The Neutron and the Bomb, page 49
There had been one earlier return to England in which Chadwick had again been closely involved. Joe Rotblat19 had decided that since it was taking the Americans so long to build the bomb, his ‘fear of the Germans being first was groundless’. He found, therefore, ‘the whole purpose of my being in Los Alamos ceased to be’, and he asked Chadwick for permission to return home. When Chadwick passed on the request to the Army authorities at Los Alamos, he had been shown a thick dossier on Rotblat containing highly incriminating evidence which pointed to him being a spy for the Russians. It appeared that he had informed a young woman of his acquaintance in Santa Fe that he intended to return to England, join the RAF to parachute out over Russia or occupied Poland, and then to establish contact with the Russians in order to betray his considerable knowledge of the Manhattan Project. In Rotblat’s words:
The trouble was that within this load of rubbish was a grain of truth. I did indeed meet and converse with a person during my trips to Santa Fe. It was for a purely altruistic purpose, nothing to do with the project, and I had Chadwick’s permission for the visits. Nevertheless, it contravened a security regulation and it made me vulnerable. Fortunately for me, in their zeal the vigilant agents had included in their reports details of conversations with dates, which were quite easy to refute and to expose as complete fabrications. The chief of intelligence was rather embarrassed by all this and conceded that the dossier was worthless. Nevertheless, he insisted that I not talk to anybody about my reason for leaving the project. We agreed with Chadwick that the ostensible reason would be a purely personal one: that I was worried about my wife whom I had left in Poland.
Chadwick20 could see other serious, political difficulties from Rotblat’s continued presence on the Hill as the bomb project came to fruition. He thought that if Rotblat’s family were still alive, they would probably be living in Russian-occupied territory after the war, raising the prospect of blackmail. He discussed this with Groves, who agreed that Rotblat should leave immediately. Chadwick also saw advantages in Rotblat’s return in that he had acquired such a close knowledge of all branches of nuclear physics which should prove useful in the postwar period. Rotblat travelled to the East Coast and stayed with Chadwick in Washington for a few days. This was just before Christmas 1944, when there was such uproar about Halban’s visit to France. Before leaving Los Alamos, Rotblat19 had packed all his research notes and personal documents into a wooden box made by his laboratory assistant. Chadwick accompanied Rotblat to Union Station, where he boarded the train for New York and together they carried the box to the train. When Rotblat arrived in New York, a few hours later, the box had disappeared and he had to sail back to Liverpool without it. The box was never recovered and it seems most likely that it was confiscated by US Army Intelligence, who still harboured suspicions about Rotblat.
The Americans were also dissatisfied with the security vetting of non-British scientists working in Montreal,21 and continued to press John Cockcroft for their biographical details. This put him in a difficult position because the British security forces were in the habit of issuing clearance notifications with no further details. Chadwick could not see why ‘a definite statement that foreigners have been thoroughly investigated by our Intelligence people’22 should not suffice, but attempted to placate General Groves by occasionally giving a brief sketch of the ‘more interesting cases in private conversation’. Groves23 found that he was ‘most punctilious in informing me of the slightest question of background, including that of German blood’. There was one case where Chadwick badly misjudged one of the British scientists. With remarkable prescience but no real suspicion, Groves24 had become concerned about the amount of time that Alan Nunn May, who was based in Montreal, had been spending at the experimental nuclear pile at Argonne, near Chicago. Chadwick interviewed May, who informed him that he had not picked up much information while working at Argonne.25 Chadwick knew May both as a research student at the Cavendish in the early thirties, and more recently from Tube Alloy research carried out jointly between Bristol and Liverpool, early in the war. May had been Powell’s chief assistant in Bristol, and had been one of the co-authors with Chadwick of the letter to Nature in 1941; Chadwick reassured Groves that May ‘was exceptionally reliable and close-mouthed’.25 Despite his own special status, even Chadwick was not above suspicion apparently: in January 1945 he was unable to visit Canada because of strong objections on the grounds of security from the Americans, which he thought ‘it would be most impolitic to ignore’.26
The divergence of opinion between Chadwick and Oliphant27 on the development of a nuclear research establishment in England rumbled on for a while. Chadwick’s constant calls for restraint plainly frustrated both Oliphant and Cockcroft at times, but in fact he became increasingly proactive as the prospect of the atomic age became ever clearer. He sent Anderson28 a detailed minute on the UK proposals at the end of May. Commenting on a suggestion of Cockcroft’s that the research station should be sited so that there was easy access to Oxford or Cambridge, Chadwick could see that there were ‘obvious advantages to be gained by promoting the interchange of visits with university laboratories, but we must avoid degrading the Establishment to a mere house of pleasure for visiting scientists’. He favoured a site in the Midlands, in proximity to an industrial centre with its manufacturing facilities, materials and supply of skilled labour. By this time Chadwick urged some immediate action to:
1. find an airfield which is reasonably suitable as regards facilities and reasonably convenient;
2. get it;
3. get on with the job of providing equipment;
4. begin to offer posts to staff.
He did not foresee any difficulty in recruiting young scientists, but thought there would be considerable competition for the ‘few senior men’ required. By this stage, Sir Edward Appleton, the Director of DSIR, had agreed with Sir John Anderson that the first chief of the new organization should be John Cockcroft. Appleton contacted Chadwick asking for his endorsement of this choice, which was granted but with some apparent reservation:29
He has many virtues which would contribute to the smooth running of the Establishment, and he would keep his hand on all the strings. In common with most scientists he also has many faults and those we must recognise from the beginning so that we can supplement him by a suitable choice of assistants. For example his knowledge is wide but it is not at all profound; his views are of rather a dull, everyday hue. On the other hand, his temper is so equable and his patience and persistence so inexhaustible that we can put in lively and relatively irresponsible men who have the real feeling for research without fear of upsetting the balance.
Chadwick himself had not had the opportunity to indulge his own love of basic research for some years. This omission contributed to the disenchantment he sometimes felt for his new status in life. In a letter to Oppenheimer, written in April 1945, he expressed some melancholy sentiments about his present duties for many of which ‘I am fitted neither by temperament, training nor ability’.30 For a while after his permanent move to Washington, Chadwick continued to supervise administrative matters concerning the British group at Los Alamos. These would include dealing with requests from scientists who wished to return to England, and with letters from their mothers in England asking what had become of them. There were also the usual wrangles over promotion and pay, and for example when Philip Moon was given an increase in salary, Chadwick31 asked him not ‘to breathe this over the loudspeaker system’ at Los Alamos. He soon realized that the arrangement was inefficient and asked Rudolf Peierls to take charge of the Los Alamos affairs. He also asked Peierls to keep him abreast of scientific progress, which Peierls did by writing a series of detailed letters, sometimes in conjunction with Moon.
Moon joined the team headed by Kenneth Bainbridge, the Harvard physicist, which was to test the implosion model of the plutonium bomb in Project Trinity. Bainbridge had chosen as a proving ground for the test an area of flat scrub land in the Jornada del Muerto valley in southern New Mexico.32 The site was about 200 miles south of Los Alamos and formed the northern limit of the huge Alamogordo Army Air Field. As the name ‘Journey of Death’ suggests, it was an intensely inhospitable place — fiercely hot and replete with poisonous snakes, spiders, and scorpions. As a rehearsal for the actual Trinity test, Bainbridge’s group detonated a 100 ton charge of high explosive mixed with a small amount of radioactive material on 7 May. Apart from serving to test functions like transportation and communication, the 100 ton shot provided the opportunity to calibrate instruments that would be used at the definitive bomb test to measure the magnitude of the explosion and the distribution of radioactive fission products. Peierls and Moon33 soon conveyed the lessons learned to Chadwick. They described the difficulties encountered in measuring the blast waves in air and seismic earth shock due to the 100 ton explosion, but thought that most instances of equipment failure had obvious causes. According to them, ‘the only surprising thing was that some mice that had been placed at about 400 yards distance were killed by lung haemorrhage. The pressure at that distance was believed to be something like 1 pound per sq. inch, which British mice withstand successfully.’
The letters continued at a rate of about one a week through May and June, and apart from including reports on contributions made by British scientists, they also gave details on the new methods available for analysing implosions (the technique developed to detonate the plutonium bomb). As a result Chadwick was conspicuously well-informed of all developments — to such an extent that Groves started to feel inadequately briefed by comparison. When he found out that Peierls was the source of Chadwick’s inside information, he sent word to Peierls asking for copies of the letters to be sent to him too. Peierls34 had been concerned that by sending the letters, he was in contravention of the strict rules governing the communication of information from Los Alamos; Groves’ flattering request brought him both relief and amusement.
Two letters written at the end of May 194535 reveal how many intricate problems relating to the fusing of a plutonium bomb remained to be solved. The methodology of implosion (whereby the central mass of plutonium would be squeezed into a critical form) required the science of explosions to be developed at a more detailed level than ever before. The theoretical solution had been provided by the Hungarian mathematician von Neumann a year earlier. The greatest English expert on fluid mechanics and the theory of explosions was Geoffrey Taylor from Cambridge — in happier times, one of Rutherford’s golf partners in the Trinity College Sunday morning foursome. Groves was so keen to have Taylor at Los Alamos that Chadwick wrote to Appleton that ‘anything short of kidnapping would be justified’.36 Another key ordnance expert whom Chadwick had prised away from London was William Penney, a mathematician from Imperial College, who had worked on the effects of high explosives for the Home Office during the blitz. Taylor and Penney both advised on the device being tested at Los Alamos, termed the lens system, which depended essentially on shaped high explosive charges (the lenses) surrounding the plutonium core of the bomb. There were two forms of explosive used with different speeds so that after detonation the pressure wave would be shaped inwards and focused onto a sub-critical arrangement of plutonium, which would experience a compression of tremendous force and be squeezed down into a solid, super-critical mass. In his letter of 23 May, Peierls35 wrote that the production of full-scale lenses was just beginning. One practical problem concerned the presence of minute flaws in the mass of explosive:
The question of the effects of gaps or cracks in the H.[igh]E.[xplosive] is being given much attention... The most spectacular effect is an acceleration of the detonation wave which is observed if the detonation wave runs along a crack of finite length. The explanation is probably that the crack acts as a cavity charge and a jet of some power runs down the crack and initiates the H.E. at the end ahead of time. This effect has been observed with gaps as small as 1/84th inch.
This gave some concern about the integrity of the explosive lenses following assembly and transportation, and Peierls mentioned that they would be lacquered and reinforced to prevent powdering or chipping.
The other crucial challenge facing the Los Alamos scientists was to initiate the nuclear chain reaction in the plutonium at the split-microsecond that it reached its maximum density. A stray cosmic neutron could not be relied upon to provide the necessary stimulus, and it was decided to incorporate a source of neutrons at the centre of the plutonium core. The device to be used was the same combination of materials, polonium and beryllium, that Chadwick had employed in his modest apparatus at the Cavendish in 1932, when he had identified the neutron for the first time. To achieve a successful initiation, the α-particles from the polonium had to be prevented from coming into contact with the beryllium metal until the instant that the chain reaction would be triggered. Peierls35 was able to report that the chemists had made remarkable progress by plating the polonium with a microscopic coating of nickel which would arrest the α-particles until disrupted by the detonation wave. Two British workers were evidently making significant contributions to this work:
In connection with the theory of initiators Fuchs has worked a great deal on the theory of the penetration of Munroe jets. An existing theory by Mott does not seem quite adequate for our purposes. Considerable help in this will come from a recent experiment by Tuck41, who succeeded in photographing with X-rays the penetration of a jet of wax into a block of wax having a sheet of lead over its surface. Tuck has a series of photographs at different times showing up the mechanism most beautifully. This apparently is the first time anyone has succeeded in photographing the penetration of a jet, although it has often been tried.
Another point which only occurred to the physicists at this late stage was that some neutrons from the polonium-beryllium mixture could be scattered back from the explosive shell as slow neutrons and ‘if they could return into the active material they would considerably decrease the critical mass’. Peierls35 was led to conclude that:
pre-detonation is determined not by the number of background neutrons during the time interval in which the gadget will be critical, but during the interval from which one of the off-spring of such a neutron has a chance of surviving until the gadget does become critical. This point is very similar to the point made by Feather a long time ago in connection with delayed neutrons. All this makes it necessary to absorb as many of the slow neutrons as possible and there are essentially three proposals.42
The first nuclear explosion at the Trinity test site had been planned for 4 July, but on 13 June Peierls37 mentioned to Chadwick that there would be a delay of a week or two because the fabrication of the explosive lenses had started later than scheduled. At the end of June, refinements were still being made in the detonators; there had been problems with the insulation of electrical wiring which was now solved by covering the leads with nylon sleeves. Tuck was starting a new programme of experiments ‘to observe detonation waves and their immediate effects in a way that will allow one to calculate detonation waves and other data directly’. There had been discrepancies between the measured velocity of shock waves and the compression produced by them, and the theoretical shock wave equations. Moon38 reported to Chadwick that these discrepancies were ‘beyond experimental error (in most cases something like 20%) and... beyond the error of the theory, supposing that the physical constant of the explosive and of the tamper have been correctly chosen and that no wrong assumption has been made by the theorists’. There were other major unresolved issues such as what effect the intense electromagnetic radiation generated in the chain reaction would have on the later stages of the nuclear reaction. Edward Teller told Moon ‘that these effects could conceivably increase the efficiency [of the bomb] manyfold’.38
As the huge scientific team now assembled on the project stretched their collective ingenuity to ensure that the uranium-235 and plutonium bombs would work as intended, there were a few individuals who stepped aside from the juggernaut and asked where it might be leading the world. Foremost amongst them was always Niels Bohr, who was regarded as the philosopher king by the scientific community, but whose ideas on sharing nuclear technology were so roundly scorned by Churchill. Bohr was still refining his proposals, which were expressed in rather abstract and circumlocutionary terms, comprehensible to theoretical physicists, but not likely to find favour with a sceptical audience of generals and politicians. In the autumn of 1944, Sir Henry Dale,39 the President of the Royal Society, had discussed with both Bohr and Chadwick their deep anxieties about the future uses to which the nuclear weapons they were helping to create might be put. By June 1945, various memoranda from Bohr were circulating in Washington and were passed to Groves and Stimson, the Secretary for War. These writings were characterized by Makins,40 a senior Foreign Office man in the Washington Embassy, in a memo to the War Cabinet Office in London, in the following terms:
As Bohr’s papers deal in ideas, and as he always avoids or evades questions on the actual working of control, inspection, etc., he has really shot his bolt and can, in the opinion of Chadwick and myself, do no more good at the moment over here either on the political or scientific side. On the other hand, his continued presence may cause some prejudice to our relations with the Americans for a general and special reason.
