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

by Close, Frank

  Until this time, every knowledgeable physicist could keep abreast of global developments in the field. There was no need for spies. The major secret in the West was the discovery made by Frisch and Peierls. Bruno knew nothing of this. The Soviets, however, thanks to Khariton and Zel’dovich had discovered it for themselves. The big unknown, of course, was what was going on in Germany. The potential of a chain reaction to fulfill the Promethean dream of creating unlimited amounts of atomic energy was common knowledge, and a year earlier, in April 1939, Germany had banned the export of uranium and started conducting experiments on fission. That was enough to spur the allies to develop nuclear technology.

  It would be two years before Bruno rejoined the world of cutting-edge physics. During that time much would change, as nuclear physics became a tool of war, its research classified as secret. This secrecy applied not only to novel results but also to the nature of the quest itself.

  BRUNO THE OIL PROSPECTOR

  When the Pontecorvos arrived in the United States in August 1940, Enrico Fermi was based at Columbia University, in New York. One of the first things Bruno did upon arrival was visit the Fermis at their home in Leonia, New Jersey, two miles from Manhattan, across the Hudson River. Years later, Enrico’s wife, Laura, remembered that this visit had disturbed her.

  She recalled that Bruno had come alone. He explained that Marianne was “worn out” from the sea crossing and needed to rest. Laura had never met Marianne or Gil, and was disappointed that Bruno hadn’t brought them along. She added that although Marianne’s fatigue was understandable so soon after an “ocean journey on a boat crowded with European refugees,” she was perturbed that Bruno declined her offer to visit Marianne, or to help her in any way.11

  Laura Fermi wrote her account in 1953, at a time when Pontecorvo had disappeared behind the Iron Curtain, his whereabouts still unknown. Her account helped fuel the media myth that Marianne was a mysterious Svengali, silent because she had so many secrets to conceal. However, the real explanation of her behavior is perhaps much simpler, and also more tragic. As we have seen, Marianne had been through a painful pregnancy and miscarried just days before crossing the Atlantic. Laura Fermi was presumably unaware of this. Marianne could also be painfully shy, at ease with friends but uncomfortable with strangers.

  After a brief stay in New York at the home of Bruno’s brother Paolo, the Pontecorvos set off on the final leg of their journey: 1,300 miles southwest to Tulsa, Oklahoma, the home of Well Surveys, where Bruno was to become an oil prospector.

  BUILD A BETTER MOUSETRAP AND THE WORLD WILL BEAT A PATH TO your door.

  In the 1930s, as the need for gasoline grew in the United States, the “world” meant the likes of Standard Oil, Texaco, and Phillips; the mousetrap, a means to locate precious new oil fields.

  Well Surveys of Oklahoma was a company that specialized in oil prospecting; their big idea was that the natural radioactivity in rocks might reveal the geological formations where oil could be found. To realize this dream they needed an expert in nuclear physics and radioactivity, and, thanks to Emilio Segrè’s recommendation, they hired Bruno Pontecorvo.12

  Two of the scientists at Well Surveys—who were instrumental in hiring Bruno—were colorful characters whose backgrounds would later attract the attention of the nation’s security agencies. Their names were Jakov (also known as Jake or Jack) Neufeld, a Polish émigré, and Serge Alexandrovich Scherbatskoy, a Russian who had been born in Turkey.

  Neufeld had been born in 1906, learned nuclear physics at the University of Liège in Belgium, and then took a position at Cornell University. It was from there that he joined Seismic Surveys Corporation, a forerunner of Well Surveys. At the time, Scherbatskoy was the research director of the company.

  Scherbatskoy had been born in 1908 in Constantinople, where his father, Alexander, was a diplomat at the Russian consulate. After the Russian Revolution of 1917, Alexander worked in the League of Nations offices in Berlin, and then moved to Paris. While the family was based in Germany, Serge studied engineering at Stuttgart. After the move to France, he enrolled at the Sorbonne, where he graduated in physics in 1926.

  In 1929 he arrived in the United States, just before the stock market crashed. According to some accounts he had no passport, but in any event he was admitted, Serge Alexandrovich being reborn as Serge Alexander. He worked on telephones at Bell Labs, but he was laid off in 1932 due to the effects of the Depression, after which he took a variety of jobs in electronics. In 1936 he joined Seismic Services Corporation in Tulsa.

  It was there that he met Neufeld. They used their combined experience in electronics and nuclear physics to design devices that detected gamma rays, which are emitted by radioactive atoms in rocks. Shales, which are harbingers of oil fields, contain uranium and thorium, which are radioactive. Sands and limestones, by contrast, are not. Their detectors could identify shale-rich oil fields more successfully than other techniques, which had failed or were at best inefficient.

  Standard Oil was impressed with their research, and poured in money. In 1937, this led the pair to create Well Surveys, where Scherbatskoy was research director and Neufeld was at his side. Their plan was to find new ways of applying nuclear physics to prospecting.

  Inspired by Frédéric and Irène Joliot-Curie’s discovery of induced radioactivity, and the Via Panisperna Boys’ demonstration that neutrons are especially efficient at activating it, Neufeld and Scherbatskoy decided to use this breakthrough to induce radioactivity in rocks. By this means, they believed that underground strata could be made to shine brightly—in terms of gamma rays—relative to the faint glimmer that is typical of natural radioactivity. This is what led them to hire Bruno Pontecorvo.

  Bruno now set about developing the first industrial application of neutrons in locating oil-bearing rocks, a technique still in use more than half a century later. His invention paid off handsomely for his employers. In a 1990 interview he commented wryly, “I could have been a millionaire if I had patented my discovery. Instead I did not make anything, and now the patent belongs to the company I worked for. I do not have any practical sense.”13 Soon Bruno’s invention would be used by the Manhattan Project to map the vast mineral resources in the Canadian wilderness, where it was secretly turned into a remarkably efficient way to find uranium. This would have profound implications for the development of nuclear energy, as well as for Cold War politics.

  THERE ARE THREE PARTS TO THE CHALLENGE OF USING NEUTRONS to find oil. First, you must produce neutrons; then, having lowered the neutron source down a hole, you need to detect signals coming from the interaction between those neutrons and the underground minerals. Finally—the aim of the exercise—you need to decode the signals to learn about the nature of the strata.

  For Bruno, making beams of neutrons was child’s play. Typically he used radium, which produces a steady stream of alpha particles, and mixed it with beryllium, whose atoms emit neutrons when they’re hit by alpha particles. In searching for oil-bearing rocks, the basic idea is that the neutrons interact with atoms in the vicinity of the borehole. Most bounce gently off other particles and pass through the rocks, never to be seen again, but some are reflected back toward the detector, or are absorbed by elements in the rocks. When neutrons are absorbed by a material, they convert the nuclei of the material’s atoms into miniature transmitters. The end result is that information returns to the detector in the form of radiation, such as gamma rays, or the reflected neutrons themselves.

  The whole apparatus that Bruno developed was contained in a thin cylinder, about two meters long and a mere ten centimeters in diameter, which could be lowered into deep boreholes. The cylinder contained his neutron source, an ionization chamber (similar to a Geiger counter), and an electronic amplifier for recording the faint signals created when radiation passed through the ionization chamber.

  In Rome, Bruno had been party to the discovery that heavy nuclei tend to absorb slow neutrons.14 This induces radioactivity in the nuclei, which ma
y respond by emitting gamma rays. In this new application, the spectrum of these gamma rays could be used to identify isotopes of heavy elements present in the rocks. The data reports that were obtained from the neutron irradiation of rocks in prospective oil fields were therefore known as “neutron-gamma” logs.

  Pontecorvo also developed an independent recording method—the “neutron-neutron” log—in which neutrons recoiling from the strata were detected. When a neutron hits a heavy element, the bulky atom stays where it is and the reflected neutron retains most of its energy. Light elements can also deflect neutrons, even reflecting them back to the source, but in this case the neutron has less energy than when it set out. Thus, if you could record the actual energies of the reflected neutrons, you would have a way (albeit a rough one) of differentiating between light and heavy elements.

  This basic idea was simple, but there were challenges to overcome before it could be put into practice. For example, the source produced fast neutrons, whereas the detector was most efficient at recording slow ones. To resolve this issue, Pontecorvo embedded the detector in paraffin, which slowed the neutrons. Although this made his device more efficient, it created a problem of its own: to convert the number of counts in the detector into meaningful information about the neutron intensity, he needed to know what percentage of the original fast neutrons had slowed, and by how much. To find the answer, he performed a series of experiments in the lab. First he determined how his detector responded to a known source of neutrons, and then compared data from the field with these benchmarks.

  He also needed to decide on the optimum amount of paraffin. A small amount would slow few, if any, of the neutrons to the low speeds necessitated by the detector. At the other extreme, a large amount of paraffin would absorb so many neutrons that very few would get through. Hence there must be some intermediate situation in which the intensity of slow neutrons is at a maximum. Only after these calibrations were done in the lab would it be possible to identify strata in the field.

  During his time in Italy, Bruno had learned that the amount of radioactivity induced in heavy elements depends greatly on the speed of the incident neutrons. At a certain speed a neutron could have only a moderate effect, whereas if it were slightly faster or slower, the response could be huge. In 1936 Fermi and Edoardo Amaldi had measured the absorption and diffusion of neutrons in various materials, and plotted graphs—known as Amaldi-Fermi (or AF) curves—which in effect showed how the response varied with neutron speed.15 AF curves were important in the practical interpretation of nuclear measurements, so Pontecorvo decided to build on that experience by seeing what happened when neutrons bombarded various strata containing a smorgasbord of minerals, and using the curves to help him interpret the results.

  In doing this, he had to shield his detector from the source of neutrons; otherwise, radiation from the source itself would swamp the delicate signals from the surrounding rocks. This raised another question: Could he discriminate between the radiation coming from the rocks and the radiation coming directly from the source? He discovered that the source produced a steady level of radiation, while that from the rocks varied dramatically as the device passed from one stratum to another while being lowered down the hole. The amount of variation turned out to be sharp enough for him to identify the genuine signal. In June 1941 Bruno reported on the trials he had performed, and on the ability of the data to reveal the chemical composition and porosity of rocks.16 His report showed that different rock strata give distinctive radioactive responses, providing remarkably clear differentiation between shale, limestone, and sandstone. What’s more, this method of rock assessment was superior to the existing techniques, such as the gamma-ray device that Neufeld and Scherbatskoy had first used.

  That same year, Bruno published a report in the Oil and Gas Journal, showing the superb correlations between signals in his detector and different varieties of rocks. He also identified a key asset of his device: “The strength of the neutron source can be made quite large, [so] the surveying speed which may be realized with [this] new method is very great.” This report, in the open literature, included what amounted to an advertisement: “If favorable tests continue, the new process will be offered to the trade shortly.”17

  Next Bruno had to decide on the best neutron source. His use of paraffin to slow the neutrons limited the precision of his measurements. He began looking for sources of lower-energy neutrons, so that he could do away with the paraffin entirely.18 This led him to seek out companies that provided radioactive materials such as polonium and actinium, to replace the radium-beryllium mixture he had used before. This occupied him throughout much of 1941.19 By the end of the year Bruno was sure that neutrons were a remarkable new tool, which could reveal the transitions between layers of sandstone and shale, or between layers of sandstone and limestone.

  The concept of “neutron” was entering the public awareness in unexpected ways. Periodically Bruno joined teams of engineers in the oil fields. During one of these visits, a truck suddenly stopped and its driver shouted to him, “Have you seen the neutrons?”20 The trucker was looking for one of his colleagues, who had the team’s exploration equipment, including one of Pontecorvo’s neutron sources. For the driver, this source had become simply “the neutrons.” Pontecorvo was quite humbled, and proud that “a particle dear to me, connected with my research with Fermi, had already entered the life of men, at least in the oil fields.”21 Bruno was establishing a whole new industry: one that used neutrons to locate the minerals that herald oil.

  DURING THIS TIME, BRUNO, MARIANNE, AND GIL WERE HAPPILY settled in a typical American house in Tulsa. It was considerably more spacious than the student housing they’d occupied in Paris, with its communal bathroom.22 In Tulsa they had a home to themselves, with bedrooms upstairs, a large living room, and a backyard. The local newspaper carried the story of the handsome, dark-haired Italian, his blond Swedish wife, and their curly-haired young son, who had escaped from the war in Europe. They were popular in the community, and planned to settle permanently in the United States.

  Much changed, however, when the US entered the war in December 1941. President Roosevelt declared that “an invasion or predatory incursion [was] threatened on the United States” by Germany and Italy, and Bruno had once more become an enemy alien.23 In this climate, Bruno decided to investigate the possibility of becoming a naturalized American citizen.

  IMAGE 5.1. The Pontecorvo family in Tulsa, 1940. (COURTESY GIL PONTECORVO; PONTECORVO FAMILY ARCHIVES.)

  SIX

  EAST AND WEST

  1941–1942

  IN JUNE 1941, THE NAZI ARMY INVADED THE SOVIET UNION AND headed toward Moscow. The Soviets viewed fission as a potential source of destructive power for a future war, but not relevant to the present one. The difficulty of extracting U-235 and controlling fission so as to guarantee an explosion seemed to be insurmountable challenges at the time. Pragmatism ruled: Why pursue fission, a long-term project of dubious success, when Moscow might fall within a few weeks? The Soviet work on fission ground to a halt.

  Georgii Flerov, Kurchatov’s protégé who had discovered spontaneous fission, thought this strategy was wrong. In his opinion, it would be a disaster to lose the race for the atomic bomb. He estimated that fast neutrons might initiate a chain reaction equivalent to 100,000 tons of TNT, without the need to increase the amount of U-235 through enrichment. He even designed a bomb, which, unknown to him, was quite similar to the one secretly envisioned by Frisch and Peierls in the UK. In December 1941 he sent a letter to Kurchatov, outlining his concerns. Unfortunately Kurchatov was ill with pneumonia and never replied.1

  At this stage Russia and the United States were ostensibly allies, if only in the sense that the enemy of my enemy is my friend. In reality there was mutual suspicion. The US and UK kept their plans for an atomic weapon secret from their Soviet ally. However, this did not mean that the Soviets were unaware of it.

  Klaus Fuchs was another refugee from fascism—a theor
etical physicist who in 1941 was working with Rudolf Peierls in Birmingham. Following the German invasion of the USSR, Fuchs, who was a communist, decided that the Soviet Union had the right to know what the British were working on. Fuchs contacted the military attaché in the Soviet consulate in the summer of 1941 and told him about the British plans for separating U-235.2 In addition to information obtained from spies such as Fuchs, there were also clues about the secret programs to be found in the open literature, provided that one knew how to interpret them.

  Early in 1942, Georgii Flerov was in the Soviet military. He had been removed from the relative luxury of physics and sent to fight at the front near Voronezh, about 300 miles south of Moscow. By this point, Voronezh State University had been evacuated, but the library shelves were full of the latest international journals. One day, when he had a few hours to spare, Flerov visited the library to read the latest American news on fission. He was astonished at what he found: there wasn’t any.

  Initially puzzled, Flerov flipped through the pages of the available Western journals and found papers on other areas of physics by a variety of authors, but on fission—nothing. That was only half of it: not only had papers on fission disappeared, but the leading nuclear physicists had also. None of the field’s most prominent researchers—such as Fermi, Bethe, and Bohr—had published anything for several months.

  Then Flerov realized the explanation: the papers were absent because American research on fission had become secret. This also explained the disappearance of the nuclear scientists: they were keeping silent as they worked on a nuclear weapon.