by Close, Frank
One way to find out how much angular momentum a nucleus has is to detect the gamma rays when it eventually decays.39 Several physicists had tried but failed to find the anticipated rays. This is where Pontecorvo made a notable contribution.
He suggested that electrons in the outer reaches of the atom capture the gamma rays. The energy of the gamma ray is passed to the electron, and the impact knocks the electron out of the atom. He proposed a test: look for electrons with a specific amount of energy—namely, that of the “lost” gamma ray. This is very different from the case of beta radioactivity, whose electrons emerge with a range of energies.
Bruno performed the experiment and found an example in an isotope of rhodium, where electrons always emerged with the same energy, as he had predicted.40 This result proved the hypothesis that the original isomer had a large amount of angular momentum.
By 1939 Joliot-Curie’s electrostatic accelerator at Ivry was ready.41 One of Irène’s assistants, André Lazard, had designed a Van de Graaff generator with her years before, and Frédéric had commissioned him to build the machine at Ivry. Bruno and Lazard now joined forces at Ivry, and discovered what Frédéric Joliot-Curie would call “nuclear phosphorescence.”
Ordinary phosphorescence, in molecules and atoms, occurs when light is absorbed, stored, and later released gradually as a glow, visible in the dark. In these cases, the electrons in the atoms are kicked to higher rungs on the energy ladder, and then release this energy as they fall back to ground. Pontecorvo and Lazard found analogous phenomena in atomic nuclei. Instead of visible light, they used X rays—higher-energy photons. This raised neutrons and protons up the energy ladder, while the nucleus remained intact. If one of the high-energy rungs was unusually stable, the neutron or proton stored the energy for a time, and later shed it when it fell to a lower rung, slowly emitting an X-ray in the process.
The energies of these X-rays are like a bar code that reveals the energy states of the nucleus, analogous to the atomic spectra that can reveal the electronic structure of atoms.42 Frédéric sent Bruno fulsome congratulations for this discovery, which gave him great joy. During his time in Rome, Bruno had felt that Fermi only really respected him for his prowess at tennis; Joliot-Curie’s praise assured him that he had now proved himself in physics.43
The phenomenon of isomerism was important in establishing that a nucleus is a rich collection of constituents, which can move, orbit, and vibrate relative to one another.44 It has applications in industry and medicine. The expertise that Pontecorvo gained in his studies of isomerism, in which he used neutrons and detected gamma rays, would prove invaluable throughout his career.
FISSION
Uranium nuclei are so large and fragile that a mere touch by a slow neutron is enough to split the pack: the phenomenon known as nuclear fission. This was so unexpected that when Otto Hahn and Fritz Strassmann discovered the phenomenon in Germany on December 17, 1938, they didn’t realize what they had achieved. The discovery came when they irradiated uranium with slow neutrons, and identified the light element barium among the products. Up to that time nuclei had been modified subtly, by chipping off just one or two constituents, transmuting the target into an immediate neighbor on the periodic table. The appearance of barium, far removed from uranium, was bizarre. Hahn and Strassmann announced their results, and said little more about them. Only during 1939 were the full implications of their breakthrough understood.
The Germans’ paper arrived in Paris on January 16, 1939. Joliot-Curie immediately understood what must have happened; Irène had seen a similar phenomenon the previous year, though she had not been confident enough to confront the criticism of her results, and had backed down. Now Frédéric suspected that she had been correct after all, and that the neutrons must have split the uranium in two. For the next few days the news was the hot topic of discussion throughout the group. Kowarski recalled that “nobody talked of anything else.” Irène raged at having missed out on getting credit for the discovery, and told Frédéric, “What fools we have been,” or probably used “a somewhat stronger word.”45
Frédéric Joliot-Curie wrote to certain Soviet physicists about the phenomenon.46 As a result, Igor Kurchatov immediately investigated whether any secondary neutrons were produced during the fission process, as did Joliot-Curie himself in Paris, and others in the United States. The critical question was whether more than one neutron was released for every neutron that caused the fission in the first place. If this was so, a self-sustaining reaction could occur.
A uranium nucleus contains more than 140 neutrons, more than enough to satisfy the needs of smaller nuclei, such as barium, krypton, or lanthanum, the likely debris. So it seemed plausible that, during the fission of uranium, some extra neutrons would also be liberated. Joliot-Curie immediately devised a simple experiment to detect these neutrons—and failed. The source of neutrons irradiating the uranium was so intense that his attempt to identify additional particles was like trying to detect a rain shower while standing beneath a waterfall.
Over the next few days Joliot-Curie designed a new experiment, one that would look for evidence of radioactivity in the debris when the neutrons hit uranium. Halban had left Paris to go skiing, and so missed the ensuing drama. Kowarski, however, was present.
Joliot-Curie had engineered two brass tubes, one of which was coated with uranium. He also had some Bakelite cylinders, which were larger than the tubes and could surround them like napkin rings. He had already verified that none of these tubes or rings was radioactive, even after they were irradiated with neutrons. Now all was ready for the experiment. First he placed a neutron source inside the brass tube that was free of uranium; then he placed the Bakelite ring around the tube. After a few minutes he removed the ring, took it to a Geiger counter, and verified that there was still no radioactivity. Next he repeated the exercise, but this time used the uranium-coated brass cylinder; as before, the tube was surrounded by the Bakelite ring. When he removed the ring on this occasion, and took it to the Geiger counter, it set the device clicking. This showed that radioactive fragments of uranium had adhered to the Bakelite, which proved that the uranium had been shattered.47
Joliot-Curie did this in the presence of Kowarski during the morning of January 26. He repeated the demonstration that afternoon before four witnesses, including Irène and Bruno Pontecorvo.48 The next day Bruno wrote to Marianne, “Work at Ivry goes very well and if it continues will be very important.”49
Halban returned from his ski vacation to find the laboratory in a state of excitement. He and Kowarski soon found a clever way to detect the liberated neutrons. The trick was to irradiate the uranium with slow neutrons, and use a detector sensitive only to fast ones. By this means, they could distinguish the liberated, fast neutrons from the slow ones emitted by the source. They began this experiment during the last week of February.50
Bruno was occupied with his experiments on nuclear phosphorescence, at the Ivry laboratory in the suburbs, so he did not take part in this fission experiment personally; nonetheless, he was deeply involved intellectually. Joliot-Curie was away at the ski resort of Val d’Isère. On March 3 he wrote to Kowarski to ensure that Bruno, who was due to join him at the resort along with the Joliot-Curies’ daughter, Hélène, brought the latest news. Bruno duly arrived to report great progress. On March 27 he wrote to Marianne and mentioned physics for only the second time that year: “Physics goes very well,” he wrote, underlining the words.
Fission might have been little more than a curiosity, except for two features. First, when the nucleus of a uranium atom splits, the total energy released is about a hundred times as much as that released by radioactivity, and up to a hundred million times the amounts in chemical reactions. Here was the first hint of how to liberate nuclear energy on a larger scale than had hitherto appeared possible. In April, Halban and Kowarski finally demonstrated that the fission of a uranium nucleus liberates more than one neutron. The potential consequences of this discovery were nothing sh
ort of awesome. The possibility that these freed neutrons could initiate further fissions and produce a self-sustaining nuclear reaction was out in the open.
Suppose, for example, that when a single neutron splits the uranium nucleus into two chunks, two neutrons are liberated. There is a chance that these neutrons will hit two additional uranium atoms and repeat the fission.51 If the same thing happens during this and subsequent collisions, there will now be four neutrons freed to make four fissions, leading to eight, sixteen, and so on—the number of neutrons doubles at each step. Thus it would only be necessary to irradiate uranium with a few neutrons to set off reactions that would continue spontaneously until all of the uranium was used up. This creates the potential for an immense release of energy.
As soon as this news arrived in the United Kingdom, scientists alerted the government to uranium’s strategic importance. Enrico Fermi was in the United States; he too immediately realized fission’s implications. In Germany there was a similarly intense response: all reference to atomic energy and uranium reactions was immediately censored in the German media.52
In France, Joliot-Curie sprang into action. On April 22, he announced that his team had established that a chain reaction was possible, and in the first week of May he applied for three patents. Two dealt with the potential application of fission to nuclear power, and the third, which was secret, related to explosives. On May 8, he went to Brussels to negotiate the acquisition of uranium stocks from the Belgian Congo, with a view to building a uranium bomb in the French Sahara.53
MEANWHILE WORLD EVENTS MADE THE LIKELIHOOD OF WAR MORE certain. Early in 1939, Bruno became seriously worried about the march of fascism, and its effect on his future. He had no idea when—or if—he and Marianne would have the chance to be together, and Gil’s future also weighed ever larger on his conscience. In February he wrote Marianne a long letter, which in parts takes the form of a manifesto: “(a) If democracy survives in France and if I will be paid after to live with you and Gil, I shall stay in France. (b) If there is a war, and if that war is a democratic war against fascism, provoked by the axis powers, I will take part in that war. (c) If fascism comes to France, I shall go to the USA. Write me if you find that juste.”54
She replied, and agreed with his plan.
Bruno then followed up on his manifesto and encouraged Marianne to come “immediately” to Paris: “I don’t want to influence you but if you decide to come I think it would be better as soon as possible, not only for the pleasure of you being here but also because we have many things to decide, for the little one and for us.”55 It seems that, at this stage, Gil was totally the responsibility of Bruno, although financial support came periodically from Sweden. Discussions with Frédéric Joliot-Curie had increased Bruno’s conviction that France, and indeed Europe, was on a path to disaster, and that war was probably inevitable. If Gil remained in the nursery at Montmorency, Bruno would be tied to Paris. In April, therefore, Bruno signed a legal document, which would be deposited in a sealed envelope, to the effect that if anything happened to him, “Mademoiselle Marianne Nordblom . . . [will] decide at her discretion the fate of my son Gil Pontecorvo.”56
During Marianne’s absence, Bruno discussed Marxism with his cousin Emilio Sereni and, along with Gillo, deepened his links with communist groups. Gillo’s son, Ludo, later described the situation: “My father [who believed strongly in communism] always said he felt that Bruno believed even stronger than him. Bruno was literally a big brother, five years older than my father—a similar age gap as between Emilio Sereni and Bruno.”57 Sereni influenced Bruno, and Bruno influenced Gillo. It wasn’t just physicists who took note of Bruno’s emerging intellect; Pontecorvo’s involvement with communists was noticed in fascist Italy, where the local intelligence agency opened a file on the young scientist.58 It is not known whether Swedish officials were aware of Bruno’s communist activities; in any event, he soon discovered that he was persona non grata in that country.
In June 1939, Bruno applied to visit Stockholm and “possibly Sandviken” where “I have friends.” He named Marianne as a reference for both personal and financial guarantees, and stated that he had visited there previously. There was no mention in the application that they were the parents of a one-year-old boy. The records show that Stockholm police constable Berger Hassler phoned Marianne to verify the legitimacy of the application.59 She confirmed everything and fully expected to see Bruno. But a few weeks later he learned that his application had been rejected.
IMAGE 3.2. Bruno’s unsuccessful application to visit Marianne in Sweden in 1939. (SVEN-OLAF EKMAN AND SWEDISH NATIONAL ARCHIVES.)
Bruno now started encouraging Marianne ever more desperately to return to France with an appropriate visa, asking, “Why not come to Paris immediately?” He added that he didn’t “want to influence her” but put strong pressure on her with the remark, “we have many things to decide,” for “le petit [Gil] and for us.”60
Marianne’s reply seems to have mentioned that she was not well. This was clearly a recurring theme, as Bruno replied, “But what I would like to know absolutely is—what is the illness you have?” On August 7, when he left Paris for Zurich, he had still heard nothing from her. As soon as he was back in Paris, Bruno visited Gil, as always. He wrote to Marianne the next day, having heard nothing for several weeks. He reminded her again that they must “decide many things” for Gil. Two days later a card arrived from Marianne, saying that she would arrive in Paris the next week.
The following day, August 23, Stalin agreed to a nonaggression pact with fascism’s high priest, Adolf Hitler. “A Victory for Peace” was the headline in L’Humanité, the organ of the French Communist Party. It reported that fascism had been “forced” to deal with “the very power that it has always declared to be its implacable enemy.” The USSR was portrayed as a peacemaker, which had “imposed [the pact] on Mr. Hitler.” In summary this was a “triumph of Stalinist politics.” For a twenty-six-year-old living in the heady atmosphere of socialist Paris, the message of L’Humanité was overpowering. That same day, Bruno joined the Communist Party, “to prove his faith with Russia.”61
He frequented party meetings, and took part in their passionate debates. At these meetings, all were free to speak, although “the leaders always had the last word.”62 The young, ardent physicist, who had attracted international attention with his scientific achievements, had already begun to be noticed by members of Comintern, the international communist organization. Marianne, meanwhile, was still living with her socially conservative family, far removed from such politics. She did not return to Paris until September 6, 1939, just days after the Nazis invaded Poland, and war in Europe threatened to cut Bruno off entirely. Her French visa now very correctly referred to “Mademoiselle Nordblom accompanied by Gil, born in Paris 30/7/38.”
Years later, following the Pontecorvos’ flight to the USSR, several reports in the conservative British media referred to Marianne as Bruno’s unmarried lover, or mistress, which helped bolster their portrayal of the Pontecorvos as “immoral.” In fact Bruno and Marianne married on January 9, 1940, which enabled them to obtain visas for North America when the Nazis invaded France in June.
DURING THOSE TUMULTUOUS MONTHS OF 1939, THE FISSION experiments of Bruno’s French colleagues gave tantalizing hints that a chain reaction could be made. The scientists desperately wanted to be the first to achieve this, as the promise of unlimited energy was akin to finding the philosopher’s stone. As five tons of uranium would be required for the attempt, the operation was moved from the university to the more spacious laboratory at Ivry.63 Of course, the laboratory required financial support. During the summer of 1939, as war loomed, Kowarski and Halban performed demonstrations to impress the French Ministry of Supply, rather than pursue “scientifically impeccable proofs.”64
In the fall of 1939 Time magazine was so impressed with the team’s work that it included Joliot-Curie on its front cover. The team’s actual scientific paper, which was also t
heir last open publication before secrecy enveloped nuclear physics worldwide, was less impressive. Kowarski recalls that one of Fermi’s colleagues once asked the great scientist, “What do you think of this paper by Joliot?” Fermi replied, “Not much.” Kowarski himself admitted, “Fermi was quite right. Scientifically the paper was not very impressive [although] as a demonstration [it was].”65
As the team began their work at Ivry, ordinary water was their first choice as a moderator, but it was not actually the best option. When a neutron hits a hydrogen atom in a water molecule, it may bounce off and lose energy—which is the key to its speed being “moderated,” thus raising its ability to cause fission. If that were the whole story, everything would be fine. However, the neutron may instead be captured by the hydrogen atom and lost, in which case the reaction dies. This latter effect is so probable that ordinary water kills rather than feeds the chain reaction. If instead of using ordinary water you use heavy water, however, neutrons are no longer captured. When a neutron encounters an atom of heavy water, it bounces off and slows, which is ideal.
One question that immediately struck scientists was this: If a chain reaction can indeed liberate energy explosively, why are the rocks around us, which contain uranium and are being hit continuously by cosmic rays, not liable to detonate spontaneously?
Niels Bohr’s unique ability to visualize the labyrinth of an atomic nucleus gave him a key insight about fission: the quirks of nuclear structure imply that fission is more likely in an isotope with an odd number of constituents, such as U-235, than in one with an even number, such as U-238. Slow neutrons bounce off U-238 without causing it to fission. U-238 can also act as a blanket that covers any nearby U-235, making a succession of fissions rare, and the chance of a chain reaction negligible. Only if the neutrons encounter some of the rare isotope U-235 before exiting the uranium target will fission occur.66