by Ray Monk
Similar problems attended the centrifuge method, which is today the main method used to enrich uranium, but which in 1941 was a new and relatively untested technique. The basic idea is to place the uranium, again in a gaseous form, in a cylinder, which is then rotated very quickly, forcing the heavier U-238 to the outer edge and concentrating the lighter U-235 near the centre. Considering both methods fairly promising, Urey gave it as his view that the assembly of a critical mass of U-235 was, though extremely difficult, quite achievable with sufficient resources.
An alternative to the arduous business of collecting together an appreciable amount of U-235 was to make a bomb from the newly discovered element of plutonium, or ‘element 94’, as it was then still known (it did not receive its name or its symbol, Pu, until March 1942). Plutonium does not exist in nature. It is one of the elusive ‘transuranics’ – elements heavier than uranium – that Fermi and others had been looking for, and which they thought would be the result of bombarding uranium with neutrons. That transuranic elements could be created had been believed by physicists for a long time, and that element 94 would be fissionable had been predicted by Bohr and Wheeler in their classic papers on fission in 1939.
In his January 1940 summary paper on the literature of fission Louis Turner had drawn attention to the possibility that there might be alternatives to U-235 as a fissionable material. If, instead of fissioning, an atom of U-238 captures the neutron fired at it, then it becomes U-239, which, Turner suggested, might fission. But, even if it did not, it would almost certainly be unstable and thus, by beta decay, transmute into the hitherto-unknown element 93 (that is, an element with 93 protons – one more than uranium). And this element would, in turn, decay into element 94, which, Turner predicted, would be even more fissionable than U-235.
Turner was right on all counts, as would eventually be shown by a series of experiments in 1940 and 1941. In the spring of 1940, using the 60-inch cyclotron, Ed McMillan and Phil Abelson produced element 93 – later named neptunium (Neptune being the planet beyond Uranus) – by bombarding uranium with neutrons. Astonishingly, they published their results. Their paper announcing the discovery of element 93 appeared in the Physical Review of June 1940, much to the disgust of James Chadwick, who persuaded the British embassy to make a formal protest to Berkeley. Though the British were, compared to the US, not particularly vigilant about the protection of their war secrets from Soviet espionage, the open publication of work directly helpful to the Nazi bomb project was something about which they were emphatically not prepared to take a relaxed view.
On 23 February 1941, Glenn Seaborg and his research team at Berkeley, again using the 60-inch cyclotron, made a conclusive identification of element 94 from the decay of element 93, and a month later showed that this element would, indeed, fission like U-235. This time they did not publish. Instead, Seaborg, together with Eugene Wigner at Princeton, joined Compton’s team of advisors, whom they told that element 94 was indeed more fissionable than U-235 and that it was realistic to believe that a critical mass of it could be produced in a uranium nuclear reactor.
So Compton’s report to the Schenactady meeting concluded that, according to the best scientific advice he had received, an atomic bomb was a possibility. The next person to speak was Oppenheimer, who gave his own estimate of the critical mass of U-235, which he put at about 220 pounds – more conservative than Fermi’s estimate, but more or less in the same ballpark (hundreds, rather than thousands, of pounds).
Compton’s biggest disappointment at this meeting was with the engineers who were present (at the express insistence of Bush) in order to provide practical estimates of how long the bomb would take to build and how much it would cost. The reason the meeting was held at Schenectady was that this was the site of the laboratories of General Electric, who provided the engineers. And yet, to Compton’s exasperation, the engineers refused to offer any opinion regarding the likely timescale and/or cost of the project. There was, in their opinion, simply too little data even to hazard a guess. As some kind of answer was needed, Compton himself suggested a time of three and a half years and a cost of ‘some hundreds of millions of dollars’ – an estimate that, at Bush’s suggestion, he played down in his final report, ‘lest the government should be frightened off’.
In his autobiography Compton reported that he had ‘always been rather proud of these forecasts, considering the limited data’. His estimate of the time required to build a bomb was indeed remarkably accurate. Once the project was formally under way, it did indeed take three and a half years to complete. The cost, however, would turn out to be $2 billion, largely because of the difficulties of isotope separation.
On the basis of the Schenectady meeting, Compton prepared a report which stated unequivocally that ‘a fission bomb of superlatively destructive power will result from bringing quickly together a sufficient mass of element U-235’ and recommended: ‘Full effort toward making atomic bombs is essential to the safety of the nation and the free world.’ After Compton presented this report to Bush on 6 November, Bush in turn presented it to the President on 27 November.
On 6 December, Bush gathered together a small group consisting of himself, Conant, Briggs, Lawrence and Compton to hear the President’s response to Compton’s report. This was that a new committee should be constituted. It was to be called S-1 (that is, ‘Section One’, the first section of the Office of Scientific Research and Development), and its membership should consist of Conant, Briggs, Lawrence and Compton, together with Eger V. Murphree, the director of research at Standard Oil, Columbia’s George Pegram and Harold Urey, both of whom were at that time in England learning about the creation of the parallel British project, called, with a deliberate attempt to be misleading, the ‘Tube Alloys’ project. The S-1 committee, with a budget of several million dollars, was to spend six months investigating further the possibility of making atomic bombs, and, if after that time it looked likely that such bombs were feasible, then practically unlimited funds would be made available to see the project through to completion.
Conant was made chairman of this new committee, with Briggs as vice chairman. Urey was to take charge of investigating the diffusion method, Murphree was to assume responsibility for researching the centrifuge method, and Lawrence was to investigate a new method that he had devised, using converted cyclotrons to separate the uranium isotopes electromagnetically. Compton had the dual responsibilities of the design of the bomb and research into the possible use of plutonium, which meant overseeing the construction of the world’s first nuclear reactor.
Oppenheimer was not included in the S-1 committee, which perhaps indicates that, though his contributions to the Schenectady meeting were valued, he was considered too unreliable from a political point of view to be included in such sensitive work. Lawrence was determined to involve Oppenheimer, but in this he was not helped by Oppenheimer’s apparent determination to remain involved in left-wing political activity. When Oppenheimer invited Lawrence to a meeting at his home of the American Association of Scientific Workers (a union organisation), Lawrence refused to attend and banned his staff from attending. ‘I don’t think it’s a good idea,’ he told Oppenheimer. ‘I don’t want you to join it. I know nothing wrong with it, but we’re planning big things in connection with the war effort, and it wouldn’t be right. I want no occasion for somebody in Washington to find fault with us.’ In reply, on the day that Lawrence left for Washington to receive the President’s response to Compton’s report, Oppenheimer wrote to him:
I had hoped to see you before you left, but will write this to assure you that there will be no further difficulties at any time with the A.A.S.W. I think that your own feeling about the men working directly with you will have a good deal of weight also with those scientists whose defense efforts are not in the Radiation Laboratory, and I doubt very much whether anyone will want to start at this time an organization which could in any way embarrass, divide or interfere with the work we have in hand.
In the light of this letter, it is surprising, to say the least, that on 6 December – the very day that Lawrence was in Washington receiving President Roosevelt’s response – Oppenheimer chose to attend a fund-raising event for veterans of the Spanish Civil War, which, he surely knew, would be attended by almost every high-ranking Communist Party official in California and, as such, was almost certain to bring him to the renewed attention of the FBI.
The party was held at the home of Louise Bransten, known to the FBI as an intimate friend of Gregory Kheifetz, an agent for the NKVD (the forerunner of the KGB) working undercover at the Soviet consulate in San Francisco. Kheifetz had been instructed to find out about US uranium research and so had been cultivating scientists. He was no doubt delighted to be introduced to Oppenheimer, and, according to Jerold and Leona Schecter in their controversial book Sacred Secrets, the two had lunch the next day, at which Oppenheimer expressed concern that the Germans would build an atomic bomb before the Allies. The Schecters also claim that at this lunch Oppenheimer told Kheifetz about Einstein’s famous letter to Roosevelt and about a secret project involving outstanding physicists, including Nobel Prize-winners.
If what the Schecters say is true, it would explain the reference to Oppenheimer in a letter written by a Soviet intelligence officer in 1944, which claims that Oppenheimer – ‘one of the leaders of scientific work on uranium in the USA’ – while ‘an unlisted member of the apparatus of Comrade Browder, informed us about the beginning of the work’. It is possible to be sceptical both about this letter and about the account offered by the Schecters of Oppenheimer’s lunch with Kheifetz, but, even if one takes both at face value, one still does not get a picture of Oppenheimer as engaged in Soviet espionage. He knew a lot more than he is reported as telling Kheifetz, and, though it would have been indiscreet of him to mention Einstein’s letter and the existence of the secret project, such revelations do not begin to compare with the actions of real spies like the ‘Cambridge Five’, or even with Oliphant’s behaviour in Berkeley. In any case, by the time of Oppenheimer’s alleged lunch with Kheifetz, the US and the Soviet Union were allies, since, if it took place the day after the fund-raising party for Spanish war veterans, then it occurred on 7 December 1941, the day that the Japanese attack on Pearl Harbor brought the US into the Second World War.
When the S-1 committee met on 18 December 1941, the new political and military situation had made the project in which they were engaged far more urgent. At this meeting, Pegram and Urey reported back from England on the optimism that prevailed there regarding the diffusion method of isotope separation, and Lawrence presented a convincing case for the practicality of the electromagnetic method. Urey also reported on research undertaken by Jesse W. Beams at Virginia, which suggested that the centrifugal method was entirely feasible. It says much about the state of play regarding all these methods, however, that Compton regarded the production of plutonium to be in some ways easier and more practical than any of them; after all, in order for that to work, one had to: 1. produce a nuclear chain reaction, which had never before been achieved; 2. operate this reaction at a level to produce the quantities of plutonium needed, a feat that no one at the time knew was even possible; and 3. in Compton’s words, ‘learn the chemistry and metallurgy of the new chemical element plutonium, so that when the plutonium was made within the uranium it could be extracted, reduced to metal and fashioned into the shapes required for the bomb’. Faced with the newly urgent imperative to make a success of their project and with the seemingly equal claims of each of the four methods of producing fissionable material, the S-I committee made the momentous decision to pursue them all.
As Compton points out in his autobiography: ‘The period from December 1941, when authority was given to push the atomic project, until June 1942, when the Army assumed responsibility, was critical.’ It was during this period that those, like Lawrence and Compton, who believed an atomic bomb could be built had to produce convincing evidence that it was possible, and produce a workable plan for making the bomb in three or four years.
At the next meeting of the S-1 committee, held at Compton’s house on 24 January 1942, the decision was made to base the work on the uranium chain reaction and the production of plutonium in one place, rather than keep it dispersed throughout various American universities. Pegram, naturally, wanted that place to be Columbia, where Fermi and Szilard had been working together on a nuclear reactor throughout the previous year. Lawrence argued vigorously for the project to be based at Berkeley. Princeton was also considered a possibility. In the end, Compton settled on his own university, Chicago. ‘You’ll never get a chain reaction going here,’ Lawrence scoffed. ‘The whole tempo of the University of Chicago is too slow.’ Compton countered by promising to have the chain reaction going by the end of the year. ‘I’ll bet you a thousand dollars you won’t,’ said Lawrence, but when Compton took the bet, immediately lowered the stakes to ‘a 5 cent cigar’. ‘I won the bet,’ Compton remarks in his autobiography, ‘but I haven’t yet received the cigar.’
During the spring of 1942, Fermi, Szilard and others prepared to relocate to Chicago to join what Compton had decided to call the ‘Metallurgical Laboratory’. At the same time Oppenheimer was moving, step by step, from the periphery of the project into its very centre. At every stage in this development his progress was threatened by the suspicions aroused by his radical politics and the circles of friends and associates into which those politics had placed him. During this period many of the people whose job it was to protect the US from Soviet espionage grew increasingly suspicious of Oppenheimer and there were several requests to place him under tighter surveillance. On 26 January 1942, Special Agent N.J.L. Pieper of the San Francisco office of the FBI wrote to J. Edgar Hoover, listing Oppenheimer as one of four people (the others were Addis, Chevalier and a man called Alexander Kaun) who ‘represent, in the opinion of this office, a group which is inimical to the welfare of this country’. Pieper recommended that ‘the highly confidential source of information and surveillance mentioned in Bureau teletype of recent date should be utilized’ – that is, their telephones should be bugged and microphones should be installed in their homes. ‘This group of individuals,’ Pieper went on, ‘is on such a plane that it is unlikely that any confidential Party informant now available to this office will be able to reach them and determine their actual position in the Party.’
In his reply, dated 10 February, Hoover granted permission to maintain ‘technical surveillance’ of Chevalier and Kaun, but not of Addis and Oppenheimer. Pieper tried again in March and was again refused, Hoover reminding him that in future he should ‘follow proper procedure’ and ask for such permission by phone rather than by letter. Evidently, if and when the day came for him to grant permission to tap Oppenheimer’s phone, Hoover was reluctant to have a written record of that decision.
At this time Hoover knew nothing about the US bomb project or the S-1 committee. The worries that he and his agents had about Oppenheimer were quite general ones, based on the understandable (if not infallible) assumption that anyone who regularly met the likes of Nelson, Folkoff, Schneiderman and Kheifetz was up to no good. If Hoover had known that Oppenheimer was in possession of detailed top-secret information regarding the development of a weapon that the US government was beginning to believe would win the war for them, there is surely no doubt that he would have placed him under the closest possible surveillance.
James Conant was aware of how much Oppenheimer knew, and he also, after Oliphant’s visit, had good reason for thinking that the security standards at Berkeley were not all they should have been. He was therefore a worried man. In February 1942, he summoned First Lieutenant John Lansdale of the US Army Military Intelligence Service to his office and explained to him that the US was in a race with the Germans to build an atomic weapon. ‘Whoever gets this first will win the war,’ Conant told him, adding that it was therefore important to find out if the physicists at Berkeley could be trusted to keep
a secret. As instructed by Conant, Lansdale travelled to Berkeley and, under the cover of being a law student, spent two weeks assessing the situation. What he discovered horrified Conant. Lansdale found that he was able to wander freely around the site of the unfinished 184-inch cyclotron (soon to be converted into a uranium-enriching machine), and that Lawrence’s work on a government project to build an explosive was common knowledge at Berkeley and the subject of open and casual conversations in the cafeteria. ‘Oh! . . . Oh! . . . Oh my goodness!’ Conant is reported to have uttered at the news. In response, he sent Lansdale back to Berkeley, this time in uniform, to give the physicists a dressing-down and to warn them that breaches in security could undermine the project.
By 9 March 1942, when Bush wrote a progress report to President Roosevelt, Lawrence’s electromagnetic method of enriching uranium seemed to be emerging as the front-runner among the various methods of producing U-235. Bush recommended to the President that a centrifuge plant should be built, which, he suggested, could be up and running by the end of 1943. A gaseous-diffusion plant, Bush advised, could start to deliver weapon-grade uranium by the end of 1944. An electromagnetic plant, however, could be completed by the summer of 1943.
Though Oppenheimer was not yet officially part of the US bomb project, he was, in practice, devoting all his energies to it, having been brought into it ‘through the back door’, as it were, by Lawrence. For most of his time at Berkeley, Lawrence had been in the habit of bringing to Oppenheimer theoretical problems raised by his experimental work, and he saw no reason to stop doing that now. Indeed, for Lawrence it was more important than ever to make use of Oppenheimer’s acute mind, since so much now depended on understanding the enrichment and possible fission of uranium before their German rivals.
