Of Thursday, August 20, 1942, Seaborg writes:
Perhaps today was the most exciting and thrilling day I have experienced since coming to the Met Lab. Our microchemists isolated pure element 94 for the first time! This morning Cunningham and Werner set about fuming . . . yesterday’s 94 solution containing about one microgram of 94239, added hydrofluoric acid whereupon the reduced 94 precipitated as the fluoride . . . free of carrier material. . . .1606
This precipitate of 94, which was viewed under the microscope and which was also visible to the naked eye, did not differ visibly from the rare-earth fluorides. . . .
It is the first time that element 94 . . . has been beheld by the eye of man.
By afternoon “a holiday spirit prevailed in our group.” After several hours’ exposure to air “the precipitated [plutonium] had taken on a pinkish hue.”1607 Someone photographed Cunningham and Werner at their crowded bench in the narrow, tile-walled room—trim, strong-jawed young men looking weary. The crew upstairs that muscled carboys and lead bricks shuffled in like clumsy shepherds to peer through the microscope at the miracle of the tiny pinkish speck.
* * *
In the summer of 1942 Robert Oppenheimer gathered together at Berkeley a small group of theoretical physicists he was amused to call the “luminaries.”1608 Their job was to throw light on the actual design of an atomic bomb.
Hans Bethe, now thirty-six and a highly respected professor of physics at Cornell, had resisted joining the bomb project because he doubted the weapon’s feasibility. “I considered . . . an atomic bomb so remote,” Bethe told a biographer after the war, “that I completely refused to have anything to do with it. . . . Separating isotopes of such a heavy element [as uranium] was clearly a very difficult thing to do, and I thought we would never succeed in any practical way.”1609 But Bethe may well have headed the list of luminaries Oppenheimer wanted to attract. By 1942 the Cornell physicist had established himself as a theoretician of the first rank. His most outstanding contribution, for which he would receive the 1967 Nobel Prize in Physics, was to elucidate the production of energy in stars, identifying a cycle of thermonuclear reactions involving hydrogen, nitrogen and oxygen that is catalyzed by carbon and culminates in the creation of helium. Among other important work during the 1930s Bethe had been principal author of three lengthy review articles on nuclear physics, the first comprehensive survey of the field. Bound together, the three authoritative studies came to be called “Bethe’s Bible.”
He had wanted to help oppose Nazism. “After the fall of France,” he says, “I was desperate to do something—to make some contribution to the war effort.”1610 First he developed a basic theory of armor penetration. On the recommendation of Theodor von Kármán, whom he consulted at Caltech, he and Edward Teller in 1940 extended and clarified shock-wave theory. In 1942 he joined the Radiation Laboratory at MIT to work on radar. That was where Oppenheimer found him.
Oppenheimer cleared his plan with Lee A. DuBridge, the director of the Rad Lab, then set a senior American theoretician, John H. Van Vleck, professor of physics at Harvard, to snare Bethe for the Berkeley summer study. “The essential point,” he counseled Van Vleck, “is to enlist Bethe’s interest, to impress on him the magnitude of the job we have to do . . . and to try to convince him, too, that our present plans . . . are the appropriate machinery.” Oppenheimer felt the weight of the work. “Every time I think about our problem a new headache appears,” he told the Harvard professor. “We shall certainly have our hands full.”1611 Van Vleck arranged to meet Bethe conspiratorially in Harvard Yard and succeeded in convincing him he was needed. The prearranged signal to Oppenheimer was a Western Union Kiddygram, an inexpensive standardized telegram with a message like “Brush your teeth.”1612
Oppenheimer also invited Edward Teller. In 1939 Bethe had married Rose Ewald, the attractive and intelligent daughter of his Stuttgart physics professor Paul Ewald; Edward and Mici Teller, “our best friends in this country,” had attended the New Rochelle wedding.1613 Setting out for Berkeley in early July 1942, the Bethes stopped over in Chicago to pick up the Tellers.1614 Teller showed Bethe Fermi’s latest exponential pile. “He had a setup under one of the stands in Stagg Field,” Bethe remembers—“in a squash court—with tremendous stacks of graphite.” A chain reaction that made plutonium would bypass the problem of isotope separation. “I then,” says Bethe, “became convinced that the atomic-bomb project was real, and that it would probably work.”1615
The other luminaries enlisted for the summer study were Van Vleck, the Swiss-born Stanford theoretician Felix Bloch, Oppenheimer’s former student and close collaborator Robert Serber, a young Indiana theoretician named Emil Konopinski and two postdoctoral assistants. Konopinski and Teller had arrived at the Met Lab at about the same time earlier that year. “We were newcomers in the bustling laboratory,” Teller writes in a memoir, “and for a few days we were given no specific jobs.” Teller proposed that he and Konopinski review his calculations that seemed to prove the impossibility of using an atomic bomb to ignite a thermonuclear reaction in deuterium:
Konopinski agreed, and we tackled the job of writing a report to show, once and for all, that it could not be done. . . . But the more we worked on our report, the more obvious it became that the roadblocks which I had erected for Fermi’s idea were not so high after all. We hurdled them one by one, and concluded that heavy hydrogen actually could be ignited by an atomic bomb to produce an explosion of tremendous magnitude. By the time we were on our way to California . . . we even thought we knew precisely how to do it.1616
That was not news Edward Teller was likely to hide under a bushel, whatever Oppenheimer’s official agenda. Bethe was ushered into the glare as the streamliner clicked west: “We had a compartment on the train to California, so we could talk freely. . . . Teller told me that the fission bomb was all well and good and, essentially, was now a sure thing. In reality, the work had hardly begun. Teller likes to jump to conclusions. He said that what we really should think about was the possibility of igniting deuterium by a fission weapon—the hydrogen bomb.”1617
At Berkeley the luminaries began meeting in Oppenheimer’s office, “in the northwest corner of the fourth floor of old LeConte [Hall],” an older colleague remembers. “Like all those rooms, it had French doors opening out onto a balcony, to which there was easy access from the roof. Accordingly a very strong wire netting was fastened securely over his balcony.” Only Oppenheimer had a key. “If a fire had ever started . . . in Oppenheimer’s absence, it would have been tragic.”1618 But the fires that summer were still only theoretical.
The theoreticians let Teller’s bomb distract them. It was new, important and spectacular and they were men with a compulsion to know. “The theory of the fission bomb was well taken care of by Serber and two of his young people,” Bethe explains. They “seemed to have it well under control so we felt we didn’t need to do much.”1619 The essentials of fast-neutron fission were firm—it needed experiment more than theory. The senior men turned their collective brilliance to fusion. They had not yet bothered to name generic bombs of uranium and plutonium. But from the pre-anthropic darkness where ideas abide in nonexistence until minds imagine them into the light, the new bomb emerged already chased with the technocratic euphemism of art deco slang: the Super, they named it.
Rose Bethe, who was then twenty-four, understood instantly. “My wife knew vaguely what we were talking about,” says Bethe, “and on a walk in the mountains in Yosemite National Park she asked me to consider carefully whether I really wanted to continue to work on this. Finally, I decided to do it.” The Super “was a terrible thing.” But the fission bomb had to come first in any case and “the Germans were presumably doing it.”1620
Teller had examined two thermonuclear reactions that fuse deuterium nuclei to heavier forms and simultaneously release binding energy. Both required that the deuterium nuclei be hot enough when they collided—energetic enough, violently enough in motion—to overco
me the nuclear electrical barrier that usually repels them. The minimum necessary energy was thought at the time to come to about 35,000 electron volts, which corresponds to a temperature of about 400 million degrees.1621 Given that temperature—and on earth only an atomic bomb might give it—both thermonuclear reactions should occur with equal probability. In the first, two deuterium nuclei collide and fuse to helium 3 with the ejection of a neutron and the release of 3.2 million electron volts of energy. In the second the same sort of collision produces tritium—hydrogen 3, an isotope of hydrogen with a nucleus of one proton and two neutrons that does not occur naturally on earth—with the ejection of a proton and the release of 4.0 MeV of energy.
The D + D reactions’ release of 3.6 MeV was slightly less by mass than fission’s net of 170 MeV. But fusion was essentially a thermal reaction, not inherently different in its kindling from an ordinary fire; it required no critical mass and was therefore potentially unlimited. Once ignited, its extent depended primarily on the volume of fuel—deuterium—its designers supplied. And deuterium, Harold Urey’s discovery, the essential component of heavy water, was much easier and less expensive to separate from hydrogen than U235 was from U238 and much simpler to acquire than plutonium. Each kilogram of heavy hydrogen equaled about 85,000 tons TNT equivalent.1622 Theoretically, 12 kilograms of liquid heavy hydrogen—26 pounds—ignited by one atomic bomb would explode with a force equivalent to 1 million tons of TNT. So far as Oppenheimer and his group knew at the beginning of the summer, an equivalent fission explosion would require some 500 atomic bombs.1623
That reckoning alone would have been enough to justify devoting the summer to imagining the Super a little way out of the darkness. Teller found something else as well, or thought he did, and with his usual pellmell facility he scattered it before them. There are many other thermonuclear reactions besides the D + D reactions. Bethe had examined a number of them methodically when looking for those that energized massive stars. Now Teller offered several which a fission bomb or a Super might inadvertently trigger. He proposed to the assembled luminaries the possibility that their bombs might ignite the earth’s oceans or its atmosphere and burn up the world, the very result Hitler occasionally joked about with Albert Speer.
“I didn’t believe it from the first minute,” Bethe scoffs. “Oppie took it sufficiently seriously that he went to see Compton. I don’t think I would have done it if I had been Oppie, but then Oppie was a more enthusiastic character than I was.1624 I would have waited until we knew more.” Oppenheimer had other urgent business with Compton in any case: the Super itself. Not to risk their loss, the bomb-project leaders were no longer allowed to fly. Oppenheimer tracked Compton by telephone at the beginning of a July weekend to a country store in northern Michigan where he had stopped to pick up the keys to his lakeside summer cottage, got directions and caught the next train east. In the meantime Bethe applied himself to Teller’s calculations.
The Cornell physicist’s instant skepticism gives perspective to Compton’s melodramatic recollection of his meeting with Oppenheimer:
I’ll never forget that morning. I drove Oppenheimer from the railroad station down to the beach looking out over the peaceful lake. There I listened to his story. . . .1625
Was there really any chance that an atomic bomb would trigger the explosion of the nitrogen in the atmosphere or the hydrogen in the ocean? This would be the ultimate catastrophe. Better to accept the slavery of the Nazis than to run a chance of drawing the final curtain on mankind!
We agreed there could be only one answer. Oppenheimer’s team must go ahead with their calculations.
Bethe already had. “I very soon found some unjustified assumptions in Teller’s calculations which made such a result extremely unlikely, to say the least. Teller was very soon persuaded by my arguments.”1626 The arguments—Bethe’s and others’—against a runaway explosion appear most authoritatively in a technical history of the bomb design program prepared under Oppenheimer’s supervision immediately after the war:
It was assumed that only the most energetic of several possible [thermonuclear] reactions would occur, and that the reaction cross sections were at the maximum values theoretically possible. Calculation led to the result that no matter how high the temperature, energy loss would exceed energy production by a reasonable factor. At an assumed temperature of three million electron volts [compare the 35,000 eV known for D + D] the reaction failed to be self-propagating by a factor of 60. This temperature exceeded the calculated initial temperature of the deuterium reaction by a factor of 100, and that of the fission bomb by a larger factor. . . . The impossibility of igniting the atmosphere was thus assured by science and common sense.1627
Oppenheimer returned to that good news and they proceeded with the Super. Teller recaptures the mood: “My theories were strongly criticized by others in the group, but together with new difficulties, new solutions emerged. The discussions became fascinating and intense. Facts were questioned and the questions were answered by still more facts. . . . A spirit of spontaneity, adventure, and surprise prevailed during those weeks in Berkeley, and each member of the group helped move the discussion toward a positive conclusion.”1628
There was serious trouble with Teller’s D + D Super. The reactions would proceed too slowly to reach ignition before the fission trigger blew the assembly apart. Konopinski came to the rescue: “Konopinski suggested that, in addition to deuterium, we should investigate the reactions of the heaviest form of hydrogen, tritium.” This, Teller explains, was at that time “only . . . a conversational guess.”1629 One tritium reaction of obvious interest was the fusion of a deuterium nucleus with a tritium nucleus, D + T, which results in the formation of a helium nucleus with the ejection of a neutron and the release of 17.6 MeV of energy. The D + T reaction kindled at a mere 5,000 eV, which corresponds to a temperature of 40 million degrees. But since tritium does not exist on earth it would have to be created. Neutrons bombarding an isotope of lithium, Li6, would transmute some of that light metal to tritium much as neutrons made plutonium from U235, but the only obvious source of such necessarily copious quantities of neutrons was Fermi’s unproven pile. The luminaries did, however, consider the possibility of making tritium within the Super itself by packing the bomb with a dry form of lithium, lithium deuteride.1630 But lithium in its natural form, like uranium in its natural form, contained too little of the desired isotope; to be effective, the Li6 would have to be separated. But lithium—element number 3 on the periodic table—would be much easier to separate than uranium . . . So the arguments progressed across the pleasant Berkeley summer. “We were forever inventing new tricks,” Bethe says, “finding ways to calculate, and rejecting most of the tricks on the basis of the calculations. Now I could see at first-hand the tremendous intellectual power of Oppenheimer who was the unquestioned leader of our group. . . . The intellectual experience was unforgettable.”1631
At the end of the summer, merging the Serber subgroup’s work with their own, the luminaries concluded that the development of an atomic bomb would require a major scientific and technical effort.1632 Glenn Seaborg heard Oppenheimer’s deduction from that outcome at a meeting of the Met Lab technical council in Chicago on September 29. “Fast neutron work has no home,” Seaborg paraphrases the Berkeley theoretician “[and] may need one.”1633 “Oppenheimer has plans in mind for fast neutron work,” Compton told the council. Oppenheimer was scouting a site where the bomb might be designed and assembled. He thought such an operation might find a home in Cincinnati or with the plutonium production piles in Tennessee.1634
* * *
James Bryant Conant heard the results of the Berkeley summer study at a meeting of the S-l Executive Committee in late August 1942 and jotted down a page of notes under the heading “Status of the Bomb.”1635, 1636 The fission bomb, he wrote, would explode according to the luminaries with “150 times energy of previous calculation” but, bad news, would require a critical mass “6 times the previous [estimated]
size[:] 30 kg U235.” Twelve kilograms of U235 were enough to explode, Conant noted, but inefficiently with “only 2% of energy.” News of the Super then startled the NDRC chairman to a slip of the pencil:
To denotate [sic: detonate] 5–10 kg of heavy hydrogen liquid would require 30 kg U235
If you use 2 or 3 Tons of liquid deuterium and 30 kg U235 this would be equivalent 108 [i.e., 100,000,000] tons of TNT.
Estimate devastation area of 1000 sq. km [or] 360 sq miles. Radioactivity lethal over same area for a few days.
Conant then drew a bold line with a steady hand and initialed the file note “JBC.” As an afterthought or at a later time he added: “S-l Executive Committee thinks the above probable. Heavy water is being pushed as hard as it can. [First] 100 kg of D will be available by fall of 1943 before 60 kg of U235 will be ready!”
A formal status report went off immediately from the Executive Committee to Bush. It predicted enough fissionable material for a test in eighteen months—by March 1944. It estimated that a 30-kilogram bomb of U235 “should have a destructive effect equivalent to the explosion of over 100,000 tons of TNT,” much more than the mere 2,000 tons estimated earlier. And it dramatically announced the Super:
If this [U235] unit is used to detonate a surrounding mass of 400 kg of liquid deuterium, the destructiveness should be equivalent to that of more than 10,000,000 tons of TNT. This should devastate an area of more than 100 square miles.
The committee—Briggs, Compton, Lawrence, Urey, Eger Murphree and Conant—concluded by judging the bomb project important beyond all previous estimates: “We have become convinced that success in this program before the enemy can succeed is necessary for victory. We also believe that success of this program will win the war if it has not previously been terminated.”
Making of the Atomic Bomb Page 58
