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

by Close, Frank

  In those days, few people owned a telephone, so invitations were sent in the form of brief letters. Bruno was very busy at the laboratory at the end of December, which made it difficult for him to book a table for a planned dinner on New Year’s Eve. The mail, however, was very efficient, as demonstrated by the fact that Bruno wrote to Marianne at 11:00 p.m. on December 30—to make arrangements for the thirty-first. His letter arrived in time and they successfully met, at 9:00 p.m. on New Year’s Eve; in accord with Bruno’s instructions, they wore “evening dress.”15 They dined, celebrated the arrival of the New Year, and then went to a club where they played pile ou face until 2:00 a.m.16

  IMAGE 3.1. Bruno Pontecorvo and Marianne Nordblom in Paris, c. 1937. (COURTESY GIL PONTECORVO; PONTECORVO FAMILY ARCHIVES.)

  Bruno was struck by Marianne’s “slender grace, sweet temperament and long careful silences” as well as her “childlike need for protection.”17 Everyone who knew Bruno Pontecorvo remembers the charismatic extrovert, who loved sports, games, and performing tricks such as riding a bicycle backwards or balancing a stick on his foot or nose. It would seem that Marianne’s experience was the same; after a lunch together in February, she wrote, “Italian—he’s stupid.”

  MEANWHILE, BRUNO HAD BECOME GOOD FRIENDS WITH THE JOLIOT-CURIES. Frédéric and Irène sailed in the summer, skied in the winter, and researched together at the Radium Institute. Bruno’s interest in sports and his charismatic personality made him a favorite with the couple. He was a frequent visitor to their home.

  Liberal ethics ran deep for the Joliot-Curies. Irène’s parents, Pierre and Marie Curie, decided not to patent the process for extracting and purifying radium because radium was an element that “belongs to all the people and [is] not meant to enrich any one person.”18 As a young woman, Irène Curie had accompanied Marie around the battlefields of World War I to X-ray wounded men. This helped forge her belief in pacifism.

  Irène was often blunt and undiplomatic. A telling example can be found in a letter in which she declined an invitation to some tedious function. Her secretary had included the formal mantra “regret that I cannot attend,” only for Irène to delete it. Like Edith Piaf, she declared, “Je ne regrette rien.”19 Her husband, by contrast, was thoroughly outgoing and loved spending time with people. Tall and slim, with a distinguished, lean face and dark, brushed-back hair, Frédéric had socialism in his genes. His father, who was reasonably prosperous, had been exiled from France in 1871 because he had fought for the Paris Commune, the government of the workers’ revolution. In 1880 he was granted amnesty and allowed to return.

  Frédéric Joliot, born in 1900, was the sixth and youngest child of the family, and in his youth was so enthused by Pierre and Marie Curie that he had a photograph of them mounted on his bedroom wall. In 1924, he went to Marie’s laboratory and asked for a position as her assistant. Marie assigned him to work with her austere daughter, Irène, who was just completing her doctorate.

  At first, Frédéric was an outsider in France’s hierarchy, which was very much linked by family ties. Some in the establishment felt he was selected by Irène as the “prince consort to the princess,” his “coarse good looks” fitting that image.20 But Joliot was a first-rate scientist and a good judge of ability. He correctly believed that he was better than many who sneered at him, and this made him bitter. Even after he and Irène won the Nobel Prize, his genius was only grudgingly recognized.

  ISOMERISM

  Bruno’s scholarship would last for a year.21 In 1937 he declined the chance to apply to the University of Rome for a tenured post. His reasons are not known, but the fascist situation in Italy surely did not help. By this stage, his relationship with Marianne was becoming serious, which may have also played a role, and he had begun to impress Frédéric Joliot-Curie. His work also attracted the attention of Igor Kurchatov. The link between Bruno Pontecorvo and Igor Kurchatov stemmed from a common interest in a strange nuclear phenomenon known as isomerism, which Kurchatov had recently brought to light in Moscow.22

  In 1935 Igor Kurchatov was inspired by the work of the Via Panisperna Boys, and while checking the slow-neutron phenomenon for himself, he noticed something unusual: after a neutron of a given speed hits a nucleus, the radioactivity of the resulting isotope varies from one experiment to the next, even though it has the same number of neutrons and protons. With this exception, the new isotopes appear to be identical. This phenomenon—in which isotopes of the same mass give off different levels of radioactivity—became known as isomerism, from the Greek for “equal masses.” Kurchatov discovered the first definitive example of an isomer when he bombarded bromine with neutrons in 1935. Within months, the number of isomers began to grow rapidly. The question was: What was happening?

  Frédéric Joliot-Curie held Kurchatov’s work on isomers in high regard. This respect was reciprocated: Kurchatov followed the work of the French group closely. When Bruno arrived at the Joliot-Curies’ laboratory, he started to investigate Kurchatov’s phenomenon himself. His success in this quest helped solidify his emerging reputation as a leading expert in the use of neutrons in nuclear physics.

  Since the year of Bruno’s birth, the atomic model of Niels Bohr had successfully posited that electrons in atoms cannot go wherever they please, but are restricted, like someone on a ladder who can only step on individual rungs.23 When an electron drops from a rung with high energy to one that is lower down, the excess energy is carried away by a photon of light. The spectrum of these photons reveals the pattern of energy levels within the atom.

  By the time Bruno started doing research, the fundamental explanation for this behavior had been found in the equations of quantum mechanics. When Niels Bohr proposed his model of the nucleus, built from neutrons and protons in contact, many scientists wondered if quantum mechanics not only applied to the electrons in atoms but also determined the energy levels of the neutrons and protons in atomic nuclei. Bruno’s experiments in Paris helped confirm that quantum mechanics does indeed control the atomic nucleus.

  Nuclei in “excited” states, with one or more protons or neutrons on a high rung, give up energy by emitting photons of light, much like electrons do. The main difference between the case of electrons and the case of atomic nuclei is the nature of the radiated light. In the former the light may be in the visible spectrum, made up of photons with relatively low energy, whereas in the latter (the case of nuclei) the light consists of X-rays and gamma rays, whose photons have energies that are up to a million times greater.24 Whereas light radiated by atomic electrons may be seen with the eye, the photons emitted from nuclei can only be detected with special instruments. Pontecorvo became an expert in this art.

  If this were the whole story, it might not be particularly remarkable that nuclei can form excited states, similar to those of atomic electrons. Typically, a neutron might be captured by a nucleus and form an excited state, which then decays by emitting gamma rays, leaving a highly stable state. If this stable state were the lowest rung of the ladder—the ground state—all would be straightforward. However, there was an unexpected and tantalizing development: there seemed to be various different end-states (the isomers), all with the same number of protons and neutrons, and all highly stable, lasting for more than a day in some cases.

  The addition of isomers to the rich variety of nuclear states created confusion, until quantum mechanics provided the explanation. Quantum mechanics had successfully described nature at atomic scales, 100,000 times smaller than the macroscopic world where Newton’s classical mechanics rule. The nucleus is smaller than the atom by a similar factor, so it was a revelation to discover that quantum mechanics applies there too. For example, according to the theory, if a nucleus is rotating much more rapidly—has more angular momentum—than the ground state, it can have unusual stability. The origin of so many highly stable isomers was thus explained: they were nuclei with a large amount of angular momentum.

  Bruno Pontecorvo entered this new field with enthusiasm. Driven by so
me theoretical ideas of his own, he worked mostly alone, receiving occasional advice from Frédéric Joliot-Curie. During the first half of 1937 he began what would become excellent pioneering work in the field.

  1937

  It’s said that in years past, Parisians did not include August in their diaries; the city closed down and its residents departed for a summer vacation. In 1937, Marianne accompanied the family she worked for to Boissy L’Aillerie, a village north of Paris, to stay at a delightful hotel: L’Oiseau Bleu. With a park where the children could roam freely while Marianne looked after them, the location was ideal. Bruno, meanwhile, was in the Italian Alps, and sent Marianne a postcard of San Martino di Castrozza, a beautiful mountain resort. He briefly hoped that she might be able to join him there, but on September 1 he wrote her at L’Oiseau Bleu to say that he would have to leave on the twelfth to attend a congress in Venice.

  Bruno and Marianne were apart until the start of October. That same month, Bruno delivered his first public report on his work on isomers at a congress at Paris’s Palais de la Découverte.

  Meanwhile Frédéric, now thirty-seven years old, was appointed as a professor at the Collège de France, the most prestigious post in France. His eminence brought influence along with it. He persuaded the National Funds for Scientific Research to buy an old electrical plant in Ivry-sur-Seine, a suburb southeast of Paris, and convert it into a nuclear physics laboratory. His plan was for this lab to provide man-made radioactive elements for use in research.25 He assembled a small but talented team, which included Bruno. Irène, meanwhile, remained with her own research team at the Radium Institute.

  There were two members of Frédéric’s group who would have a significant impact on Bruno’s life and career: Lew Kowarski and Hans von Halban. Kowarski was large, a veritable Russian bear who could have become a concert pianist had his fingers not grown too big for the keys.26 In 1937 Kowarski was thirty years old, and lacked confidence, fearing that he was already too old to make a mark with his talent for electronic gadgets. In Kowarski’s opinion, “Pontecorvo and von Halban were the two most outstanding personalities in the laboratory after Joliot.”27

  At the age of twenty-nine, Halban was already an established physicist. Bruno was still only twenty-four years old, a student, but, with his work on isomers now added to his earlier research in Rome, he was rapidly gaining an international reputation. Halban and Bruno shared a love of outdoor sports, especially mountain climbing and skiing. In terms of personality, however, they couldn’t have been more different. Whereas Bruno was everyone’s friend—the stereotypical warm, extroverted Italian—the arrogant Halban, who hailed from from Leipzig, was “intensely unpleasant, brutal with the weak,” and carried himself “like a Prussian officer.”28

  Kowarski had developed an electronic gadget to measure radiation over a greater range of intensity than was previously possible. Joliot-Curie had no need for it in his research at the time, so in early 1938 Kowarski consulted Bruno, who, he knew, “could teach me a lot.” Kowarski’s impression was that Bruno wasn’t exactly discouraging but nevertheless gave him the unspoken message, “Why should I be interested; it doesn’t look promising and you’re too old anyway.” This seems unlike the Bruno of most people’s memory, and it probably reveals more about Kowarski’s self-image than about Bruno’s opinion. However, Bruno did make an offer that had profound consequences: he introduced Kowarski to Halban.

  Bruno’s lack of interest in Kowarski’s overtures might also provide a glimpse of the personal pressure that Bruno was under at the time. For Bruno, the carefree joy of 1937 was now replaced by worry. His financial situation was insecure, but more serious was the looming responsibility he now had, as Marianne was expecting their child later that year. Added to this was the emotional worry regarding her visa, which was due to expire soon afterward. On January 4, 1938, Marianne left her lodgings to live with Bruno, at the Hôtel des Grands Hommes, on the Place du Panthéon.29

  On July 5, 1938, Marianne celebrated her twenty-first birthday with Bruno. On July 30, their first son, Gil, was born. Marianne’s position was grim: she was unmarried, and in six weeks she would have to leave France. It isn’t clear why Bruno didn’t marry Marianne at this time, since it would appear to have been the obvious means of enabling them to stay together with Gil in France. To make matters worse, Marianne’s baby was apparently unwelcome in her Swedish family home.30 Whatever the reasoning was behind this attitude, the reality was that Gil would be unable to come to Sweden with Marianne.

  So Marianne and Bruno placed Gil in a residential nursery, Le Nid (The Nest), on the avenue de la Terrasse in Montmorency, a northern suburb of Paris. In September, when Gil was only a few weeks old, Marianne and Bruno made their way to her home in Sweden. En route they visited Holland and Denmark, where they stayed in youth hostels, and they spent the last week of September in Copenhagen, where Bruno attended a physics conference. Here he met one of his friends from Rome, the theoretical physicist Gian Carlo Wick. Years later, Wick recalled seeing Bruno, in the company of a “very pretty Swedish girl.” He was surprised by the change in Bruno’s political attitudes. He remembered that during their time in Rome Bruno had been uninterested in politics, but now “he was very keen about international events.” Wick was especially struck by “the strength of his belief in the USSR.”31

  Bruno and Marianne at last reached Sandviken, her hometown in Sweden. Two years earlier, shortly after her nineteenth birthday, Marianne had left home, carefree and set on adventure. Now she returned to the small, conservative town, twenty-one years old, her Italian boyfriend in tow, their six-week-old son left behind in a French nursery. In 1938, being an unmarried mother was not accepted as easily as it is now.32 Bruno returned to Paris alone, after a few days. This would be the only time he visited Marianne’s family.

  Later in her life, Marianne would suffer from depression and experience chronic periods of mental breakdown, which required treatment in a sanatorium. One can only imagine her distress in 1938, as a new mother subject to postpartum depression, her child a thousand miles away, across borders closed to her by the bureaucracy of visas.

  AFTER LEAVING SANDVIKEN, BRUNO MEANDERED BACK TO PARIS. HIS first stop was Stockholm, where he attended another physics congress. On October 11, he went to dinner at the home of Professor Manne Siegbahn, the Swedish Nobel laureate; of the thirty guests, only Bruno and Gian Carlo Wick were not in evening dress.33 The next day he met Lisa Meitner and Niels Bohr.

  Meitner, who was Jewish, had escaped from Nazi Germany just two months earlier. She had been working in the same laboratory as Otto Hahn, with whom she had performed experiments that would lead to the discovery of nuclear fission in December. Bohr, having offered the world both his conceptual picture of the nuclear atom and his celebrated explanation of nuclear structure, had established himself as the foremost theorist in atomic physics. Bohr was based in Copenhagen, but visited Stockholm regularly. (It would be on one such visit early in 1939 that he would learn from Meitner about fission’s awesome potential.) Bohr’s presence in October 1938 was the excuse for another “grand dinner,” as Bruno described it to Marianne in his second letter from Stockholm. He ended with a P.S.: “Don’t fail to give my best wishes to your parents and to your brother.”34

  Bruno’s journey back to Paris was difficult. He traveled via Cologne, Germany, and then spent five hours at the Belgian border because he didn’t have a visa. He had to return to the Belgian consulate in Germany to obtain one. When he finally arrived at the Gare du Nord in Paris, he spent four hours with the police commissariat and paid 150 francs to amend his French visa due to a recent change in the law.35 The next day he visited Gil at the nursery.

  The following Saturday, he was invited to dinner at Halban’s home. The invitation, which asked him to bring “the blond Scandinavian lady,” suggests that Bruno had kept some distance between Marianne and his colleagues. On Sunday Bruno again visited the nursery in Montmorency, and wrote to Marianne about Gil’s progress.
During the ensuing months, Bruno’s letters to Marianne would be written on the way back from Montmorency; they contained reports of visits to Gil, occasional allusions to progress in physics, and plans to obtain a visa for one or the other, allowing them to get together. Bruno’s only mention of politics came in November, when he confessed his fear that “fascism will come to France.” The correspondence is in line with the opinion of family friends who insist that Marianne had no interest in politics. There are, however, clues that all was not well with her. On more than one occasion Bruno alluded to Marianne’s health: “I hope to find in your letter that you are not sick at all”; “are you ill?”; “what is the illness you have?”

  During the rest of 1938 and much of 1939, Bruno remained in Paris, apart from Marianne. Bruno’s brother Gillo—still a teenager—had stayed behind in Italy, where he “continued to pretend to be a playboy—granted with the defect of being Jewish.”36 Bruno sent Gillo tickets, allowing his brother to join him in Paris. It was during this period that Bruno introduced Gillo to communism. It was also the time of his most significant work on nuclear isomers.

  VIEWED WITH THE EXPERIENCE AND HINDSIGHT OF EIGHTY YEARS, Pontecorvo’s first entrance into nuclear isomerism was as a descriptive taxonomist, who collects species and classifies them, but does not offer any insights into their behavior and evolution, let alone their DNA. He predicted that relatively stable nuclear isomers should exist,37 and he found the first example when he bombarded cadmium with fast neutrons.38 In 1938 and 1939, he performed two significant experiments. One helped establish the correlation of angular momentum with stability; the other measured nuclear energy levels. In due course, these results helped others develop the theory of nuclear structure.