons, before the Japanese surrender and before the Russians could play a role in their surrender.”22 As events transpired, this was completely accurate. Hiroshima was bombed on August 6 and Nagasaki on August 9. Late on the evening of the eighth, Stalin declared war on Japan. Soon after midnight Soviet troops invaded and annexed Manchuria, Inner Mongolia, and North Korea. Japan surrendered to the Americans on September 2, 1945.
NINE
MANEUVERS
1945–1950
THE BOMBING OF HIROSHIMA AND NAGASAKI NOT ONLY BROUGHT A conclusion to World War II; it also ended the collaborative wartime work on nuclear physics, and heralded the start of national nuclear energy programs—with both peaceful and military goals. In the UK, the development of nuclear power for peaceful purposes began at a new facility called the Atomic Energy Research Establishment, or Harwell.
Harwell’s goal was to design and build the first nuclear reactors in the UK and, indeed, in Western Europe. It was operated by the United Kingdom Atomic Energy Authority, and on January 1, 1946, was established under the formal charge of the Ministry of Supply. John Cockcroft left Chalk River to become the first director of the British reactor project. Bruno had been invaluable in the design of the reactor at Chalk River, so in 1945 the British invited him to join their own new venture.
That same year, Bruno had been offered several highly prestigious and lucrative positions at American institutions, but in the end he chose Harwell. His decision raised eyebrows, including those of Emilio Segrè, who later told the FBI that he believed Bruno’s choice to have been influenced by communist relatives in Europe, with dark hints of Soviet malevolence. According to Oleg Gordievsky, a KGB double agent who came over to the United Kingdom in 1985, Bruno Pontecorv
o had been a valuable source for the KGB since 1943. If this is true, then Segrè’s suspicions may have been correct.
Bruno had visited the University of Michigan, which wanted to hire him, and had received very attractive offers from the University of Rochester and the Radiation Laboratory at the University of California, Berkeley, home of Emilio Segrè. He also exchanged letters with luminaries such as Hans Bethe at Cornell, Robert Marshak at Rochester, Arnold Siegert at Syracuse, and the chairman of the physics department at MIT. Bruno dithered about his decision. In part this was the natural reaction of a talented physicist, much in demand, who wanted to weigh his options to achieve the best outcome for his career and his family. However, the correspondence also hints that Bruno was unsure about when he would be able to decide, as if he was not completely in charge of his own destiny.1
Several letters came from the University of Rochester. They offered Bruno a position as an associate professor at $6,000 a year, to begin “any time at your convenience or availability between now and Sept 1946.” The facilities were superb: there was “already a small cyclotron operating” and the university “[hoped] to build a uranium neutron reactor.” In addition to these obvious advantages, the authors of the letters pointed out that Rochester was only ninety miles from Cornell, home of leading nuclear theorist Hans Bethe. In the meantime, Bruno told Michigan that he would decide in January 1946, after seeing some people “at the British project.” This referred to Bruno’s discussions with James Chadwick about the restrictions Harwell would impose on him.
This was a difficult time for Bruno. For several months, he and other non-British members of the Canadian team had been severely restricted in their freedom. Security had become very tight as the test of the first atomic bomb approached. General Groves was suspicious of international collaborations, and the FBI was concerned about the number of foreign scientists involved, especially in Canada. This tension had been present for some time, but now it was growing worse. After the Normandy invasion in June 1944 and the liberation of Paris in August, several of the French scientists on the Anglo-Canadian project wished to return home and celebrate, but permission for these trips was denied.
In September 1944, Churchill wanted all visas for non-British members of Tube Alloys to be withdrawn. Around this same time, Halban visited France and talked to Joliot-Curie about developments in fission. He did this because he believed that he and Joliot-Curie held patent rights in the field. General Groves was furious about the visit, which created trouble for everyone. Bruno, like his French colleagues, wished to visit his relatives in Europe as soon as possible, but travel for all non-British members had been curtailed, and in the summer of 1945 Bruno’s request to visit Italy was rejected.
At this stage, James Chadwick was directing British nuclear policy from Washington. He decreed that any offer of employment at Harwell should carry the condition that Pontecorvo was forbidden to travel to mainland Europe to visit his parents. Bruno, however, would not tolerate such a long-term lack of liberty, especially now that the war had ended, and just before Christmas 1945 he visited Chadwick in Washington, DC, to discuss this.2 This initial meeting proved inconclusive.
In 1946, Bruno visited New York and Washington from January 23 to January 29 for the meeting of the American Physical Society, and it was then that his decision seems to have crystallized.3 During the conference, he was courted by several potential employers. General Electric thought he was still in the market, and on February 7 they wrote him with an offer. Bruno immediately declined, saying it would be a “pity to leave the Canadian pile just as research rather than engineering starts” and that he would stay in Canada “for at least one year.” At the conference Arnold Siegert, a physicist at Syracuse University, also inquired about Bruno’s availability, but on February 13 Bruno thanked him for asking but stated unequivocally, “I am staying here.”
Meanwhile, on February 4, Otto Frisch had written from Los Alamos to extol the scientific excitement that the new laboratory in Harwell would offer, and encouraged Bruno to accept a position there. Frisch also mentioned “Uncle James” (Chadwick), and remarked that “the big show” in Canada—namely, the NRX reactor—was expected to start in mid-March.
Bruno finally informed Harwell that he would accept their offer only if he was allowed to travel. He stressed that one of Harwell’s attractions for him was that it would allow him to be closer to his Italian roots and family, after ten years of absence. Harwell must have eventually agreed to his terms, as on February 21 Otto Frisch wrote of his pleasure that Bruno had accepted the job in England.4
Hiring Bruno Pontecorvo was a coup for British science, despite the government’s reluctance to employ non-British nationals. This xenophobic attitude, which had surfaced when Bruno had been recommended to Chalk River in 1943, now came up again. Harwell needed him desperately, but his Italian origins threatened to kill the deal.
The question of his reliability was raised with General Groves. Chadwick confirmed that Pontecorvo would be employed by the British government at the Harwell nuclear establishment, and, to break the impasse, he suggested that Pontecorvo would “probably take steps to acquire British citizenship.” In fact, Bruno became a naturalized British citizen later, on February 7, 1948, on the basis that he had been involved in a British project for five years. A report to Prime Minister Clement Attlee, specifically about Bruno, commented, “It is believed Pontecorvo had already taken out first papers in the US but had expressed a preference for British naturalization if he could get it.” In hindsight, this may reflect Bruno’s concerns regarding American attitudes toward communism.5
The lengths to which the British went to obtain Bruno Pontecorvo—even sending a report to the prime minister—shows how important the Italian physicist was for them. The number of quality institutions that had bid for him in North America also testifies to his international stature. Bruno held all the cards. In 1946 he could make a career wherever he wanted.
Despite claims that Bruno’s move to Harwell was related to espionage, many of his colleagues have argued that there were innocent reasons for his choice. Given that John Cockcroft left Canada to become the director at Harwell in 1946, and that several other Canadian colleagues made the move too, continuity in research was one obvious advantage. One Harwell scientist, Godfrey Stafford, who later became the director of Harwell’s neighbor, the Rutherford Laboratory, judged that the atmosphere at Harwell was key: “At Harwell at that time there was a blank canvas,” he explained. He recalled that an attraction of Harwell was the implicit invitation, “Come here and do what you like.”6 So it is plausible that Harwell, with its close proximity to the leading physics departments at the Universities of Oxford, Bristol, and London, had a competitive edge, notwithstanding Bruno’s North American offers. Being nearer to Italy after his decade-long absence was also attractive, as he himself claimed.
Segrè’s allegations that Bruno’s move had communist motives, which he made in 1949 to the security authorities, may have been sour grapes because Bruno had chosen Harwell over Berkeley; nonetheless they would later have consequences. Segrè’s report coincided with the growing anticommunist hysteria in the United States, fueled by Senator Joseph McCarthy and the House Un-American Activities Committee, who pursued witch hunts against communists in government, science, and Hollywood. Eventually, their spotlight would shine on Bruno Pontecorvo.
MEANWHILE, ATTRACTIVE OFFERS OF EMPLOYMENT IN THE UNITED States continued to come Bruno’s way. In 1947 Bruno visited Cornell, and Hans Bethe offered him a position as a tenured associate professor at a salary of $7,000 a year, requiring him to teach “no more than 6 hours a week.”
The tone of Bethe’s letter, written as if Bruno was still open to offers, suggests that even then Bruno’s mind was not yet totally made up. This may indeed have been the case, as Bruno stayed in Canada until the end of 1948, while acting as a consultant to Harwell. The Production Pile Discussion Group was formed to advise the British reactor design team. James Kendal
l, the engineer responsible for pile design at Harwell, reported back from one meeting in Canada that the help from Pontecorvo “was worth that of all the others put together.”
PRIVATE LIFE IN CANADA
Bruno’s time in Canada was when fault lines in his public image first began to appear. For example, he presented himself as apolitical to friends and colleagues, but he was in reality a member of the Communist Party. His reasons for keeping this a secret are obvious, of course. The fact that he succeeded so comprehensively, however, shows that he was not naive in these matters. What’s more, this ability to keep inconvenient truths hidden shows that he was more sophisticated than he often appeared to friends and colleagues. As a family man, his persona was that of a naive extrovert, the life and soul of the party, always pleased to be in the limelight; his home life, on the other hand, was more complex.
Laura Fermi recalled two incidents that reveal the nature of the man and his private life.
In January 1944, during a visit to Chicago, Bruno called on the Fermis at their home. He had broken his leg skiing, and was hopping about on crutches. This made him the center of attention, which stimulated him to prance around even more dramatically.7 Having gained people’s attention, he milked it. Such was the public image of Bruno Pontecorvo, the showman. However, even though everyone perceived him as happy-go-lucky, he described himself as having an inferiority complex.8 This he traced back to his childhood, when, as the fourth of eight talented children, he had to compete for attention. His handsome exterior and bubbly personality were thus cloaks that helped mask his insecurity.