The Neutron and the Bomb Read online

  The Neutron and the Bomb

  A biography of Sir James Chadwick

  by Andrew Brown

  Published by Plunkett Lake Press, August 2018

  © Andrew Brown, 1997

  Cover by Susan Erony

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  Contents

  Preface

  Acknowledgements

  1 ~ Obscure origins

  References (chapter 1)

  2 ~ Manchester — the nuclear nursery

  References (chapter 2)

  3 ~ Germany and war

  References (chapter 3)

  4 ~ A new beginning

  References (chapter 4)

  5 ~ Sparks don’t fly

  References (chapter 5)

  6 ~ Discovery of the neutron

  References (chapter 6)

  7 ~ International renown

  References (chapter 7)

  8 ~ Liverpool and the Nobel Prize

  References (chapter 8)

  9 ~ His own man — and Rutherford’s

  References (chapter 9)

  10 ~ The post-heroic age

  References (chapter 10)

  11 ~ The big question

  References (chapter 11)

  12 ~ The MAUD report

  References (chapter 12)

  13 ~ Transatlantic travails

  References (chapter 13)

  14 ~ The new world

  References (chapter 14)

  15 ~ The scientist-diplomat

  References (chapter 15)

  16 ~ A different world

  References (chapter 16)

  17 ~ Sir Atom

  References (chapter 17)

  18 ~ Liverpool and Europe

  References (chapter 18)

  19 ~ The mastership

  References (chapter 19)

  20 ~ Final reflections

  References (chapter 20)

  Appendix 1

  Appendix 2

  Preface

  One spring afternoon in the early 1950s, an economist and a nuclear physicist were sauntering idly along the backs of the River Cam. Just as a couple of schoolboys on the same walk might have argued about the batsmanship of Denis Compton and Len Hutton, the two young dons passed the time making comparisons between academic figures. The economist asked the physicist for his opinion of two leading Cambridge physicists, Sir George Thomson and Sir James Chadwick. Both were Nobel Laureates and the Masters of Cambridge Colleges — Corpus Christi and Gonville and Caius, respectively. Sir George was of the Cambridge purple: the son of J.J. Thomson, himself a Nobel prizewinner and discoverer of the electron. In the physicist’s opinion, George Thomson was a first class scientist, who thoroughly deserved the recognition he had received for his work on the diffraction of electrons passing through very thin films of metals. By contrast, Chadwick, the discoverer of the neutron, was quite simply one of the makers of modern physics.

  By coincidence Chadwick’s father shared J.J. Thomson’s initials and even came from the same part of England, but worked in a cotton mill and died in his seventies, unmourned by his eldest son. James Chadwick was a very close man when it came to personal matters, and I have not been able to discover the reason for the estrangement from his family. He was a product of the late Victorian age, and the aspiration to better himself was a natural one, so that it cannot have been just the urge to shake off his working class origins that was responsible.

  Chadwick’s keen intelligence was nurtured by remarkably good state schools, and his scientific career blossomed under Rutherford’s inspirational leadership, first in Manchester and then at the Cavendish Laboratory in Cambridge. A less fortunate legacy of being born an Englishman in the last decade of the nineteenth century was the duty to fight, and often die, for King and Country in the 1914-1918 war. As readers will see, Chadwick avoided this terrible opportunity by being interned in Germany, where he had been studying with Hans Geiger. Geiger along with many other German scientists went off to the trenches, as did their English counterparts, but the personal warmth and regard for former scientific colleagues never dimmed, as is made plain in some of Chadwick’s and Rutherford’s wartime correspondence. By the time of the second world war, knowledge in the field of nuclear physics had advanced so far, in large part due to the experimental work carried out and supervised by Rutherford and Chadwick at the Cavendish, that scientists were too valuable to risk in mere combat; by degrees, they came to realize that they held the potential to unleash a new weapon of unimaginable power on the world. Chadwick’s life was emblematic of these changes: indeed he was unique in history as the only man to witness both the Kaiser announcing the outbreak of war to Berliners in 1914 and the explosion of the first atomic bomb in the New Mexico desert in 1945. Between these two cataclysmic events Chadwick discovered the subatomic particle that launched the atomic age; he was centrally involved in the bomb’s development from the first months of the war.

&
nbsp; There were no such possible ramifications in Chadwick’s mind while he carried out his dogged search for the neutron. It is remarkable how the Cavendish Laboratory in the 1920s and 1930s managed to retain the carefree approach of happier times, with scarcely a thought given to the utility of any of its discoveries. In Chadwick’s view they were competing with nature to unlock its secrets. Apart from some banter between him and Rutherford about alchemy, commercial considerations were entirely absent; nor were practical applications of major concern. As captain of the vessel, Rutherford set a bold course to explore the innermost depths of the nucleus, and Chadwick as his helmsman stuck to his directions tenaciously. The impenetrability of their chosen route led to criticism from more faint-hearted and worldly bodies, but they were not to be deflected. Their purity of purpose meant that the only ethical dilemma that confronted them concerned the means of their search — were they being scrupulously objective in their observations — and this was the subject of a spirited controversy with the school of physics in Vienna. There were no qualms about the outcome — knowledge could not be inherently dangerous.

  Chadwick was abruptly thrown from this state of innocence when he realized that the energy stored in the uranium nucleus could be released in an explosion of devastating power. It seemed highly probable to him that his former colleagues in Germany would have reached the same conclusion, and so despite any personal misgivings, it was necessary to pursue this idea as vigorously as possible to the point of production of a weapon. His efforts towards this end both as a scientific leader and in the unaccustomed role of a trusted diplomat were immeasurable. When the first atomic bomb was successfully detonated at the Trinity test, Germany was no longer a combatant and the United States was clearly the only country in possession of the weapon, but Chadwick thought its use against Japan was justified. In the immediate aftermath, he held a press conference in Washington and warned an unheeding American public that the US monopoly in nuclear weapons would be short-lived.

  Unlike Bohr, Oppenheimer, Teller or his own Liverpool research student Joseph Rotblat, Chadwick refrained from any other public statements on the politics of the bomb. In part this was because of innate shyness, but mainly because he regarded such policies as the prerogative of statesmen rather than of scientists. He once wrote that ‘Rutherford’s genius was in getting knowledge not in expounding it’, and regarded his own talents in a similar, if lesser, light. On the question of controlling nuclear weapons, it was clear to Chadwick that the pitfalls were so cavernous and the stakes so high that there was never going to be an easy and guaranteed solution. He saw the problems of verification and international enforcement more clearly than many of his scientific contemporaries. There was a broad streak of realism, tainted with pessimism, running through his core. His lack of outward enthusiasm and unfailingly critical nature may have been the reason that General Groves seized on him as the most reliable scientist on the Manhattan Project. His unquestioned integrity enabled him to maintain the close relationship with Groves without losing the respect of his fellow scientists.

  Even in the presence of such dominant personalities as Rutherford and Groves, Chadwick was always his own man. With Rutherford, any disagreements were short-lived, and caused Chadwick more anguish than they did his chief. Groves was a more arrogant and ruthless character than Rutherford, but Chadwick stood up to him quietly and implacably. Nowhere was this more evident than in their first meeting in 1943 in Washington, where Chadwick demolished the General’s cherished compartmentalization approach to security. Chadwick’s own attitude to security was complex and individual. He first confronted the question in Liverpool early in the war, where two of his key workers — Frisch and Rotblat — were classified as aliens. He thought the petty restrictions placed on their movements and activities were absurd and encouraged the two young men to flout them. He organized visits for Frisch to other universities so that he could share his thoughts on nuclear fission with other leading experts, and allowed Rotblat to include nuclear fission in lectures to undergraduates. Yet he stopped short of showing them the Maud Report he was compiling for the government on the prospects for an atomic weapon. Later in the war, he interviewed Frédéric Joliot-Curie after the liberation of Paris, and scrupulously observed the letter of the Quebec Agreement, which forbade any disclosure about the bomb to a non-Anglo-American.

  Chadwick recognized that the fertilization of ideas depends on free communication, and he held to this philosophy even in the quasi-military setting of the Manhattan Project. After a while, General Groves seemed to appreciate his approach and asked to be supplied with some of Chadwick’s correspondence, since he found it was the most efficient way of gleaning what important advances were being made. Chadwick’s trust was betrayed by two members of the British contingent — the atomic spies Fuchs and Nunn May. The former seems to have fooled everyone, whereas from records I examined in the American National Archives, Groves suspected Nunn May, just from monitoring his movements between Montreal and the Argonne Laboratory. In her new biography of Lise Meitner, Ruth Sime refers to an evening in February 1946 which Meitner spent with Chadwick in Washington. Meitner was disappointed because he would not be drawn on questions about the bomb, and his sparse comments on the subject seemed to be intended to cut off any further questions. While this episode again reflected his adherence to the Quebec Agreement, his reticence was surely reinforced by the growing hysteria that week in DC, as the news of the Canadian spy ring became public; Chadwick knew that there would soon be more disclosures about Nunn May’s role in particular. As was often the case during the war, Chadwick was preoccupied by sensitive information which he could not discuss with any of his scientific peers. Despite this, his natural predilection for open debate remained, and the following year he wrote a delicious letter of reprimand to the Cabinet Office in London for labelling as ‘secret’ a public lecture given by the head of the newly formed US Atomic Energy Commission.

  Although I cannot pretend to retain an objective view of my subject, it would seem to me that he led an extraordinarily eventful life and that his stature as a physicist, not to mention his subsequent distinguished war service, easily merit historical study. Why is this the first biography of Chadwick to be written? He never sought the limelight and was always exceedingly modest about his own accomplishments. In the copious press stories last summer commemorating the fiftieth anniversary of the bomb, I noted Chadwick’s name just once in the New York Times, and not at all in the British newspapers. As a man he was not easy to approach and the reputation he earned tended to be one of dourness and acerbity. This persona is dismantled by his own hand in correspondence, which apart from providing a concise and stylish record of contemporary events, also reveals a sharp wit and often surprising sensitivity about the feelings of others. The seventh Duke of Devonshire, founder of the Cavendish Laboratory, was described in an admittedly servile Vanity Fair portrait as: ‘Shy and reserved, he has been unjustly accused of supercilious reserve and want of geniality, but in fact there is no man more ready than he to do a kind or a generous act’. I hope I will leave the reader with the impression that Chadwick was very much in this Cavendish mould.

  The other problem with relating Chadwick’s life is that he was a pioneer in nuclear physics, an incomprehensible subject to all but a few. I am not a physicist and have not presumed to teach physics during the course of the book to those who might understand even less than I. It is fortunate from my point of view that most of Chadwick’s work was not mathematical in nature, and depended on Newtonian rather than quantum mechanics so that it can be described to a large extent in non-technical language. I have tried to present an accurate account of his scientific contributions, and if I have succeeded at all, it is because his original papers were so clearly written. The letter to Nature (see Appendix 1) announcing the discovery of the neutron illustrates this perfectly. I believe that even a non-scientist, with no knowledge of the fascinating story leading up to the discovery, could read this and recognize
the incisiveness of the argument, constructed with all the drama of an end-game by a grand master in chess.

  I have made free use of Chadwick’s letters to fellow scientists, including Rutherford, Bohr, and Lawrence. Most of these have not appeared in print before, and they convey the excitement felt by some of the leading figures as the boundaries of nuclear physics were pushed back. Similarly his wartime letters to Oliphant, Peierls, Rotblat, Kinsey, Feather, and sundry civil servants remind us of the imminent dangers faced in wartime Britain and the obstacles (some remarkably petty) confronting all of them. The vital but often ramshackle British effort, which sparked American interest in the bomb, succeeded despite a shortage of scientists and skilled manpower with its participants doing without telephones, petrol, cash, uranium and sleep. Chadwick kept them productive, if not always happy, by adroit tact, sympathetic cajoling, occasional withering rebukes and his own ceaseless example.

  His letters are scattered through various archives and piecing them together was a satisfying, but not always straightforward, process. I mentioned to Sir Rudolf Peierls that I had found some letters in which Chadwick suggested a search of German journals as a way of determining whether the German physicists were still publishing and lecturing at their usual universities, or whether there was an ominous silence that would suggest they were collaborating on a Nazi atomic bomb programme. Peierls replied that his recollection was that the examination of German periodicals was his idea. More thorough searching on my part revealed a letter that made it plain the same idea had occurred to Chadwick and Peierls independently and at the same time. I was able to send this evidence to Sir Rudolf, who replied with great charm: ‘It is of course of no consequence who first suggested the idea, only I found it disturbing that my memory should be so wrong, and I find it comforting that it was right after all. At the same time there are flaws in one’s memory — I had completely forgotten that this search was done with the cooperation of Fuchs’. Sir Rudolf, who died last year, was just one of a distinguished cadre of senior academics who willingly provided me with invaluable remembrances of Chadwick, and my sincere thanks to each of them are recorded in the Acknowledgements.