Half-Life: The Divided Life of Bruno Pontecorvo, Physicist or Spy

by Close, Frank

  In his logbook, Bruno suggests using the magnetic fields of the cyclotron to curve the path of “the electrons” (the beta particles produced when the H4 isotope decays), and from this deduce the transient presence of quadium.20 If its existence was established, methods of producing the isotope in greater quantities could be developed. The discussion continued on November 3, when Bruno noted the possibility of performing the experiment with an arrangement of electronic counters.

  Pontecorvo also evaluated ways of detecting pions. On this same date, he had some ideas on “Fission [caused] by mu meson.” However, this last phrase is crossed out and not developed further. In addition, he recorded some thoughts about the strange particles—thoughts that would mature a couple of years later. And, tantalizingly, he considered the possibility that the decay of a muon produces two neutrinos of different characters. This set of ideas is a rough outline of Bruno’s initial hopes for his particle physics experiments at Dubna, which would have no immediate military significance.

  After three days, Bruno’s flow of ideas is interrupted, as the November entries suddenly end halfway down page 9 of the logbook. The writing on the lower half of the page is inverted. The explanation for this is given below.

  The above burst of activity seems to reflect an initial period of brainstorming, after Bruno’s arrival at Dubna. At the end of this period, he was apparently assigned another task until September 1951. Whatever he did in the intervening ten months was not part of that original program, and was thus not recorded in the logbook.21 When he resumed writing in the logbook ten months later, he turned it upside down so that the final page became the first. He then maintained his daily research log for Dubna until, by March 1952, he had worked his way “forward” to page 9. When Bruno first resumed writing, in September 1951, experiments at the Dubna cyclotron were under way, and his logbook was quickly filled with data. Indeed, his particle physics research, which the authorities would later claim to be his sole activity in the USSR, occupied him full-time from the end of that year onward.

  Bruno’s decision to record these data in the rear of the original logbook, separate from the initial inquiries, is a deliberate act. It would be natural for him to retain space in the logbook for any further work on the original questions; hence the new material is recorded from the rear. However, it is clear that he had given his employers all the information they required on the question of fission and H4 particles, as there is no further mention of them.

  The first notebook having been filled, Bruno began a new one: “начато 1952” (started 1952). This and subsequent logbooks record his ongoing research interests, which from then on appear exclusively to involve experiments at the Dubna cyclotron. Other pages constitute lesson plans or drafts of research papers. This continues until his “coming out” in 1955.

  So what can we conclude about Bruno Pontecorvo’s first year in the Soviet Union? For one thing, the questions involving H4 and fission, which occupied Bruno during his first days at Dubna, are in marked contrast to his subsequent work there.

  The entries from those first days of November appear to be responses to problems that Dubna had grappled with before Bruno’s arrival. It is possible to view them as genuinely “pure” physics questions, but their nature and scope make them more obviously applicable to strategic issues related to the release of energy from atomic nuclei. Specifically, the first two entries suggest that the Soviet scientists are looking for ways to increase the energy released by the fission of strategically important elements (that is, elements of relevance to energy release in weapons or reactors, including, but not limited to, uranium and thorium). Such an interest would be esoteric in the context of pure nuclear physics. And to identify the phenomenon with certainty, very precise measurements of the energies of neutrons would be needed; hence a reason for the initial entry. The logbooks suggest that, upon arriving at Dubna, Bruno was consulted on questions relating to the production of atomic and thermonuclear bombs.22

  Once this particular task was completed, he was consulted on other secret matters, which explains the ten-month gap in the record.23 We know that, in general, Bruno discussed aspects of nuclear reactors with Pomeranchuk and others. If he performed any detailed work for the Soviets on reactor physics (or uranium), it must have taken place during this gap, between November 1950 and September 1951. After that, he then took up full-time work on particle physics at Dubna.24

  STRANGE PARTICLES

  In 1951, Bruno was thirty-eight years old, and in the prime of life. Over and above his initial significance for the Soviet government’s nuclear program, he played an active role in the work at Dubna. He both inspired new lines of research, and helped drive existing lines forward. The most significant new line of inquiry he was involved in dealt with the so-called strange particles.

  Three years before Bruno fled to the USSR, physicists had discovered “strange” particles in cosmic rays. They were dubbed strange because they lived for about a hundred-millionth of a second, which, although short by everyday standards, is about a million billion times longer than expected. To illustrate the unexpected duration of these particles, one scientist said, “It’s as if Cleopatra fell off her barge in 40 BC and hasn’t hit the water yet.”25

  As we saw earlier, one goal of the research at Dubna was to understand pions—the particles that are the embodiment of nuclear energy. A single pion contains about one-seventh of the energy normally locked within a proton or neutron, similar to the amount liberated in a single fission of uranium. So the discovery of a “strange” sibling, about three times heavier (and thus possessing three times the energy of a pion), was tantalizing. This new particle became known as the kaon, or K-meson.

  The Dubna synchrocyclotron was powerful enough to make pions, but in 1950 did not have enough energy to make the more massive kaon, and solve the mystery of strangeness. The enigma began to be unraveled after another strange particle turned up in the debris from cosmic collisions in 1951: the Lambda. A Lambda is like a neutron that carries this mysterious strangeness. And, in the Soviet Union, it was Bruno who made the breakthrough that helped scientists understand it.

  In 1951, soon after the discovery of the Lambda, Bruno wrote a classified paper in which he drew attention to the anomalous properties of the strange particles. This articulated the possibility that “the process of formation of these particles is not the reverse of their decay.”26 In other words, even though strange particles are produced by the strong force in pairs, they decay individually due to the weak force—the same universal weak force that Bruno helped identify in 1947. A kaon and a Lambda are born together but die alone. Bruno proposed the idea of what is now called “associated production”: this states that strange particles are born in pairs, but then part company. The strong interaction only operates when the two of them are close together, after which they are freed from its snare and the weak force takes over. The weak force is very feeble, however, compared to the strong, which is why an isolated strange particle can survive for an unexpectedly long time.

  Bruno may have been the first scientist to make that insight, but he remained flummoxed as to what made particles such as the kaon and Lambda “strange,” whereas the proton, neutron, and pion were “normal.” Other scientists would solve that conundrum.

  The answer is that, just as some particles carry electric charge, while others do not, some particles carry this attribute of strangeness. You cannot create a positive or negative electric charge in isolation; every positive charge must be balanced by a negative charge, and vice versa. A similar idea applies to strangeness. The kaon has, let’s say, one unit of positive strangeness, and the Lambda has a negative unit of the same value. The protons and neutrons in the nucleus have no strangeness, so when a proton in a cosmic ray or an accelerator collides with a nucleus, the creation of a Lambda (negative strangeness) must be counterbalanced by the simultaneous appearance of a kaon (positive strangeness). The total strangeness remains zero.

  Bruno, however,
had not conceived of positive and negative strangeness, so the system he developed only required that strange particles appear in pairs. For example, we now know that a collision between two neutrons cannot spawn two Lambdas, as the latter pair carries two negative units of strangeness, whereas Bruno mistakenly thought that this reaction was possible.

  Although Dubna lacked the energy necessary to make kaons, it did have enough to produce Lambdas. In a Dubna report from 1953, which was classified as secret, he proposed a search for the reaction described above, in which two colliding neutrons spawn two Lambdas. However, when Bruno and his colleagues carried out the experiment in 1954, there was no sign of Lambda particles. Indeed, no such reaction has been seen to this day.

  By 1954 the Cosmotron had begun operating in the United States, smashing protons into targets at energies far in excess of what Dubna could achieve. The Cosmotron could even make beams of pions, and direct them at atomic nuclei. The result: a kaon emerged along with a Lambda. Thus Bruno Pontecorvo had correctly identified the phenomenon of associated production, but had failed to realize its deeper significance.27

  In any case, he received no credit for his insight. Like the tree that falls in the forest with no one to hear, Pontecorvo’s work was unknown outside of Dubna. That same year, in the US, theorist Abraham Pais had independently come up with the idea, and many textbooks credit him alone with the discovery.28

  The hypothesis had far-reaching consequences. In 1961, the concept of strangeness inspired American physicist Murray Gell Mann to propose the “Eightfold Way” scheme for classifying strongly interacting particles. This in turn led to discovery of a deeper layer of reality that exists within these particles: they are made of smaller particles called quarks.29

  LIFE IN THE USSR

  When the Pontecorvos arrived in Dubna in 1950, barrack dormitories surrounded the town. Beria’s Gulag prisoners, with uniforms and shaved heads, were building the roads. Bruno saw these workers, but gave them no special attention. Years later he said that he didn’t know who they were or why they had been imprisoned, and that “there are laws in every country and when broken this can lead to imprisonment.” Yet he was also aware that “there had been great trials and death sentences. I knew people were detained in camps and prisons. I thought, as a million other communists, that this was the inevitable consequence of the class struggle still in process.”30

  Bruno too was still a prisoner, albeit in a gilded cage. On one hand, living in Dubna gave him plentiful access to food and conveniences of a higher quality than those available to the general Soviet populace at that time. However, if anyone wanted to leave the limited area around the laboratory encampments, they had to obtain special permission. For Bruno the restrictions were even tighter. No documents or books could be taken from the lab for work at home. His logbooks were locked in the laboratory safe overnight. And there were other, more serious, constraints as well.

  Whenever he left his house, even for the short walk to the laboratory, two guards accompanied him, supposedly for protection. Given that Dubna was a restricted area whose inmates—for at times that is how Bruno saw them—were specially selected, the concept of protection seemed absurd. The guards did play an important role, however: they prevented Bruno from speaking to strangers as he walked. One of the guards had a habit of whistling, which annoyed Bruno. When this man was assigned to accompany him, Bruno would walk fast and get to his destination quickly to minimize the time spent in his presence.

  This enforced isolation even extended to his social life.

  In the West, it had been customary for scientists to visit one another’s homes, for their families to have dinner together and take communal trips on the weekends. Bruno experienced none of this during his five secret years at Dubna. He worked closely with colleagues at the laboratory, had lunch with them in the canteen, and then returned to his home in the woods, always accompanied by his KGB minders. Social contact with his fellow scientists, whose houses were in glades among the trees near his own, was nonexistent. No one ever invited him to their home. Bruno, the gregarious extrovert who thrived in company, was trapped by a cordon sanitaire.

  BRUNO MAXIMOVITCH PONTECORVO

  We have seen how Pomeranchuk would refer to Bruno only as “the professor.” Even in Dubna itself, this denial of Pontecorvo’s identity was the norm. He was simply the professor, whose first name was Bruno.

  IMAGE 15.2. Bruno and Marianne at Dubna. (COURTESY GIL PONTECORVO; PONTECORVO FAMILY ARCHIVES.)

  Russians traditionally address one another by their given name and their patronymic—this second name essentially meaning “son or daughter of X.” Bruno, however, was called simply “Bruno,” the only identity allowed for the professor. This caused embarrassment. The formalities of the Soviet Union in the 1950s meant that colleagues could no more address him as “Bruno” than a member of the general public in England could have greeted the prime minister as “Winston.” The problem was solved when one of Bruno’s senior colleagues asked him for the first name of his father. On learning that it was Massimo, his colleagues agreed to call him “Bruno Maximovitch.” Once his presence was officially acknowledged in 1955, he would be known socially and professionally as B. M. Pontecorvo.

  Even his children had lost their family name. At school they were known as Gil, Tito, and Antonio Ivanov. Gil confirmed that Bruno’s presence in Dubna was a state secret, necessitating the peculiar name change: “Being called Ivanov was strange, but we didn’t care much.”31 Bruno’s assessment was more blunt: “Some things had to be kept secret.”32

  Bruno and his family were effectively in exile. In the judgment of a former head of MI5, it sounded like the Soviets didn’t trust him.33 There are similarities here to the cases of Guy Burgess and Donald Maclean, of the infamous Cambridge Five spy ring. Following their defection to the USSR in 1951, Burgess and Maclean were exiled to the industrial city of Kuibishev for several months, while “a thorough check of their credentials was made to ensure that they had not been turned.” Their presence in the Soviet Union was also initially kept secret; Philby’s role in both their defection and Bruno’s had to be protected. According to Chapman Pincher, “The KGB always put the safety of its agents first, [and preferred] silence to any short-term propaganda gain.”34

  Whereas Bruno suffered, Marianne considered the isolation “not a problem,” as she had been almost pathologically averse to social life to begin with.35 In the United States, when she had balked at meeting the Fermis, Bruno had admonished her. In Dubna, there were no social appointments to avoid. Marianne, who was “always silent and a little distant,” was beginning to “forget names, appointments and things.” She would “read for hours, or remain lying on the bed, gazing out of the window at the trees.”36 In these descriptions, given to Miriam Mafai years later, Bruno reveals Marianne in a state of depression, en route to the psychological breakdown that would eventually lead to long periods in a sanatorium.

  BRUNO AND POLITICS

  For Bruno’s first five years in the Soviet Union, he was unknown to all but a handful of people there. To the Western media, his disappearance remained an enigma, as it did to his family and friends.

  Bruno recalled, “I read Pravda each day, and occasionally saw L’Unità. Even so, it was difficult if not impossible to get a rounded view of world events.”37 An example of this difficulty occurred in 1953, shortly before Stalin died, during the coverage of the so-called Doctors’ Plot, in which several Jewish doctors were arrested “for aiming to remove by harmful treatment the lives of the active leaders of the USSR.” Bruno read this coverage in the cloistered confines of Dubna. He decided that the claims might be true: “I believed it. Not the actual words, as they were simply propaganda, but the substance could have been true. In history there are plenty of examples of political assassinations around the world. It was 1953, the height of the Cold War [and] of rebuilding the Soviet economy. Killing Stalin could have been a real objective.”38

  One of the doctors died in
prison, and the others would undoubtedly have been shot if Stalin himself had not died on March 5, 1953.39 Within weeks, the state news agency announced that the whole case had been a “misunderstanding” and that the surviving doctors had been released. Speaking in 1990, Bruno interpreted this as a sign of a “political struggle taking place within the Kremlin. I did not know much about this fight. It might seem inexplicable now, but [at the time] I believed [the reports].”40

  Stalin maintained control by the periodic use of terror. Bruno was not overly concerned about this. He defended Stalin’s harshness on the grounds that it had saved the October Revolution and defeated Nazism, drawing analogies to the Jacobin Reign of Terror that had saved the French Revolution.

  For many in the Soviet Union, Stalin was a hero, who had saved them from the Nazis. Nonetheless, he feared the power of the United States, as well as the other Western nations, who had attempted to crush the Bolshevik Revolution in its infancy, and then developed atomic weapons without informing their Soviet ally. One reason Fuchs, Nunn May, and other sympathetic Western scientists had passed atomic secrets to the Soviets was because they believed it was vital for the West’s power to be balanced by that of the USSR, which would require both sides to possess nuclear weapons.

  At the time of Stalin’s death, Bruno, like millions of others in the USSR, still had great faith in the communist state and its leaders. The entire nation went into mourning. Hundreds of thousands filed past Stalin’s coffin, and many died of suffocation within the crush of people in Red Square. Bruno, however, was not among them; he was still trapped in Dubna, listening to the radio’s coverage of the events. Gil, by then fourteen years old, took note of the occasion: there was “a week or ten days of solemn music on the radio. It was the first time I appreciated classical music.”41