Nishina continued isotope studies for the Army, deciding on March 19 to focus on thermal diffusion as the only practical separation technology at a time of increasing national shortages. He spoke to his staff of processing several hundred tons of uranium after first building laboratory-scale diffusion apparatus. He envisioned a major program run in parallel, as the Manhattan Project was beginning to be, with weapon design and development proceeding simultaneously with U235 production.
Meanwhile a different branch of the Navy, the Fleet Administration Center, sponsored a new project in atomic bomb development at the University of Kyoto, where Tokutaro Hagiwara had made his startling early prediction of the possibility of a thermonuclear explosive. The university won support in 1943 to the extent of 600,000 yen—nearly $1.5 million—much of which it budgeted to build a cyclotron.
* * *
Robert Oppenheimer moved to Santa Fe with a small team of aides on March 15, 1943, brisk early spring. Scientists and their families arrived by automobile and train during the next four weeks. Not much was ready on the mesa, which they began to call the Hill. Groves wanted no breaches of security in the lobbies of Santa Fe hotels; the Army commandeered guest ranches in the area for quarters suitably remote and bought up Santa Fe’s feeble stock of used cars and jitneys to serve as transportation through ruts and mud up and down the terrifying unbarricaded dirt switchback of the mesa access road. After flat tires and mirings, hours could be short on the Hill. Box lunches assembled in Santa Fe gave cold comfort when the delivery truck made it through.
The hardships only mattered because they slowed the work. Oppenheimer had sold it as work that would end the war to end all wars and his people believed him. The unit of measurement for wasted hours was therefore human lives. Construction crews unwilling to vary the specifications of a laboratory door or hang an unauthorized shelf initially bore the brunt of the scientists’ impatience. John Manley remembers inspecting the chemistry and physics building. It needed a basement at one end for an accelerator and a solid foundation at the other end for the two Van de Graaffs—which end for which was unimportant. Rather than adjust the construction plans for terrain the contractor had drilled the basement from solid rock and used the rock debris as fill for the foundation. “This was my introduction to the Army Engineers.”1782
Fuller Lodge, a Ranch School hall elegantly assembled of monumental hand-hewn logs, was kept to serve as a dining room and guest house. The pond south of the lodge—predictably named Ashley Pond after the Ranch School’s founder—offered winter ice-skating and summer canoeing and the easeful harmonic wakes of swimming ducks. The engineers preserved the stone icehouse beside the pond that the school had used to store winter cuttings of ice and the row of tree-shaded faculty residences northeast of the lodge. Across the dirt main road that divided the mesa south of the pond the Tech Area went up in a style the Army called modified mobilization: plain one-story buildings like elongated barracks with clapboard sides and shingled roofs. T Building would house Oppenheimer and his staff and the Theoretical Physics Division; behind T, connected by a covered walkway, would be the much longer chemistry and physics building with its Van de Graaffs; behind that the laboratory shops. Farther south near the rim of the mesa above Los Alamos canyon contractors would hammer up a cryogenics laboratory and the building that would shelter Harvard’s cyclotron. West and north of the Tech Area the first two-story, four-unit family apartments, painted drab green, urbanized last year’s pastures and fields; more apartments, and dormitories for the unmarried, would follow.
At the beginning of April Oppenheimer assembled the scientific staff—“about thirty persons” at that point of the hundred scientists initially hired, says Emilio Segrè, who was one among them—for a series of introductory lectures.1783 Robert Serber, thin and shy, delivered the lectures with authority despite the distraction of a lisp; they summed up the conclusions of the Berkeley summer study and incorporated the experimental fast-fission work of the past year. Edward U. Condon, the crew-cut, Alamogordoborn theoretician from Westinghouse whom Oppenheimer had chosen for associate director, revised his notes of Serber’s lectures into the new laboratory’s first report, a document called the Los Alamos Primer that was subsequently handed to all new Tech Area arrivals cleared for Secret Limited access.1784 In twenty-four mimeographed pages the Primer defined the laboratory’s program to build the first atomic bombs.
Serber’s lectures startled the chemists and experimental physicists whom compartmentalization had kept in the dark; the scientists’ euphoria at finally learning in detail what they had only previously guessed or heard hinted measures the extent to which secrecy had contorted their emotional commitment to the work. Now, following the lead of their mentors—their average age was twenty-five; Oppenheimer, Bethe, Teller, McMillan, Bacher, Segrè and Condon were older men—they could apply themselves at last with devotion. In that heady new freedom they seldom noticed the barbed wire. Similarly confined but kept uninformed because Oppenheimer and Groves decided it so, the wives served harder time.
“The object of the project,” Condon summarizes what Serber told the scientists, “is to produce a practical military weapon in the form of a bomb in which the energy is released by a fast neutron chain reaction in one or more of the materials known to show nuclear fission.”1785 Serber said one kilogram of U235 was approximately equal to 20,000 tons of TNT and noted that nature had almost located that conversion beyond human meddling: “Since only the last few generations [of the chain reaction] will release enough energy to produce much expansion [of the critical mass], it is just possible for the reaction to occur to an interesting extent before it is stopped by the spreading of the active material.”1786 If fission had proceeded more energetically the bombs would have slept forever in the dark beds of their ores.
Serber discussed fission cross sections, the energy spectrum of secondary neutrons, the average number of secondary neutrons per fission (measured by then to be about 2.2), the neutron capture process in U238 that led to plutonium and why ordinary uranium is safe (it would have to be enriched to at least 7 percent U235, the young theoretician pointed out, “to make an explosive reaction possible”).1787, 1788 He was already calling the bomb “the gadget,” its nickname thereafter on the Hill, a bravado metonymy that Oppenheimer probably coined.1789 The calculations Serber reported indicated a critical mass for metallic U235 tamped with a thick shell of ordinary uranium of 15 kilograms: 33 pounds. For plutonium similarly tamped the critical mass might be 5 kilograms: 11 pounds. The heart of their atomic bomb would then be a cantaloupe of U235 or an orange of Pu239 surrounded by a watermelon of ordinary uranium tamper, the combined diameter of the two nested spheres about 18 inches. Shaped of such heavy metal the tamper would weigh about a ton. The critical masses would eventually have to be determined by actual test, Serber said.
He went on to speak of damage. Out to a radius of a thousand yards around the point of explosion the area would be drenched with neutrons, enough to produce “severe pathological effects.”1790 That would render the area uninhabitable for a time. It was clear by now—it had not been clear before—that a nuclear explosion would be no less damaging than an equivalent chemical explosion. “Since the one factor that determines the damage is the energy release, our aim is simply to get as much energy from the explosion as we can. And since the materials we use are very precious, we are constrained to do this with as high an efficiency as is possible.”1791
Efficiency appeared to be a serious problem. “The reaction will not go to completion in an actual gadget.”1792 Untamped, a bomb core even as large as twice the critical mass would completely fission less than 1 percent of its nuclear material before it expanded enough to stop the chain reaction from proceeding. An equally disadvantageous secondary effect also tended to stop the reaction: “as the pressure builds up it begins to blow off material at the outer edge of the [core].”1793 Tamper always increased efficiency; it reflected neutrons back into the core and its inertia—not its
tensile strength, which was inconsequential at the pressures a chain reaction would generate—slowed the core’s expansion and helped keep the core surface from blowing away. But even with a good tamper they would need more than one critical mass per bomb for reasonable efficiency.
Detonation was equally a problem. To detonate their bombs they would have to rearrange the core material so that its effective neutron number, which corresponded to Fermi’s k, changed from less than 1 to more than 1. But however they rearranged the material—firing one subcritical piece into another subcritical piece inside the barrel of a cannon seemed to be the simplest option—they would have no slow, smooth transition as Fermi had with CP-1. If they fired one piece into another at the high velocity of 3,000 feet per second it would take the pieces about a thousandth of a second to assemble themselves. But since more than one critical mass was necessary for an efficient explosion the pieces would be supercritical before they had completely mated. If a stray neutron then started a chain reaction, the resulting inefficient explosion would proceed from beginning to end in a few millionths of a second. “An explosion started by a premature neutron will be all finished before there is time for the pieces to move an appreciable distance.”1794 Which meant that the neutron background—spontaneous-fission neutrons from the tamper, neutrons knocked from light-element impurities, neutrons from cosmic rays—would have to be kept as low as possible and the rearrangement of the core material managed as fast as possible. On the other hand, they did not have to worry that a fizzle would drop an intact bomb into enemy hands; even a fizzle would release energy equivalent to at least sixty tons of TNT.
Predetonation would reduce the bomb’s efficiency, Serber repeated; so also might postdetonation. “When the pieces reach their best position we want to be very sure that a neutron starts the reaction before the pieces have a chance to separate and break.”1795 So there might be a third basic component to their atomic bomb besides nuclear core and confining tamper: an initiator—a Ra + Be source or, better, a Po + Be source, with the radium or polonium attached perhaps to one piece of the core and the beryllium to the other, to smash together and spray neutrons when the parts mated to start the chain reaction.
Firing the pieces of core together, the Berkeley theoretician continued, “is the part of the job about which we know least at present.”1796 The summer-study group had examined several ingenious designs. The most favorable fired a cylindrical male plug of core and tamper into a mated female sphere of tamper and core, illustrated here in cross section from the Los Alamos Primer:
The target sphere could be simply welded to the muzzle of a cannon; then the cylinder, which might weigh about a hundred pounds, could be fired up the barrel like a shell:
The highest muzzle velocity available in U.S. Army guns is one whose bore is 4.7 inches and whose barrel is 21 feet long. This gives a 50 lb. projectile a muzzle velocity of 3150 ft/sec. The gun weighs 5 tons. It appears that the ratio of projectile mass to gun mass is about constant for different guns so a 100 lb. projectile would require a gun weighing about 10 tons.1798
For a mechanism eight times lighter or with double the effective muzzle velocity they could weld two guns together at their muzzles and fire two projectiles into each other. Synchronization would be a problem with such a design and efficiency might require four critical masses instead of two, a demand which would significantly delay delivering a usable bomb.
Serber also described more speculative arrangements: sliced ellipsoidal core-tamper assemblies like halves of hard-boiled eggs that slid together; wedge-shaped quarters of core/tamper like sections of a quartered apple mounted on a ring. That was an odd and striking design, sketched in the mimeographed Primer as probably on a blackboard before, and it did not go unnoticed. “If explosive material were distributed around the ring and fired the pieces would be blown inward to form a sphere”:1799
Autocatalytic bombs—bombs in which the chain reaction itself, as it proceeded, increased the neutron number for a time—looked less promising. The cleverest notion incorporated “bubbles” of boron-coated paraffin into the U235 core; as the core expanded it would compress the neutron-absorbing boron and render it less efficient, freeing more neutrons for fission chains. But: “All autocatalytic schemes that have been thought of so far require large amounts of active material, are low in efficiency unless very large amounts are used, and are dangerous to handle. Some bright ideas are needed.”1801
Their immediate work of experiment, Serber concluded, would be measuring the neutron properties of various materials and mastering the ordnance problem—the problem, that is, of assembling a critical mass and firing the bomb. They would also have to devise a way to measure a critical mass for fast fission with subcritical amounts of U235 and Pu239. They had a deadline: workable bombs ready when enough uranium and plutonium was ready. That probably gave them two years.
The Japanese physics colloquium in Tokyo had decided in March 1943 that an atomic bomb was possible but not practically attainable by any of the belligerents in time to be of use in the present war. Robert Serber’s lectures at Los Alamos in early April asserted to the contrary that for the United States an atomic bomb was both possible and probably attainable within two years. The Japanese assessment was essentially technological. Like Bohr’s assessment in 1939, it overestimated the difficulty of isotope separation and underestimated U.S. industrial capacity. It also, as the Japanese government had before Pearl Harbor, underestimated American dedication. Collective dedication was a pattern of Japanese culture more than of American. But Americans could summon it when challenged, and couple it with resources of talent and capital unmatched anywhere else in the world.
The Europeans at Los Alamos complained of the barbed wire. With the exception, apparently, only of Edward Condon, who found security so oppressive he quit the project within weeks of his arrival and went back to Westinghouse, the Americans accepted the fences around their work and their lives as a necessity of war. The war was a manifestation of nationalism, not of science, and such did their duty on the Hill appear at first to be. There was “relatively little nuclear physics” at Los Alamos, Bethe says, mostly cross-section calculations.1802 They thought they were assembled to engineer a “practical military weapon.” That was first of all a national goal. Science—a fragile, nascent political system of limited but increasing franchise—would have to wait until the war was won. Or so it seemed. But a few among the men and women gathered at Los Alamos—certainly Robert Oppenheimer—sniffed a paradox. They proposed in fact to win the war with an application of their science. They dreamed further that by that same application they might forestall the next war, might even end war as a means of settling differences between nations. Which must in the long run have decisive consequences, one way or the other, for nationalism.
* * *
By the time Robert Serber finished his orientation lectures at Los Alamos in mid-April most of the scientific and technical staff was on hand, many lodged temporarily in the surviving buildings of the Ranch School. Now began a second phase of the conference, to plan the laboratory’s work. “If there were any ground-breaking ceremonies at Los Alamos like champagne or cutting ribbons,” John Manley comments, “I was unaware of them.1803 Most of us who were there felt that the conference in April, 1943, was really the ground-breaking ceremony.”1804 Rabi, Fermi and Samuel Allison arrived from Cambridge and Chicago to serve as senior consultants. Groves appointed a review committee—W. K. Lewis again, an engineer named E. L. Rose who was thoroughly experienced in ordnance design, Van Vleck, Tolman and one other expert—to follow planning and advise. Groves despite his formidable competence as an organizer and administrator was intellectually insecure around so many distinguished scientists, as who would not be?
They laid their plans, often during hikes into the uninhabited wild surroundings of the mesa. They had to rely heavily on theoretical anticipations of the effects they wanted to study; that was their basic constraint. Any experimental device that demonstrat
ed a fast-neutron chain reaction to completion would use up at least one critical mass: there could be no controlled, laboratory-scale bomb tests, no squash-court demonstrations. They decided they had to analyze the explosion theoretically and work out ways to calculate the stages of its development. They needed to understand how neutrons would diffuse through the core and the tamper. They needed a theory of the explosion’s hydrodynamics—the complex dynamic motions of its fluids, which the core and tamper would almost instantly become as their metals heated from solid to liquid to gas.
They needed detailed experiments to observe bomb-related nuclear phenomena and they needed integral experiments to duplicate as much as possible the full-scale operation of the bomb. They had to develop an initiator to start the chain reaction. They had to devise technology for reducing uranium and plutonium to metal, for casting and shaping that metal, possibly for alloying it to improve its properties. Particularly with plutonium, they had to discover and measure those properties in the first place and do so quickly when more than microgram quantities began to arrive. As a sideline, because they agreed that work on the Super should continue at second priority, they wanted to construct and operate a plant for liquefying deuterium at −429°F—the cryogenics plant to be built near the south rim of the mesa.
Ordnance work was crucial. From the April discussions came immediate breakthroughs. An Oppenheimer recruit from the National Bureau of Standards who had been a protégé at Caltech, a tall, thin, thirty-six-year-old experimental physicist named Seth Neddermeyer, imagined an entirely different strategy of assembly.1805 Neddermeyer could not quite remember after the war the complex integrations by which he came to it. An ordnance expert had been lecturing. The expert had quibbled at the physicists’ use of the word “explosion” to describe firing the bomb parts together. The proper word, the expert said, was “implosion.” During Serber’s lectures Neddermeyer had already been thinking about what must happen when a heavy cylinder of metal is fired into a blind hole in an even heavier metal sphere. Spheres and shock waves made him think about spherically symmetrical shock waves, whatever those might be. “I remember thinking of trying to push in a shell of material against a plastic flow,” Neddermeyer told an interviewer later, “and I calculated the minimum pressures that would have to be applied. Then I happened to recall a crazy thing somebody had published about firing bullets against each other. It may have had a photograph of two bullets liquefied on impact. That is what I was thinking when the ballistics man mentioned implosion.”1806
Making of the Atomic Bomb Page 64
