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

by Close, Frank

  In Moscow, Gil had “hated the loneliness.” Now he attended the local school, where he enjoyed the company of other children. On his first day of school in Dubna, “all problems vanished.”30 Over sixty years later he still lives there.

  FIFTEEN

  EXILE

  THE ATOMIC NUCLEUS HAD REVEALED ITS AWESOME POWER IN THE explosions at Hiroshima and Nagasaki; exploring its deepest structure was the obvious next step for the world’s postwar governments, and an intellectual challenge for scientists.

  In 1944 Soviet scientist Vladimir Veksler had shown that it was possible to create stable beams of high-energy particles, which could be used to bombard atomic nuclei and shatter them. The following year, in the United States, Edwin McMillan independently discovered the technique. This breakthrough raised the possibility of revealing the deep secrets of the nucleus by bombarding it with pions produced by a high-energy particle accelerator. This line of research soon became a top priority in the West, and Igor Kurchatov, the father of the Soviet atomic bomb, urged his superiors to make it a priority in the USSR as well. As a result, the Soviet government decided in August 1946 to build a special laboratory—Dubna—that would contain the world’s largest high-energy particle accelerator, known as a synchrocyclotron.

  Up to that time, energy had been extracted from the nucleus under a limited set of circumstances, which required either a rare isotope of uranium or artificially created plutonium. Even though the results of these methods could be explosively dramatic, the amount of energy they liberated was still less than 1 percent of what was locked inside the nucleus by the powerful nuclear forces.

  The discovery of pions in 1947 gave scientists a new understanding of those forces. Just as photons are the material embodiment of electromagnetic fields, so are pions the material embodiment of the much-stronger nuclear fields. Scientists initially studied pions out of curiosity; their research had no immediate military significance, and high-energy particle physics in the West was developed in the open. Even so, some thought that pions might be able to unleash nuclear energy in quantities that would make all previous methods pale in comparison, which was one reason for Western governments to support this new field.

  The motivations of the scientists at Dubna were probably no different than those of their counterparts in the West; the Soviet government, however, mindful of the strategic possibilities, kept the existence of the Dubna accelerator a secret. Stalin’s agenda was to create atomic and hydrogen bombs, for military purposes. He distrusted intellectuals, but realized that he needed physicists to do the job. The advice of Lavrenti Beria, Stalin’s security chief, was characteristically direct: “Let them get on with it; we can always shoot them later.”1

  Once the Soviets decided to build the accelerator, a site had to be chosen. Beria set up a meeting, where three possible locations were discussed. The choices didn’t include Dubna, however. Beria, who enjoyed hunting near Dubna, then pointed at the map and announced that the laboratory would be built—right there!

  The Dubna area was hardly an ideal place to construct a particle accelerator, as it was full of swamps. To this objection, Beria announced, “We will drain them.” As for the lack of roads: “We will build them.”2 And, as for the workforce, Beria had an answer for that too: the area was full of forced labor camps, part of the Gulag. Throughout the 1950s, in Dubna’s early years, scientists traveling from Moscow would routinely see prisoners with shaved heads building the roads.

  And so Dubna was born. The project’s purpose was disguised by the name Hydro Technical Laboratory. The construction was completed in December 1949, and in January 1950 physicists from Moscow began conducting experiments at what was then the world’s highest-energy accelerator.3 It retained this honor until 1953, when the Cosmotron accelerator at New York’s Brookhaven National Laboratory took the blue ribbon.

  THERE WAS GREAT EXCITEMENT AT DUBNA WHEN, IN THE FALL OF 1950, the senior management learned that Bruno Pontecorvo—“student of the famous Enrico Fermi”—had arrived at the laboratory. According to Venedict Dzhelepov, who later became Dubna’s director, the fact “that such a talented and well-known scientist was to work in the then small scientific community of our laboratory was very valuable.”4

  Only a select few knew of Bruno Pontecorvo’s presence in Dubna. Irina Pokrovskaya, who served as Bruno’s secretary at the laboratory for forty years, initially knew him only as “the professor,” a man with no name.

  Nonetheless, the Western media was sure that Pontecorvo was in the USSR, and over the next five years reporters made some fanciful claims.

  Often, half-truths and rumors were elevated to the level of supposedly factual stories. One notable example occurred in November 1951, when newspapers in Rome claimed that Pontecorvo had been arrested by the Russians in an effort to stop their atomic secrets from being leaked to the United States; Pontecorvo, apparently, was suspected of being a double agent. The article quoted unnamed Russian sources. The story’s genesis was apparently President Truman’s announcement that atomic explosions had taken place in the USSR; as these explosions were meant to be secret, the Soviets thought Truman must have a source, a Western spy among their top-ranking scientists. A Harwell spokesman commented that if the news of the explosions was true, it was intriguing that it had percolated through the Iron Curtain.5 A closer analysis of the facts, however, reveals the Pontecorvo rumors to be nonsensical. The White House had indeed made the announcement regarding atomic explosions in Russia—but this had happened in the fall of 1949, nearly a year before Pontecorvo disappeared.

  Another story, popular in North America, was that Pontecorvo was in China’s Xinjiang Province, working at a “huge atomic stronghold” that Russia was setting up there. Such stories were accepted unquestioningly by the Los Angeles Times, Chicago Tribune, and Christian Science Monitor. The Glasgow Bulletin was more skeptical, announcing that Swedish and Finnish sources had poured cold water on the story. The reports of Pontecorvo’s presence in China had been ascribed to refugees who had escaped from the USSR to Helsinki and Stockholm. However, inquiries showed that no Soviet refugees had actually reached those cities since Pontecorvo’s disappearance. Even so, in 1951 a US congressional committee declared Pontecorvo to be the “second deadliest” spy in history (Klaus Fuchs being the first). The committee also claimed that Fuchs and Pontecorvo had advanced the Soviet weapons program by eighteen months.6

  The bizarre suggestions that Pontecorvo was working on helium weapons or atomic fogs are best seen as science fiction, and never had any scientific credibility. Reports that he was used in the Soviet quest for uranium are harder to dismiss, as his expertise in this area meshed so well with the USSR’s needs at the time. Indeed, uranium mining soon became one of the jobs performed by forced laborers in the Gulag.7

  Bruno always denied having worked on atomic weapons at any stage. Although this may be literally true, Isaak Pomeranchuk, head of the nuclear-reactor research program in Moscow, consulted Bruno frequently during the latter’s first five years in Russia. Boris Ioffe recalls that Pomeranchuk “often visited Dubna at that time and many times said after returning that he discussed such and such a question with ‘a professor,’ or ‘a professor said this.’”8 Samoil Bilenky, who would later work closely with Bruno, was a young scientist at that time, and a student of Pomeranchuk. He later recalled a car journey he took with Pomeranchuk and another senior scientist. Pomeranchuk kept repeating, “Professor said this; professor said that.” Bilenky remembered the incident because it had seemed so strange. “Why did he not say the name of the professor? Naturally I knew not to ask.”9

  Bilenky and Ioffe both stressed the fact that Pomeranchuk “never said who the professor was.” Ioffe added that “mentioning Pontecorvo’s name was taboo” until 1955.10 Only later did Pomeranchuk confirm what Ioffe already suspected: “the professor” was Bruno Pontecorvo.

  POMERANCHUK’S QUEST

  What did Pomeranchuk need from Pontecorvo? Why did he consult him so frequent
ly?

  In 1945 Pomeranchuk and three colleagues had worked on mathematical problems relating to “the tube”—a conceptual method in which deuterium and tritium could be used to make a thermonuclear weapon. A conventional atomic explosion would heat the tritium, which would then provide the spark to ignite the rest of the bomb, which consisted of a tube full of deuterium. Only a small amount of tritium was needed, and because deuterium was cheap, the tube could be made as long as necessary. The plan was for a shock wave to pass down its length and cause the nuclei of deuterium to fuse explosively.11

  Until 1949, all the physics research in the USSR had been geared toward making a traditional, fission-based atomic bomb. This was because the Soviets were eager to demonstrate their power to the West, and because a fission explosion is needed to ignite the tritium in a hydrogen bomb—so mastering fission explosions was a necessary first step on the path toward their ultimate goal of a hydrogen bomb. After 1949 the Soviet quest for a hydrogen bomb began in earnest. The basic physical principles behind such a weapon were clear; what was uncertain was whether the reaction would explode or fizzle. One of the unknowns involved how energy would spread through the device. If too much escaped, there would be no explosion. During 1949 and 1950, a group at Arzamas-16, the “Soviet Los Alamos,” investigated this intensely. They focused on how energy, carried by gamma-ray photons, would dissipate as the photons bounced off of electrons in the device—a phenomenon known as Compton scattering.

  Compton scattering was one of many processes that could be studied using the new breakthroughs in quantum electrodynamics (QED), the quantum theory of light and matter. The theory of QED had been successfully completed in 1947 by theorists in Japan and the US, and subsequently published in the literature. Around the world, physicists investigated its implications. In fact, this is exactly what Boris Ioffe was studying for his PhD thesis. Individual electrons and photons can spin in flight. In 1950 Ioffe was told to calculate how certain properties of Compton scattering depended on the relative orientations of the particles’ intrinsic spins. This was, apparently, a question of academic interest, an application of a new theory, which could be used to test its limitations. However, it also had considerable relevance to the innards of a thermonuclear weapon. In order to maintain secrecy, the examining committee for Ioffe’s thesis was carefully chosen. At the end of the examination, one of the members, L. V. Groshev, agreed that the thesis was sound but didn’t understand one point: Why it was so secret? The chairman, Lev Artsimovich, replied that it was “very good that you didn’t understand it.”12

  Ioffe was part of Pomeranchuk’s team. In the summer of 1950, Pomeranchuk had been sent to Arzamas on a “long assignment.” However, he wanted to discuss the revolutionary discoveries in QED with his colleagues in Moscow. He managed to convince the authorities that it would be best if his group returned to Moscow, where he could work on both QED and “the problem,” as the bomb project was known. The team’s expertise was mainly in reactors, and their research was subject to the highest level of security—“Top Secret Special Folder.” At this level, the protocol was so restrictive that reports were not typed by the carefully vetted special secretarial staff, but were written longhand by the scientists themselves.

  In Moscow, Pomeranchuk’s group took on the task of assessing how much energy in “the tube” would be lost due to Compton scattering. By 1952 they had the answer: so much energy would be lost that the bomb would not work.

  Thus, at the start of the 1950s, Pomeranchuk’s interests included QED as it applied to the H-bomb, and the design of nuclear reactors for making tritium. In the realm of pure physics, he was also interested in how particles scatter off one another at high energy. Which brings us back to our question: What did Pomeranchuk need from Pontecorvo?

  In general, Bruno’s focus was different from that of Pomeranchuk. His interests lay in the neutrino, whose existence had yet to be proven; in cosmic rays and the “strange” particles that had been discovered within them; and in the relationship between electrons, muons, and the weak force of radioactivity. Moreover, Bruno was primarily an experimentalist, with little to offer a first-rank theorist like Pomeranchuk on subjects like QED or high-energy scattering. If Pomeranchuk needed advice on theoretical physics, he had considerable talent at his disposal in Moscow. It is also unlikely that he made the visits to Dubna to discuss basic physics for the purpose of pure research: Pontecorvo and Pomeranchuk never published a joint paper on basic physics during their lifetimes, nor did their primary interests overlap strongly. It is known that Pomeranchuk asked Bruno about the strange particles, which had interested physicists in Moscow, but this was a transient interest and Pomeranchuk played no role in this field. However, the two men did have a mutual interest in nuclear reactors.

  Pomeranchuk was, in Ioffe’s opinion, the “main contributor” in the Soviet Union to the theory of nuclear reactors. In 1947 Pomeranchuk wrote the first text in the world on the principles of nuclear reactors.13 He was a theoretical physicist, and the reigning expert on the underlying concepts. However, as we saw during the building of the reactor at Chalk River, there is no substitute for hands-on experience. In a way, building a nuclear reactor is like learning to drive: you can read books on the subject, but until you actually go out on the highway, you won’t get the experience necessary to pass the test. For the Soviets, it was imperative to build a heavy-water reactor that actually worked. Pomeranchuk had read the books, but he’d never been on the highway. In this regard, the presence of Bruno Pontecorvo, who had already passed the driving test, was invaluable.

  Indeed, considered from this angle, the reasons for Pomeranchuk’s visits become obvious. When the first reactors were built in the West, a new problem emerged, known as “creep.” The intense neutron bombardment, combined with the heat produced by the reactor, deformed the metal in the cooling system and threatened to kill the reactor entirely. By 1946 Kurchatov had four institutions working on ways to seal the uranium rods and avoid this problem.14 For these purposes, information from agents in the West was useful, but limited. Much of the research at Chalk River and Harwell was not formally recorded, but “done on chalkboards and by coffee-housing” and would have been “taken to the USSR in Bruno’s head.”15 Indeed, in 1955, when Pontecorvo’s presence in Russia was finally acknowledged, he would admit, “A few years ago I had occasion to discuss with Soviet colleagues some problems regarding radiation protection for nuclear power plants intended for peaceful purposes.”16

  Given the paramount secrecy of the atomic project, it is obvious that Bruno would not have been told the real agenda behind Pomeranchuk’s questions. And when we consider the fact that the Soviets’ top priority was to construct reactors that could breed tritium, it is naive to suppose that none of Pomeranchuk’s discussions with Pontecorvo had any relevance to the “atomic problem.” It is also naive to assume that Bruno Pontecorvo would not have deduced what was going on.

  THE SECRET NOTEBOOKS

  For Bruno’s first five years in the USSR, he recorded his work at Dubna in classified logbooks. At the end of each day, a letter-sized journal, with секрет (“secret”) stamped on its maroon cover, would be deposited in the laboratory safe. There it would remain until Bruno returned.17

  Each journal consisted of two hundred pages, with every page numbered so that it would be apparent if any were removed. The date of Bruno’s arrival in Dubna is established by his first journal entry. The front cover of his first logbook declares, “начато 1950” (started 1950); the handwritten date on the first page is “1 ноябрь” (November 1). The dates are the only entries in Russian, and appear to have been written by someone other than Bruno. His personal log from each day is written in English.

  IMAGE 15.1. Cover of Bruno Pontecorvo’s first secret logbook in the USSR, 1950. (COURTESY OF GIL PONTECORVO AND THE PONTECORVO CENTENARY EXPOSITION, UNIVERSITY OF PISA.)

  The first logbook begins with a brief entry on how to measure the energies of neutrons v
ery precisely. The application of this question becomes apparent in the next entry, on page 2: “Fission from highly excited states.”

  By this point in his life, Bruno Pontecorvo was a world-leading authority in this field of instrumentation. In Oklahoma, he’d designed a neutron detector for oil prospectors; in Canada, this device had been used to find uranium; at Harwell, he’d developed more sophisticated detectors. Normal fission happens when high-energy neutrons hit atomic nuclei, which are in their most stable state. In the logbook, Bruno comments, “As the fission of medium A [nuclei in the middle of the periodic table] shows, there must [occasionally] be fissions arising from very highly excited states”—states in which one of the constituent neutrons or protons has been raised up the energy ladder temporarily. (Italics added.) He then notes, “These fissions must . . . release plenty of energy in uranium and thorium.”

  Halfway down the second page we come to another question discussed on Bruno’s first day at Dubna: “Is it possible to detect H4 particles inside the chamber?”

  In 1955, H4, or quadium, would play a prominent role in the satirical novel The Mouse That Roared as the isotope powering the “Q-bomb.”18 Today we know that quadium is so highly unstable as to be effectively nonexistent and useless for military purposes. However, the Soviets’ attempt to isolate this exotic isotope was reasonable, given the tritium shortage that hindered their production of thermonuclear bombs. The scientists at Dubna hoped that when beams of deuterons, or alpha particles, smashed into suitable targets, H4 particles might be produced.19 If they were, this might be a fast-track solution to Stalin’s challenge. But first the scientists would have to successfully detect H4 particles, and for this they turned to Bruno Pontecorvo.