The Joliot-Curies had worked independently for the two years since their marriage in 1927; in 1929 they decided to work in collaboration. They first developed new chemical techniques for separating polonium, and by 1931 had purified a volume of the element almost ten times more intense than any other existing source. With their powerful new source they turned their attention to the mystery of beryllium.
Chadwick’s student H. C. Webster had progressed in the meantime, by the late spring of 1931, beyond recapitulation to discovery: he found, says Chadwick, “that the radiation from beryllium which was emitted in the same direction as the . . . alpha-particles was more penetrating than the radiation emitted in a backward direction.”584 Gamma radiation, an energetic form of light, should be emitted equally in every direction from a point source such as a nucleus, just as visible light radiates equally from a lightbulb filament. A particle, on the other hand, would usually be bumped forward by an incoming alpha. “And that, of course,” Chadwick adds, “was a point which excited me very much indeed, because I thought, ‘Here’s the neutron.’ ”585
With twin daughters now, Chadwick had become a family man of regular habits. Among the most sacred of these was his annual June family vacation. The possibility of finding his long-sought neutron was not sufficient cause to change his plans. It might have been, but he thought he needed a cloud chamber for the next step in the search, and the one immediately available to him at the Cavendish was not in working order. He found a cloud chamber in other hands; its owner agreed to help Webster use it when he had finished using it himself. Still assuming that the neutron was an electron-proton doublet with enough residual electrical charge to ionize a gas at least weakly, Chadwick wanted Webster to aim the beryllium radiation into the cloud chamber and see if he could photograph its ionizing tracks. He left his student to the work and went off on holiday.
“Of course,” Chadwick said in retrospect of the neutron he was hunting for, “they should not have seen anything” in the cloud chamber, nor did they. “They wrote and told me what had happened, that they hadn’t found anything, which disappointed me very much.”586 When Webster moved on to the University of Bristol, Chadwick decided to take over the beryllium research himself.
First he had to shift his laboratory to a different part of the Cavendish building, and that delayed him; then he had to prepare a strong polonium source. In the matter of polonium he was lucky. Norman Feather had spent the 1929–30 academic year in Baltimore, in the physics department at Johns Hopkins, and there befriended an English physician who was in charge of the radium supply at Baltimore’s Kelly Hospital. The physician had stored away several hundred used radon seeds; “together,” Feather remembers, “they contained almost as much polonium as was available to Curie and Joliot in Paris.”587 The hospital donated them to the Cavendish and Feather brought them home. Chadwick accomplished the dangerous chemical separation that autumn.
Irène Joliot-Curie reported her first results to the French Academy of Sciences on December 28, 1931. The beryllium radiation, she found, was even more penetrating than Bothe and Becker had reported. She standardized her measurements and put the energy of the radiation at three times the energy of the bombarding alpha particle.
The Joliot-Curies decided next to see if the beryllium radiation would knock protons out of matter as alpha particles did. “They fitted their ionization chamber with a thin window,” explains Feather, “and placed various materials close to the window in the path of the radiation. They found nothing, except with materials such as paraffin wax and cellophane which already contained hydrogen in chemical combination. When thin layers of these substances were close to the window, the current in the ionization chamber was greater than usual. By a series of experimental tests, both simple and elegant, they produced convincing evidence that this excess ionization was due to protons ejected from the hydrogenous material.”588 The Joliot-Curies understood then that what they were seeing were elastic collisions—like the collisions of billiard balls or marbles—between the beryllium radiation and the nuclei of H atoms.
But they were still committed to their previous conviction that the penetrating radiation from beryllium was gamma radiation. They had not thought about the possibility of a neutral particle. They had not read Rutherford’s Bakerian Lecture because such lectures were invariably, in their experience, only recapitulations of previously reported work. Rutherford and Chadwick alone had thought seriously about the neutron.
On January 18, 1932, the Joliot-Curies reported to the Academy of Sciences their discovery that paraffin wax emitted high-velocity protons when bombarded by beryllium radiation. But that was not the title and the argument of the paper they wrote. They titled their paper “The emission of protons of high velocity from hydrogenous materials irradiated with very penetrating gamma rays.” Which was as unlikely as if a marble should deflect a wrecking ball. Gamma rays could deflect electrons, a phenomenon known as the Compton effect after its discoverer, the American experimental physicist Arthur Holly Compton, but a proton is 1,836 times heavier than an electron and not easily moved.
At the Cavendish in early February Chadwick found the Comptes Rendus, the French physics journal, in his morning mail, discovered the Joliot-Curie paper and read it with widening eyes:
Not many minutes afterward Feather came to my room to tell me about this report, as astonished as I was. A little later that morning I told Rutherford. It was a custom of long standing that I should visit him about 11 a.m. to tell him any news of interest and to discuss the work in progress in the laboratory. As I told him about the Curie-Joliot observation and their views on it, I saw his growing amazement; and finally he burst out “I don’t believe it.” Such an impatient remark was utterly out of character, and in all my long association with him I recall no similar occasion. I mention it to emphasize the electrifying effect of the Curie-Joliot report. Of course, Rutherford agreed that one must believe the observations; the explanation was quite another matter.589
No further duty interposed itself between Chadwick and his destiny. He went fervently to work, starting on February 7, 1932, a Sunday: “It so happened that I was just ready to begin experiment [when he read of the Joliot-Curie discovery]. . . . I started with an open mind, though naturally my thoughts were on the neutron. I was reasonably sure that the CurieJoliot observations could not be ascribed to a kind of Compton effect, for I had looked for this more than once. I was convinced that there was something quite new as well as strange.”
His simple apparatus consisted of a radiation source and an ionization chamber, the chamber connected to a vacuum-tube amplifier and thence to an oscilloscope. The radiation source, an evacuated metal tube strapped to a rough-sawn block of pine, contained a one-centimeter silver disk coated with polonium mounted close behind a two-centimeter disk of pure beryllium, a silver-gray metal that is three times as light as aluminum.590 Alpha particles from the polonium striking beryllium nuclei knocked out the penetrating beryllium radiation, which, Chadwick found immediately, would pass essentially unimpeded through as much as two centimeters of lead.
The half-inch opening into the small ionization chamber that faced this radiation source was covered with aluminum foil. Within the shallow chamber, in an atmosphere of air at normal pressure, a small charged plate collected electrons ionized by incoming radiation and moved their pulses along to the amplifier and oscilloscope. “For the purpose at hand,” explains Norman Feather, “such an arrangement was ideal. If the amplifier were carefully designed, it was possible to ensure that the magnitude of the oscillograph deflection was directly proportional to the amount of ionization produced in the chamber. . . . The energy of the recoil atom producing the ionization could thus be calculated directly from the size of the deflection on the oscillograph record.”591
Chadwick mounted a sheet of paraffin two millimeters thick in front of the aluminum-foil window into the ionization chamber; immediately, he wrote in his final report on the experiment, “the number of deflec
tions recorded by the oscillograph increased markedly.” That showed that particles ejected from the paraffin were entering the chamber. Then he began interposing sheets of aluminum foil between the wax and the chamber window until no more kicks appeared on the oscilloscope; by scaling the absorptions of aluminum compared to air he calculated the range of the particles as just over 40 centimeters in air; that range meant “it was obvious that the particles were protons.”592
Thus repeating the Joliot-Curie work prepared the way. Now Chadwick broke new ground. He removed the paraffin sheet. He wanted to study what happens to other elements bombarded directly by the beryllium radiation. Elements in the form of solids he mounted in front of the chamber window: “In this way lithium, beryllium, boron, carbon and nitrogen, as paracyanogen, were tested.”593 Elements in the form of gases he simply pumped into the chamber to replace the ambient air: “Hydrogen, helium, nitrogen, oxygen, and argon were examined in this way.”594 In every case the kicks increased on the oscilloscope; the powerful beryllium radiation knocked protons out of all the elements Chadwick tested. It knocked about the same number out of each element. And, most important for his conclusion, the energies of the recoiling protons were significantly greater than they could possibly be if the beryllium radiation consisted of gamma rays. “In general,” Chadwick wrote, “the experimental results show that if the recoil atoms are to be explained by collision with a [gamma-ray photon], we must assume a larger and larger energy for the [photon] as the mass of the struck atom increases.”595 Then, quietly, in what in fact is a devastating criticism of the Joliot-Curie thesis, invoking the basic physical rule that no more energy or momentum can come out of an event than went into it: “It is evident that we must either relinquish the application of the conservation of energy and momentum in these collisions or adopt another hypothesis about the nature of the radiation.” When they read that sentence the Joliot-Curies were deeply and properly chagrined.
The hypothesis Chadwick proposed adopting should come as no surprise: “If we suppose that the radiation is not a [gamma] radiation, but consists of particles of mass very nearly equal to that of the proton, all the difficulties connected with the collisions disappear, both with regard to their frequency and to the energy transfer to different masses. In order to explain the great penetrating power of the radiation we must further assume that the particle has no net charge. . . . We may suppose it [to be] the ‘neutron’ discussed by Rutherford in his Bakerian Lecture of 1920.”
Chadwick then worked the numbers to show that his hypothesis was the correct one to explain the facts.
“It was a strenuous time,” he said afterward.596 From beginning to end the work took ten days and he kept up his Cavendish responsibilities besides. He averaged perhaps three hours of sleep a night, labored over the weekend of February 13–14 as well, finished probably on the seventeenth, a Wednesday, the day he sent off a first brief report to Nature to establish priority of discovery. He titled that report, published as a letter to the editor, “Possible existence of a neutron.” “But there was no doubt whatever in my mind or I should not have written the letter.”597
“To [Chadwick’s] great credit,” writes Segré in tribute, “when the neutron was not present [in earlier experiments] he did not detect it, and when it ultimately was there he perceived it immediately, clearly and convincingly.598 These are the marks of a great experimental physicist.”
A young Russian, Peter Kapitza, had come up to Cambridge in 1921 to work at the Cavendish. He was solid, dedicated, charming and technically inventive and he soon made himself the apple of Rutherford’s eye, the only one among all the boys, even including Chadwick, who could convince the frugal director to allow large sums of money to be spent for apparatus. In 1936 Rutherford would attack Chadwick angrily for encouraging the construction of a cyclotron at the Cavendish; but already in 1932 Kapitza had a separate laboratory in an elegant new brick building in the Cavendish courtyard for his expensive experiments with powerful magnetic fields. As Kapitza had settled in at Cambridge he had noticed what he considered to be an excessive and unproductive deference of British physics students to their seniors. He therefore founded a club, the Kapitza Club, devoted to open and unhierarchical discussion. Membership was limited and coveted. Members met in college rooms and Kapitza frequently opened discussions with deliberate howlers so that even the youngest would speak up to correct him, loosening the grip of tradition on their necks.
That Wednesday Kapitza wined and dined the exhausted Chadwick into what Mark Oliphant calls “a very mellow mood,” then brought him along to a Kapitza Club meeting.599 “The intense excitement of all in the Cavendish, including Rutherford,” Oliphant remembers, “was already remarkable, for we had heard rumors of Chadwick’s results.” Oliphant says Chadwick spoke lucidly and with conviction, not failing to mention the contributions of Bothe, Becker, Webster and the Joliot-Curies, “a lesson to us all.”600 C. P. Snow, who was also present, remembers the performance as “one of the shortest accounts ever made about a major discovery.” When tall and birdlike Chadwick finished speaking he looked over the assembly and announced abruptly, “Now I want to be chloroformed and put to bed for a fortnight.”601
He deserved his rest. He had discovered a new elementary particle, the third basic constituent of matter. It was this neutral mass that compounded the weight of the elements without adding electrical charge. Two protons and 2 neutrons made a helium nucleus; 7 protons and 7 neutrons a nitrogen; 47 protons and 60 neutrons a silver; 56 protons and 81 neutrons a barium; 92 protons and 146 (or 143) neutrons a uranium.
And because the neutron was as massive as a proton but carried no electrical charge, it was hardly affected by the shell of electrons around a nucleus; nor did the electrical barrier of the nucleus itself block its way. It would therefore serve as a new nuclear probe of surpassing power of penetration. “A beam of thermal neutrons,” writes the American theoretical physicist Philip Morrison, “moving at about the speed of sound, which corresponds to a kinetic energy of only about a fortieth of an electron volt, produces nuclear reactions in many materials much more easily than a beam of protons of millions of volts energy, traveling thousands of times faster.”602 Ernest Lawrence’s cyclotron, spiraling protons to million-volt energies for the first time the same month that Chadwick made his fateful discovery, fortunately proved to be adaptable to the production of neutrons. More than any other development, Chadwick’s neutron made practical the detailed examination of the nucleus. Hans Bethe once remarked that he considered everything before 1932 “the prehistory of nuclear physics, and from 1932 on the history of nuclear physics.”603 The difference, he said, was the discovery of the neutron.
Word of the discovery reached Copenhagen in the midst of preparations for an amateur theatrical, a parody of Goethe’s Faust, to celebrate the tenth anniversary of the opening of Bohr’s Institute for Theoretical Physics. The postdoctoral dramatists gave the new particle the last word. They had cast Wolfgang Pauli, a corpulent man with a smooth, round face and protuberant, heavy-lidded eyes who resembled the actor Peter Lorre, as Mephistopheles, Bohr as The Lord. Eclectically they cast Chadwick in absentia as Wagner and an anonymous illustrator drew him into the script, “the personification of the ideal experimentalist” according to the stage directions, balancing a vastly magnified neutron on his finger:604
In Copenhagen, as before in Cambridge, Chadwick reports his discovery briefly and succinctly:
The Neutron has come to be.
Loaded with Mass is he.605
Of Charge, forever free.
Pauli, do you agree?
Pauli steps forward to dispense his Mephistophelean blessing:
That which experiment has found—
Though theory has no part in—
Is always reckoned more than sound
To put your mind and heart in. . . .606
And a chorus of clowning, friendly physicists, Bohr’s brilliant young crew, dances out to sing a finale and bring the curtain d
own:
Now a reality,
Once but a vision.607
What classicality,
Grace and precision!
Hailed with cordiality,
Honored in song,
Eternal Neutrality
Pulls us along!
It was the last peaceful time many of them would know for years to come.
7
Exodus
“Antisemitism is strong here and political reaction is violent,” Albert Einstein wrote Paul Ehrenfest from Berlin in December 1919.608 The letter coincides with Einstein’s discovery by the popular press, the beginning of his years of international celebrity. “A new figure in world history,” the Berliner Illustrirte Zeitung described him under a cover photograph on December 14, “. . . whose investigations signify a complete revision of our concepts of nature, and are on a par with the insights of a Copernicus, a Kepler, a Newton.”609 Immediately the anti-Semites and fascists set to work on him.
Einstein was already, at forty-three, respected in the first rank of theoretical physicists. He had been nominated for the Nobel Prize in all but two years since 1910, the secondings increasing in number after 1917; Max Planck, who was not given to exaggeration, wrote the Nobel Committee in 1919 that Einstein “made the first step beyond Newton.”610, 611 The award might have come sooner than in 1922 (belatedly for 1921: the 1922 prize was Bohr’s) had relativity been less paradoxical a revelation.
Physically Einstein was not yet the amused, grandfatherly notable of his later American years. His mustache was still dark and his thick black hair had only begun to gray. C. P. Snow would observe “a massive body, very heavily muscled.”612 The Swabian-born physicist’s friends thought his loud laugh boyish; his enemies thought it rude. “A powerful sensuality,” Snow suspected, suspecting also that Einstein took his sensuality to be “one of the chains of personality that ought to be slipped off.”613 Nor had he yet learned, in the psychoanalyst Erik Erikson’s words, “to look into cameras as if he were meeting the eyes of the future beholders of his image.”614 In the past year Einstein had endured a stomach ulcer, jaundice and a painful divorce; he had lost and partly regained fifty-six pounds; his mother was dying of cancer: fatigue stained his expressive face. Leopold Infeld, a young Polish physicist who knocked at his door in postwar Berlin seeking a letter of recommendation, found him “dressed in a morning coat and striped trousers with one important button missing.” Infeld knew Einstein’s face from magazines and newsreels. “But no picture could reproduce the shining glow of his eyes.”615 They were large and dark brown, and the diffident young visitor was one of many—Leo Szilard was another—who found comfort in those cold days in their honest warmth.