Before the Fallout

by Diana Preston

  The experimenters of the Cavendish Laboratory were less immediately impressed by the deluge of fresh ideas. James Chadwick recalled that "it took quite a time to absorb the meaning of the new quantum mechanics. It was rather slow. . . . there was no immediate application to the structure of the nucleus, which was what we were interested in." Rutherford was frankly skeptical of the complex new mathematical theories, preferring to scent new discoveries in some unexpected experimental result rather than indulge in abstract theorizing. Only in the late 192os did he concede somewhat grudgingly that wave mechanics might aid the understanding of the nucleus. In the meantime his laboratory remained the greatest center of experimental physics in the world. His only rivals were Lise Meitner and Otto Hahn in Berlin and Marie Curie and Irene and Frederic Joliot-Curie—as the pair chose to be known to emphasize their close collaboration—in Paris.* All the other major players were theorists.

  Rutherford had been convinced for many years that an undetected particle at the heart of the nucleus, the "neutron," as he called it, was the great, unclaimed prize. As early as June 1920 he had talked to the Royal Society of the possible existence of such a particle. His discovery, the year before, of the positively charged proton, residing in the nucleus of every atom, had provided tantalizing clues. For example, the simplest, lightest atom—hydrogen—had one single, positively charged proton counterbalanced by one external, negatively charged electron. The next-heaviest atom—helium—had two protons and two orbiting electrons. However, its mass, or atomic weight, was not, as might have been expected, double that of hydrogen. It was quaduple. This could only mean that it had to have one or more electrically neutral particles, equivalent in mass to, and complementing, the two protons. Rutherford speculated intuitively that the missing piece of the jigsaw, his "neutron," consisted of electrons and protons parceled together.

  Although Rutherford continued to think about the neutron throughout the 1920s and undertook experiments when he could, he was frequently distracted by other work, including the pressures of university administration and serving on national public committees. His ennoblement in 1931 by King George V as Baron Rutherford only added to the commitments of a man who was still considerably shaken by the sudden death in 1930 from a blood clot of his only child, his daughter, Eileen. She had left four children, to whom Rutherford was deeply attached, from her marriage to a Cavendish mathematician.

  Realizing that domestic pressures and public duties would continue to hamper his search for the neutron, Rutherford entrusted more and more of the hunt to James Chadwick, who had already been working on the topic for him since the mid-1920s and who, in his own words, "just kept on pegging away" and "did quite a number of quite silly experiments" just in case they turned something up. In fact, he worked obsessively. His efforts attracted affectionate satire from junior members of the laboratory, who staged a show raucously lampooning the hunt for the elusive "Fewtron."

  Chadwick made his breakthrough in January 1932, precipitated by a paper by the Joliot-Curies in the French journal Comptes Rendus. This described how, building on work by the German scientist Walther Bothe, they had bombarded the light element beryllium—a hard, silvery, toxic metal—with an intense source of polonium, causing an unusually penetrating radiation to stream out of the beryllium. The Joliot-Curies experimented with various substances, including wax, to see whether they could halt the rays from the beryllium, but the rays not only passed through the barriers but appeared to get stronger. The puzzled Joliot-Curies concluded in their paper that the radiation had to consist of some particularly powerful form of gamma rays—the most penetrating of the three types of radiation emitted by radioactive substances. Rutherford read their conclusions and roared, "I don't believe it." Chadwick, too, "knew in his bones" that they were wrong. Their description of the pattern and path of the radiation they had observed convinced him that it consisted of uncharged or neutral particles knocked out of the nuclei of the beryllium—in other words, neutrons. Chadwick rushed to replicate their experiments.

  Applying the classic "sealing wax and string" principles of the Cavendish to make his equipment the simplest fit for the purpose, an excited but careful Chadwick worked day and night. He violated Rutherford's rule that all work in the laboratory should cease by 6 p.m., partly through irrepressible enthusiasm but also so that his sensitive counting equipment would not be affected by other work going on in the laboratory. After three weeks he had shown that radiation from bombarded beryllium was powerful enough to knock particles out of hydrogen, helium, lithium, beryllium, carbon, and argon. The particles expelled from the hydrogen were clearly protons, and the others were whole nuclei of the target substance. His measurements of their penetrating power and velocity proved that gamma rays could never have caused the ejection of particles of such energy. The only viable conclusion was that the radiation flowing so powerfully from the bombarded beryllium consisted of "particles of mass 1 and charge o"—neutrons.

  Chadwick chose the Kapitza Club as the forum for revealing his findings. There was an air of keen anticipation as Chadwick, gray-faced from lack of sleep but plainly exhilarated, addressed his audience. Mark Oliphant captured the moment in the restrained language of the day: "Kapitza had taken him to dine in Trinity beforehand, and he was in a very relaxed mood. His talk was extremely lucid and convincing, and the ovation he received from the select audience was spontaneous and warm. All enjoyed the story of a long quest, carried through with persistence and vision."* At the end, the exhausted Chadwick asked "to be chloroformed and put to bed for a fortnight." In fact, he was up again the next morning writing to Niels Bohr and, a month after first reading the Joliot-Curies' paper, sending a letter to Nature cautiously headed "The possible existence of the neutron." His entry in the notebook recording presentations to the Kapitza Club was similarly guarded; it read, "Neutron?" Chadwick was instinctively cautious. Yet however he hedged his findings, he knew in his heart he was right.

  Chadwick was not, as he freely acknowledged, the first to produce neutrons. Walther Bothe had done so in Germany in the 1920s. So had the Joliot-Curies, following in Bothe's wake. However, none of them had interpreted their experiments correctly and established the existence of the neutron. Chadwick's achievement, in the words of the distinguished Italian physicist Emilio Segre, was "immediately, clearly and convincingly" to recognize neutrons for what they were—the true hallmark "of a great experimental physicist." Chadwick put it more modestly and prosaically: "The reason that I found the neutron was that I had looked, on and off, since about 1923 or 4. I was convinced that it must be a constituent of the nucleus."

  The discovery was a blow to Frederic Joliot-Curie, who wrote privately of his frustration: "It is annoying to be overtaken by other laboratories which immediately take up one's experiments." However, his public response was gracious and generous. It was "natural and just" that the final steps of the journey toward the neutron were undertaken at the Cavendish, since "old laboratories with long traditions have . . . hidden riches."

  Chadwick's achievement marked a watershed. Nuclear physics (the study of the atom's nucleus) as opposed to atomic physics (the study of atoms) had been in the doldrums. Scientists had faced difficulties of interpretation that arose far more swiftly than they could be resolved. Chadwick's discovery provided the all-important clue to many unresolved problems. For example, the neutron added to the understanding of isotopes (discovered in 1913 by Frederick Soddy). Until then, no one had known exactly what differentiated isotopes from their "sister" element. The suspicion was that the difference lay in the nucleus, but it took Chadwick's findings to prove that suspicion correct; what made isotopes different was the number of neutrons in their nuclei. But most exciting of all was the realization that the neutron, which carried no electrical charge, would not be deflected by the positive nuclear charge. It was the ideal missile with which to bombard and probe elements, as it could hurtle on until it penetrated the nucleus of the atom.

  Across Europe scientists to
ok note. In Germany the physicist Hans Bethe, later the head of theoretical physics at Los Alamos and an architect of the atomic bomb, decided that the discovery of the neutron made nuclear physics the field in which to work. In Rome the Italian scientist Enrico Fermi—yet another of the fraternity who had studied under Max Born in Gottingen in the 1920s and till then a theoretical physicist—plunged into experimental nuclear physics, setting up a small group to explore the interactions of neutrons "with any elements he could get hold of."

  What none of them yet knew was that the neutron was also the catalyst for achieving an explosive nuclear chain reaction. Curiously, though, that very year, 1932, Harold Nicolson published a novel, Public Faces, abouc a catastrophically destructive new weapon made from a powerful raw material. This substance could transmute itself with such violence that it could cause an explosion "that would destroy all matter within a considerable range and send out waves that would exterminate all life over an indefinite area." "The experts," Nicholson wrote in his novel, "had begun to whisper the words . . . 'atomic bomb.'" They claimed it could "destroy New York."

  · · ·

  Neutrons were by no means the only reason 1932 would be recalled as a spectacular year in the history of science. In January, just a few weeks before Chad­wick's coup, the American chemist Harold Urey made another discovery that Rutherford had long predicted. Working at Columbia University, Urey found that natural hydrogen consisted of 99.985 percent ordinary hydrogen but also of 0.015 percent "heavy hydrogen"—an isotope given the name "deu­terium"—which also existed naturally in combination with oxygen in water. This so-called "heavy water"—which appeared to the naked eye identical to ordinary water—boiled and froze at different temperatures and was 1 o percent heavier. A decade later it would become a substance much sought after by the Nazis, and people would die to deny it to them.

  But in 1932 Urey thought of deuterium as a "delightful plaything for physicists" to use in bombarding other more complex atoms so that they could better understand nuclear structure. He speculated whether heavy water itself might be "valuable in understanding more of living processes," perhaps even in the study of cancer since some initial research showed that yeast cells, which had some similarities to cancer cells, multiplied less quickly in heavy water than in ordinary. This proved impracticable. Nevertheless, heavy water caught the American public's attention. In a 1935 fictional murder mystery, the villain killed by persuading the victim to enter a swimming pool filled with heavy water, which the author described as "lethal."* In a review a scientist wrote, "It is the most expensive murder on record. . . . at the present cost that pool of heavy water would have cost about $ 200 million."

  On 21 April 1932, a few weeks after the neutron discovery, Rutherford reported another Cavendish triumph, writing exuberantly to Bohr, "It never rains but it pours." John Cockcroft and Ernest Walton had just become the first scientists to split the atom using a man-made machine, an accelerator—the device Rutherford had asked them to develop some time earlier. They had created it lovingly and carefully, smoothing plasticine—an innovative new material which had replaced the sealing wax previously used for this purpose—over the joints to create a vacuum. Fearing that rivals might overtake them, Rutherford had urged them to stop perfecting it and "do what he'd told them to do months ago"—start experimenting. His bullying paid off. Cock­croft and Walton bombarded lithium with accelerated protons and succeeded in disintegrating the lithium nucleus into two helium nuclei. According to one of his colleagues, John Cockcroft, "normally about as much given to emotional display as the Duke of Wellington," ran through Cambridge shouting, "We've split the atom! We've split the atom!" An additional excitement was that the energies of the particles measured by Cockcroft and Walton provided the first experimental confirmation of the validity of Einstein's proposal that E = mc2.

  Rutherford asked Cockcroft and Walton to temper their jubilation in favor of discretion to allow them time to exploit their discovery without alerting rivals. However, with a media increasingly hungry for further revelations about nuclear physics following Chadwick's recent discovery of the neutron, soon it seemed only sensible to court press attention. The team chose the Marxist science correspondent of the Manchester Guardian to announce their achievement.

  · · ·

  Rutherford had been right to fear competition. The Cavendish might easily have been upstaged by Ernest Lawrence at Berkeley. While Cockcroft and Walton had been busily massaging plasticine over the joints of their accelerator, Lawrence had been developing a successor to his small octopus-armed pillbox. His new cyclotron was an eleven-inch version. In August 1931 his assistant, Milton Stanley Livingston, achieved an energy of over one million electron volts with the new machine-surely enough to accelerate particles to split atoms. Livingston asked Lawrence's secretary to send him a telegram, which read, "Dr. Livingston has asked me to advise you that he has obtained 1,100,000 volt protons. He also suggested that I add 'Whoopee!'" When he received it, Lawrence "literally danced around the room," pale blue eyes shining with excitement and already planning yet bigger, more powerful devices.

  Ernest Lawrence's eleven-inch cyclotron

  It was therefore a shock to Lawrence, honeymooning happily in Connecticut in the summer of 1932, to learn that Cockcroft and Walton's linear accelerator had become the first device to disintegrate the nucleus with accelerated particles. He sent agitated telegraphic orders to Berkeley: "Get lithium from chemistry department and start preparations to repeat with cyclotron. Will be back shortly." Success was not far off. A few weeks later, the president of the university dispatched a jubilant message to the governor of California: "In September of 1932 artificial disintegration was first accomplished outside of Europe in the Laboratory of Professor Ernest O. Lawrence. This laboratory has taken the lead, in all the world, in the disintegration of the elements."

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  Lawrence had been joined at Berkeley in the autumn of 1929 by a young scientist who shared his ambition to help the United States take "the lead in all the world": the twenty-five-year-old J. Robert Oppenheimer. Slenderly built, with intensely blue eyes, friends thought him "both subtly wise and terribly innocent." He was also sensitive, conceited, often neurotic, but charismatically engaging. Though passionate about physics, he was a Renaissance man with obsessions ranging from Hindu philosophy to Dante's Inferno.

  Oppenheimer had grown up in New York, the product of a wealthy, cultured Jewish family whose Riverside Drive apartment was hung with paintings by impressionist masters. He had been, in his own words, "an abnormally, repulsively good little boy." After attending New York's exclusive Ethical Culture School, he went on to Harvard. Like many contemporaries in continental Europe, Oppenheimer's early years were not free of anti-Semitism, albeit differently expressed. He arrived at Harvard shortly after its president had recommended a quota for Jewish undergraduates. When he applied to go and study under Rutherford at the Cavendish, his Harvard professor's letter of recommendation concluded, in character with the times: "As appears from his name, Oppenheimer is a Jew, but entirely without the usual qualifications of his race. He is a tall, well set-up young man, with a rather engaging diffidence of manner, and I think you need have no hesitation . . . in considering his application."

  Rutherford, who would never have dreamed of being influenced by matters of race and had a deep contempt for racists, accepted Oppenheimer but was unimpressed by his abilities as an experimentalist. Bohr, while visiting the Cavendish, asked an obviously unhappy Oppenheimer how his work was going. Oppenheimer replied that he was having difficulties. When Bohr asked whether his problems were mathematical or physical, he despairingly said that he didn't know. Bohr replied with devastating if unhelpful honesty, "That's bad." Oppenheimer spent tortured days standing by a blackboard, chalk in hand, but unable to write anything. He could hear himself saying, over and over, "The point is. The point is. The point is. . . ."

  Such were Oppenheimer's inner frustration and turmoil that, during a
reunion with a friend, Francis Fergusson, in Paris he became so enraged by something Fergusson said that he leaped on him and tried to strangle him, forcing the more powerfully built Fergusson to fend him off. Back in Cambridge a contrite Oppenheimer wrote to Fergusson, seeking forgiveness for his bizarre behavior and explaining how his failure to live up to "the awful fact of excellence" was tormenting him. He remained troubled, depressed, and occasionally deluded. On one occasion he insisted that he had left a poisoned apple on the desk of a colleague at the Cavendish. For a while a psychiatrist treated him for dementia praecox. There are conflicting stories about why the treatment ended in 1926. According to one, the psychiatrist warned that continuing would do more harm than good. According to the other—and this sounds more likely—Oppenheimer decided he understood more about his condition than his doctor and canceled further sessions. When Max Born visited the Cavendish in 1926 and invited him to Gottingen, Oppenheimer accepted with gratitude but little confidence in his own abilities.

  However, Oppenheimer shook off the worst of his depression and mood swings and flourished at Gottingen. More than at either Harvard or Cambridge he felt, in his words, "part of a little community of people who had some common interests and tastes and many common interests in physics." His passion was theoretical physics, and Gottingen was the focus of the theoretical physics world with all of its leaders teaching there or regularly visiting. Oppenheimer wrote to a friend, "They are working very hard here, and combining a fantastically impregnable metaphysical disingenuousness with the go-getting habits of a wall paper manufacturer. The result is that the work done here has an almost demonic lack of plausibility to it and is highly successful."