Excerpt: The Thicket's Prodigy: The Extraordinary Life of an Improbable Genius
With the exception of Albert Einstein, who had been assessed as a security risk, a more extreme collection of the world’s top physicists working together in a single isolated location was beyond any reasonable imagination.37
Among the first to arrive was a group of Nobel Prize winners and future prizewinners referred to by Oppenheimer as his “luminaries.” This first assemblage included Enrico Fermi, Leo Szilard, Hans Bethe, John Van Vleck, Edward Teller, Robert Serber, Emilio Segre, Edwin McMillan, Robert Bacher, Isidor Rabi, J. M. B. Kellogg, Edward Condon, L. D. P. King, Richard Tolman, Donald Kerst, and mathematicians J. Carson Mark and Stanislaw Ulam. And more were to come.
Many had fled Europe’s totalitarian regimes or Germany’s progressing war. To satisfy several of these individuals’ concerns over “anything military,” program management control, initially planned to be under US Army supervision, was revised to purportedly be under the supervision of the University of California and Dr. Glenn Seaborg. Actual oversight remained under General Groves and the Department of the Army.
All were brilliant scientists, but not all were theoretical physicists. For most, physics related to nuclear fission was new. Addressing the issue, a series of introductory lectures was initiated by Robert Serber, a Theoretical Physics Division group leader. Dr. Serber’s lectures were subsequently published in a book called The Los Alamos Primer and provided to incoming staff.
The Gadget
Prior to the completion of secure Main Tech Area facilities, conceptual development brainstorming sessions were often inconveniently required to be held while construction was in process.38 To avoid raising worker concerns that a project of mega-proportions was in the offing, “Gadget” was euphemistically substituted during discussions for “bomb.” The term remained through the Trinity test detonation
Conclusions summarized in the MAUD report and agreed as workable in the Oppenheimer conferences had settled on a gun-type trigger design—firing one piece of fissile material into another to result in achieving the critical mass required for detonation.
Planning focused on two bomb formats—the Mark 1 bomb was to make use of U-235 as the fissile material; the Mark 2 was to use PU-239:
Note: Materials capable of sustaining a chain reaction are called “fissile;” the two fissile materials used in atomic weapons are U-235 and PU-239. Uranium’s most common isotope, Uranium-238, composes 99.3% of natural, or unenriched, uranium. U-238 is fissionable but not fissile, meaning it cannot sustain a chain reaction by itself. The uranium isotope U-235 is the only naturally occurring substance which can break apart. It can split because the U-235 nucleus is unstable. When it breaks apart, the atom’s neutrons are released, hitting other U-235 atoms and causing them to also split. With enough intensity, such as in a bomb, the effect is an impressive explosion. The potential for that condition had been determined mathematically by Lise Meitner before being confirmed by Hahn and Strassmann.
The Mark 1 bomb, code-named “Little Boy,” was not particularly problematic by Manhattan Project standards. Enrico Fermi’s experiments had proven U-235’s potential, and Britain’s MAUD Report had concluded that one pound of uranium achieving full fission was capable of yielding an explosive force of eight kilotons—sixteen million pounds—of TNT. A proper mechanism needed to be developed, but, as applied to a uranium bomb’s probable success, concerns were limited.
The Mark 2 bomb, codenamed “Thin Man,” proved unworkable. Test results supported an already open skepticism raised by the Tube Alloys group in England. The MAUD Report had raised doubts over plutonium’s suitability for making a nuclear bomb at all; plutonium’s physical properties were too inconsistent. Now Manhattan Project scientists had cause for their own concerns.
In one of the Oppenheimer-sponsored, pre-Manhattan Project discussions, a conference participant, physicist Richard Tolman, had suggested the concept of an implosion-type device but had raised no support among conference participants.
But taking a cautious view, Oppenheimer had anticipated a need to mitigate risks associated with plutonium as a bomb component. While maintaining priority on the gun-type trigger format during Thin Man bomb testing, a separate group under physicist Seth Neddermeyer had been investigating implosion as a plutonium bomb trigger.
Neddermeyer’s belief was that Tolman’s idea had merit; implosion could be practical by making use of a hollow cylinder of plutonium surrounded by an explosive shell. The conceptual use of shaped charges in three-dimensional explosive “lenses” was reported to have earlier been broached by British physicist James L. Tuck. And Robert Christie was credited with calculations that concluded plutonium sphere compression could result in critical density. But complications in dealing with both considerations were profoundly complex and currently unsolved.
In September 1943, Oppenheimer made one of his more astute managerial conclusions when he requested mathematician John von Neumann to provide his thoughts on implosion.39 Von Neumann, regarded as the world’s most brilliant mathematician at the time, had developed an expertise in mathematically difficult-to-model explosions and was the leading authority on the mathematics of “shaped-charges.” A Princeton mathematics professor and member of Princeton’s Institute for Advanced Study, von Neumann had joined the institute, along with Albert Einstein, in 1935, following both of their escapes from Germany’s threat.
A review of Dr. Neddermeyers’s study results prompted von Neumann to discuss his observations with Edward Teller. Teller’s input concerning the potential for compressing PU-239 to a fissionable core density was convincing. Von Neumann concluded that the use of high-explosive shaped charges to implode a plutonium sphere could result in sufficient fission to create a bomb.
Further, his mathematical conclusions were that the implosion design, which later would be used on the Trinity and Nagasaki bombs, was likely to be more efficient than the Mark 1 bomb gun-type trigger design.
The following month, in October 1943, George Kistiakowsky, a Ukrainian-born explosives expert and Harvard professor, was persuaded to join the Manhattan Project, replacing Dr. Neddermeyer as head of the Explosives Division. Kistiakowsky’s work focused on the shape and composition of the high-explosive lenses calculated by von Neumann to be the most promising design for implosion bomb success.
In December 1943, the Brits joined the effort when a Tube Alloys contingent, led by Sir Rudolf Ernst Peierls and including Niels Bohr, Otto Frisch, Klaus Fuchs, and Ernest Titterton, arrived to add their expertise to Manhattan Project activities.40 At that point, there was expressed concern by Manhattan Project participants that their activity was proceeding satisfactorily without the Tube Alloys group’s assistance. The reaction would likely have been even more strident if it had been known Klaus Fuchs would one day become one of the Manhattan Project’s most notorious participants.
Several months later, on July 17, 1944, a final decision was made to abandon prospects for a plutonium bomb to use the gun-type trigger. The hypothetical plutonium fission-by-implosion problem’s status remained unchanged, but it was no longer simply an option; it had to be solved. Manhattan Project research was reorganized to focus efforts entirely on a third bomb format: The Mark 3 bomb, codenamed “Fat Man,” was to trigger plutonium detonation by implosion.41 That much was certain.
Two requirements still had to be addressed: unsolved difficulties associated with how to enable casting a spherical plutonium core and the need to overcome significant plutonium inconsistencies. Both were concerns, as had been stated in the MAUD Report. The ultimate tasks of the metallurgists were first, to cast plutonium into the shape of a perfect sphere, and second, to eliminate the corrosive effects of exposure to air.
Assuming plutonium inconsistencies could be overcome, the plutonium sphere’s compression had to be precise, meaning the explosive lenses had to be perfectly configured for uniformity of the compression wave required to condense the plutonium into a critical mass. The implosion process would squeeze a softball-sized solid plutonium sphere into a tennis ball size as dimensionally precise as the softball had been.
Assuming both a way could be found to cast plutonium into a perfect sphere and an answer to plutonium’s susceptibility to atmospheric degradation could be discovered, existing technology was still incapable of meeting the explosive lens detonation timing required. To be successful, the lenses had to detonate precisely within a fraction of a millionth of a second. If the implosion was even the slightest asymmetric, the plutonium core might be caused to shoot out one side, producing a fizzle—a considerably reduced result.
Intense testing of lens shapes, explosives compositions, and manners of detonation would be required to study the behavior of the shockwave needed to uniformly compress the plutonium sphere. The question for testing was how. Robert Serber, who had earlier been instrumental in his lectures defining a nuclear bomb’s physics, had the answer: radioactive lanthanum as a plutonium surrogate.42
Lanthanum-140, a lanthanum isotope, was chosen because of its short half-life and ready availability. Barium-140, a fission product produced in uranium slugs at Oak Ridge’s X-10 graphite reactor, was separated from the uranium, then shipped in lead-lined containers to Los Alamos where the lanthanum-140 and barium-140 were separated at the TA-10 Site for use in Bayo Canyon RaLa shots.
The first RaLa test was conducted on September 22, 1944, by a team led by Italian experimental physicist Bruno Rossi. Several generations of RaLa program testing solved detonation circuit timing difficulties, explosive lens composition, and lens configuration. Meanwhile, plutonium inconsistency problems had also been separately resolved. To overcome decomposition resulting from atmospheric exposure, the plutonium core was cast in two nickel-plated hemispheres.
Combined, the results led to a February 1945 conclusion: test participants were satisfied that an atomic explosion using PU-239 could be produced. Robert Oppenheimer, General Groves, James Conan, George Kistiakowsky, and three others met in Robert Oppenheimer’s office on February 28, 1945, to approve the implosion bomb’s design.43
Mark 3 was now referred to as "Fat Man."
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37. Hawkins, “The Los Alamos Project Y, Vol. I,”, 41.
38. Jogekalar, “What John von Neumann Really Did . . .”
39. Hawkins, “The Los Alamos Project Y,” 29 and 50.
40. Truslow and Smith, Manhattan Project Y, Volume II, 82.
41. Ibid., 126, 186, 1250.
42. Atomic Heritage Foundation, “Electronics and Detonators,” 1-9.
43. Atomic Archive, “Design of Two Bombs,” 2.
Hawkins, David. “MANHATTAN DISTRICT HISTORY PROJECT Y THE LOS ALAMOS PROJECT VOL. I INCEPTION UNTIL AUGUST 1945.” United States. 1961. https://doi.org/10.2172/1087644.
https://www.osti.gov/servlets/purl/1087644.
Jogalekar, Ashutosh. “What John von Neumann really did at Los Alamos.” 3 Quarks Daily, October 26, 2020. https://3quarksdaily.com/3quarksdaily/2020/10/what-john-von-neumann-really-did-at-los-alamos.html.
Truslow, Edith C., and Ralph C. Smith. Manhattan District History, Project Y, The Los Alamos Project Volume II, August 1945 -December 1946. Los Alamos: U.S. Atomic Energy Commission (Contract W-7405-ENG). Written in 1947, released in 1961.
Electronics and Detonators.” Atomic Heritage Foundation, July 11, 2017. https://www.atomicheritage.org/history/electronics-and-detonators.
Atomic Archive. “Designs of Two Bombs.” Accessed May 13, 2019. https://www.atomicarchive.com/history/atomic-bombing/hiroshima/page-2.html.
