Making of the Atomic Bomb

by Richard Rhodes

  Rutherford’s assistant James Chadwick attended this lecture and found cause for disagreement.560 Chadwick was then twenty-nine years old. He had trained at Manchester and followed Rutherford down to Cambridge. He had accomplished much already—as a young man, two of his colleagues write, his output “was hardly inferior to that of Moseley”—but he had sat out the Great War in a German internment camp, to the detriment of his health and to his everlasting boredom, and he was eager to move the new work of nuclear physics along.561 A neutral particle would be a wonder, but Chadwick thought Rutherford had deduced it from flimsy evidence.

  That winter he discovered his mistake. Rutherford invited him to participate in the work of extending the nitrogen transmutation results to heavier elements. Chadwick had improved scintillation counting by developing a microscope that gathered more light and by tightening up procedures. He also knew chemistry and might help eliminate hydrogen as a possible contaminant, a challenge to the nitrogen results that still bothered Rutherford. “But also, I think,” said Chadwick many years later in a memorial lecture, “he wanted company to support the tedium of counting in the dark—and to lend an ear to his robust rendering of ‘Onward, Christian Soldiers.’ ”562

  “Before the experiments,” Chadwick once told an interviewer, “before we began to observe in these experiments, we had to accustom ourselves to the dark, to get our eyes adjusted, and we had a big box in the room in which we took refuge while Crowe, Rutherford’s personal assistant and technician, prepared the apparatus.563 That is to say, he brought the radioactive source down from the radium room, put it in the apparatus, evacuated it, or filled it with whatever, put the various sources in and made the arrangements that we’d agreed upon. And we sat in this dark room, dark box, for perhaps half an hour or so, and naturally, talked.” Among other things, they talked about Rutherford’s Bakerian Lecture. “And it was then that I realized that these observations which I suspected were quite wrong, and which proved to be wrong later on, had nothing whatever to do with his suggestion of the neutron, not really. He just hung the suggestion on to it. Because it had been in his mind for some considerable time.”

  Most physicists had been content with the seemingly complete symmetry of two particles, the electron and the proton, one negative, one positive. Outside the atom—among the stripped, ionized matter beaming through a discharge tube, for example—two elementary atomic constituents might be enough. But Rutherford was concerned with how each element was assembled. “He had asked himself,” Chadwick continues, “and kept on asking himself, how the atoms were built up, how on earth were you going to get—the general idea being at that time that protons and electrons were the constituents of an atomic nucleus . . . how on earth were you going to build up a big nucleus with a large positive charge? And the answer was a neutral particle.”

  From the lightest elements in the periodic table beyond hydrogen to the heaviest, atomic number—the nucleus’ electrical charge and a count of its protons—differed from atomic weight. Helium’s atomic number was 2 but its atomic weight was 4; nitrogen’s atomic number was 7 but its atomic weight was 14; and the disparity increased farther along: silver, 47 but 107; barium, 56 but 137; radium, 88 but 226; uranium, 92 but 235 or 238. Theory at the time proposed that the difference was made up by additional protons in the nucleus closely associated with nuclear electrons that neutralized them. But the nucleus had a definite maximum size, well established by experiment, and as elements increased in atomic number and atomic weight there appeared to be less and less room in their nuclei for all the extra electrons. The problem worsened with the development in the 1920s of quantum theory, which made it clear that confining particles as light as electrons so closely would require enormous energies, energies that ought to show up when the nucleus was disturbed but never did. The only evidence for the presence of electrons in the nucleus was its occasional ejection of beta particles, energetic electrons. That was something to go on, but given the other difficulties with packing electrons into the nucleus it was not enough.

  “And so,” Chadwick concludes, “it was these conversations that convinced me that the neutron must exist. The only question was how the devil could one get evidence for it. . . . It was shortly after that I began to make experiments on the side when I could. [The Cavendish] was very busy, and left me little time, and occasionally Rutherford’s interest would revive, but only occasionally.”564 Chadwick would search for the neutron with Rutherford’s blessing, but the frustrating work of experiment was usually his alone.

  His temperament matched the challenge of discovering a particle that might leave little trace of itself in its passage through matter; he was a shy, quiet, conscientious, reliable man, something of a neutron himself. Rutherford even felt it necessary to scold him for giving the boys at the Cavendish too much attention, though Chadwick took their care and nurturing to be his primary responsibility. “It was Chadwick,” remembers Mark Oliphant, “who saw that research students got the equipment they needed, within the very limited resources of the stores and funds at his disposal.”565 If he seemed “dour and unsmiling” at first, with time “the kindly, helpful and generous person beneath became apparent.”566 He tended, says Otto Frisch, “to conceal his kindness behind a gruff façade.”567

  The façade was protective. James Chadwick was tall, wiry, dark, with a high forehead, thin lips and a raven’s-beak nose. “He had,” say his joint biographers, colleagues both, “a deep voice and a dry sense of humour with a characteristic chuckle.”568 He was born in the village of Bollington, south of Manchester in Cheshire, in 1891. When he was still a small boy his father left their country home to start a laundry in Manchester; Chadwick’s grandmother seems to have raised him. He sat for two scholarships to the University of Manchester at sixteen, an early age even in the English educational system, won them both, kept one and went off to the university.

  He meant to read mathematics. The entrance interviews were held publicly in a large, crowded hall. Chadwick got into the wrong line. He had already begun to answer the lecturer’s questions when he realized he was being questioned for a physics course. Since he was too timid to explain, he decided that the physics lecturer impressed him and he would read for physics. The first year he was sorry, his biographers report: “the physics classes were large and noisy.”569 The second year he heard Rutherford lecture on his early New Zealand experiments and was converted. In his third year Rutherford gave him a research project. His timidity again confounded him, this time almost fatally for his career: he discovered a snag in the procedure Rutherford had recommended to him but could not bring himself to point it out. Rutherford thought he missed it. Man and boy found their way past that misunderstanding and Chadwick graduated from Manchester in 1911 with first-class honors.

  He stayed on for his master’s degree, working with A. S. Russell and following the research in those productive years of Geiger, Marsden, de Hevesy, Moseley, Darwin and Bohr. In 1913, taking his M.Sc., he won an important research scholarship that required him to change laboratories to broaden his training. By then Geiger had returned to Berlin; Chadwick followed. Which was a pleasure while it lasted—Geiger made a point of introducing Chadwick around, so that he became acquainted with Einstein, Hahn and Meitner, among others in Berlin—but the war intervened.

  A reserve officer, Geiger was called up early. He fortified Chadwick with a personal check for two hundred marks before he left. Some of the young Englishman’s German friends advised him to leave the country quickly, but others convinced him to wait to avoid the danger of encountering troop trains along the way. On August 2 Chadwick tried to buy a ticket home by way of Holland at the Cook’s Tours office in Berlin. Cook’s suggested going through Switzerland instead. That struck Chadwick’s friends as risky. He again accepted their advice and settled in to wait.

  Then it was too late. He was arrested along with a German friend for allegedly making subversive remarks—merely speaking English would have done the job in those first
weeks of hysterical nationalism—and languished in a Berlin jail for ten days before Geiger’s laboratory arranged his release. Once out he returned to the laboratory until chaos retreated behind order again and the Kaiser’s government found time to direct that all Englishmen in Germany be interned for the duration of the war.

  The place of internment was a race track at Ruhleben—the name means “quiet life”—near Spandau. Chadwick shared with five other men a box stall designed for two horses and must have thought of Gulliver. In the winter he had to stamp his feet till late morning before they thawed. He and other interns formed a scientific society and even managed to conduct experiments. Chadwick’s cold, hungry, quiet life at Ruhleben continued for four interminable years. This was the time, he said later, making the best of it, when he really began to grow up.570 He returned to Manchester after the Armistice with his digestion ruined and £11 in his pocket. He was at least alive, unlike poor Harry Moseley. Rutherford took him in.

  Some of the experiments Chadwick conducted at the Cavendish in the 1920s to look for the neutron, he says, “were so desperate, so far-fetched as to belong to the days of alchemy.”571 He and Rutherford both thought of the neutron, as Rutherford had imagined it in his Bakerian Lecture, as a close union of proton and electron. They therefore conjured up various ways to torture hydrogen—blasting it with electrical discharges, searching out the effects on it of passing cosmic rays—in the hope that the H atom that had been stable since the early days of the universe would somehow agree to collapse into neutrality at their hands.

  The neutral particle resisted their blandishments and the nucleus resisted attack. The laboratory, Chadwick remembers, “passed through a relatively quiet spell. Much interesting and important work was done, but it was work of consolidation rather than of discovery; in spite of many attempts the paths to new fields could not be found.”572 It began to seem, he adds, that “the problem of the new structure of the nucleus might indeed have to be left to the next generation, as Rutherford had once said and as many physicists continued to believe.”573 Rutherford “was a little disappointed, because it was so very difficult to find out anything really important.”574 Quantum theory bloomed while nuclear studies stalled. Rutherford had felt optimistic enough in 1923 to shout at the annual meeting of the British Association, “We are living in the heroic age of physics!” By 1927, in a paper on atomic structure, he was a little less confident.575 “We are not yet able to do more than guess at the structure even of the lighter and presumably least complex atoms,” he writes.576 He proposed a structure nonetheless, with electrons in the nucleus orbiting around nuclear protons, an atom within an atom.

  They had other work. In hindsight, it was necessary preparation. The scintillation method of detecting radiation had reached its limit of effectiveness: it was unreliable if the counting rate was greater than 150 per minute or less than about 3 per minute, and both ranges now came into view in nuclear studies.577 A disagreement between the Cavendish and the Vienna Radium Institute convinced even Rutherford of the necessity of change. Vienna had reproduced the Cavendish’s light-element disintegration experiments and published completely different results. Worse, the Vienna physicists attributed the discrepancy to inferior Cavendish equipment. Chadwick laboriously reran the experiments with a specially made microscope with zinc sulfide coated directly onto the lens of the microscope’s objective, which greatly brightened the field. The results confirmed the Cavendish’s earlier count. Chadwick then went to Vienna. “He found,” write his biographers, “that the scintillation counting was done by three young women—it was thought that not only did women have better eyes than men but they were less likely to be distracted by thinking while counting!” Chadwick observed the young women at work and realized that because they understood what was expected of the experiments they produced the expected results, unconsciously counting nonexistent scintillations.578 To test the technicians he gave them, without explanation, an unfamiliar experiment; this time their counts matched his own. Vienna apologized.

  Hans Geiger, among others, turned back to the electrical counter he had devised with Rutherford in 1908 and improved it. The result, the Geiger counter, was essentially an electrically charged wire strung inside a gas-filled tube with a thinly covered window that allowed charged particles to enter. Once inside the tube the charged particles ionized gas atoms; the electrons thus stripped from the gas atoms were drawn to the positively charged wire; that changed the current level in the wire; the change, in the form of an electrical pulse, could then be run through an amplifier and converted to a sound—typically a click—or shown as a jump in the sweep of a light beam on the television-like screen of an oscilloscope. The electrical counter could operate continuously and could count above and below the limits possible to fallible physicists peering at scintillation screens. But the early counters had a significant disadvantage: they were highly sensitive to gamma radiation, much more so than zinc sulfide, and the radium compounds the Cavendish used as alpha sources gave off plentiful gamma rays. Polonium, the radioactive element that Marie Curie had discovered in 1898 and named after her native Poland, could be an excellent alternative. It was a good alpha source and with a gamma-ray background 100,000 times less intense than radium it was much less likely to overload an electrical counter. Unfortunately, polonium was difficult to acquire. A ton of uranium ore contained only about 0.1 gram, too little for commercial separation. It was available practically only as a byproduct of the radioactive decay of radium, and radium too was scarce.

  There was time in those years to recover from the bleakness of the war and get on with living. In 1925 Chadwick married Aileen Stewart-Brown, daughter of a family long established in business in Liverpool. He had been living at Gonville and Caius College; now he made plans for permanent residence. A year later, in the midst of house-building, when Rutherford asked him and another Cavendish man to take on part of the work of revising Rutherford’s old textbook on radioactivity, he fitted in the duty at night, working bundled in an overcoat at a writing table moved close to the fireplace of a drafty temporary rental. When the fire burned low he even pulled on gloves.

  At the end of the decade the Rutherfords suffered a personal tragedy. Their daughter Eileen, twenty-nine years old and the mother of three children—she was married to a theoretician, R. H. Fowler, who kept up that end of physics at the Cavendish—gave birth to a fourth; one week later, on December 23, she was felled by a lethal blood clot. “The loss of his only child,” writes A. S. Eve, “whom he loved and admired, aged Rutherford for a time; he looked older and stooped more. He continued his life and work with a manful purpose, and one of the delights of his life was his group of four grandchildren. His face always lit up when he spoke of them.”579

  Rutherford was elevated to baron in the New Year’s Honours List of 1931, the year he would turn sixty. A kiwi crested his armorial bearings; they were supported on the dexter side by a figure representing Hermes Trismegistus, the Egyptian god of wisdom who was supposed to have written alchemical books, and on the sinister side by a Maori holding a club; and the two crossed curves that quartered his escutcheon traced the matched growth and decay of activity that gives each radioactive element and isotope its characteristic half-life.580

  Around 1928 a German physicist, Walther Bothe, “a real physicist’s physicist” to Emilio Segrè, and Bothe’s student Herbert Becker began studying the gamma radiation excited by alpha bombardment of light elements.581 They surveyed the light elements from lithium to oxygen as well as magnesium, aluminum and silver. Since they were concentrating on gamma radiation excited from a target they wanted a minimum gamma background and used a polonium radiation source. “I don’t know how [Bothe] got his sources,” Chadwick puzzles, “but he did.”582 Lise Meitner had generously sent polonium to Chadwick from the Kaiser Wilhelm Institutes, but it was too little to allow Chadwick to do the work Bothe was doing.

  The Germans found gamma excitation with boron, magnesium and aluminum, as they ha
d more or less expected, because alpha particles disintegrate those elements, but they also and unexpectedly found it with lithium and beryllium, which alphas in this reaction did not disintegrate. “Indeed,” writes Norman Feather, one of Chadwick’s colleagues at the Cavendish, “with beryllium, the intensity of the . . . radiation was nearly ten times as great as with any other element investigated.”583 That was strange enough; equally strange was the oddity that beryllium emitted this intense radiation under alpha bombardment without emitting protons. Bothe and Becker reported their results briefly in August 1930, then more fully in December. The radiation they had excited from beryllium had more energy than the bombarding alpha particles. The principle of the conservation of energy required a source for the excess; they proposed that it came from nuclear disintegration despite the absence of protons.

  Chadwick set one of his research students, an Australian named H. C. Webster, to work studying these unusual results. A French team began the same study a little later with better resources: Irene Curie, Mme. Curie’s somber and talented daughter, then thirty-three, and her husband Frédéric Joliot, two years younger, a handsome, outgoing man trained originally as an engineer whose charm reminded Segré of the French singer Maurice Chevalier.

  Marie Curie’s Radium Institute at the east end of the Rue Pierre Curie in the Latin Quarter, built just before the war with funds from the French government and the Pasteur Foundation, had the advantage in any studies that required polonium. Radon gas decays over time to three only mildly radioactive isotopes: lead 210, bismuth 210 and polonium 210, which thus become available for chemical separation. Medical doctors throughout the world then used radon sealed into glass ampules—“seeds”—for cancer treatment. When the radon decayed, which it did in a matter of days, the seeds no longer served. Many physicians sent them on to Paris as a tribute to the woman who discovered radium. They accumulated to the world’s largest source of polonium.