Trinity, Los Alamos, and fission weapons (1943-1945)

A learning journey by Arthur Lee, assisted by Google NotebookLM and Microsoft CoPilot

16 July 2025

Photo caption: Robert Oppenheimer and Manhattan Project commanding general Leslie R. Groves visited the Trinity site and found only the reinforcing rods of the tower footings left unvaporized.

Source: Los Alamos Primer, 2020 edition, from the archives of the Los Alamos National Laboratory.

On 16 July 1945, 5:29 am Mountain War Time, scientists of the Manhattan Project measured and witnessed the first man-made nuclear explosion sited in the Jornada del Muerto desert in New Mexico. The nuclear age was born eighty years ago.

I am continuing my learning journey. I have written about this history in previous years, re-engaging with my early career roots #nuclearweapons #nuclearpower.

The 80th anniversary is an important year to commemorate this momentous event in human history. Writing about it and reviewing some of the original documents of this history always impressed me how they completed all these monumental tasks, from the tiniest bit of precision machining to the fitting of the largest and heaviest components of a nuclear core. Accomplishing the most complex calculations and thinking through the initial theory of the physics, and translating the data from the crudest of measurements, without sophisticated computing machinery and precision digital mechanisms, amazed me even more.  

For this learning journey, I reviewed three original references, beginning with the mimeograph of Robert Serber’s lecture notes and Robert Serber’s book.

·       Robert Serber’s mimeographed notes of five lectures, compiled and edited by Edward Condon, the associate director of the Los Alamos project.

·       Robert Serber’s book, annotated by him, published in 1992, University of California Press. The second edition, published in 2020, has a foreword by Richard Rhodes.

·       David Hawkins, Manhattan District History: Project Y, the Los Alamos Project, New Mexico, Volume I, inception until August 1945. Written in 1946 and 1947. LAMS-2532(Vol. I), Special Distribution.

I generated two AI-podcasts of this review. The complex history of the Los Alamos Project seems to me to deserve two crucial segments of a learning journey. The two podcasts are organized like so. To get a good historical flow, I recommend listening to the order of the podcasts I laid out below.

·       In the first AI-podcast, I asked the two AI-podcasters to focus more on the known science up to April 1943, the fundamentals and the physical data that Robert Serber elucidated.

·       In the second AI-podcast, I asked the two AI-podcasters to describe and review the actual work, the incremental successes and failures up to the Trinity test in July 1945, including the ‘crisis’ of discovering plutonium-240 impurities which led to a major reorganization of the lab and its workflow.

All these events occurred against the backdrop of a war when every day counted, when every day meant tens of thousands of lives, when every day meant Werner Heisenberg’s nuclear program in Nazi Germany was potentially one day closer to a German atomic bomb.

In the beginning, there were five lectures.

In late 1942 and early 1943, under the leadership of J. Robert Oppenheimer, Project Y the code name for Los Alamos, New Mexico, the secret atomic bomb development site of the Manhattan Project, opened its doors. Scores of scientists and engineers arrived. How to onboard them quickly (in today’s corporate business language)?  Robert Serber, the postdoctoral researcher who studied under Oppenheimer, one of Oppenheimer’s earliest students, was tasked with giving lectures to these new recruits. He gave five seminal lectures. The lecture notes have been mimeographed and circulated widely amongst physicists at Los Alamos. After the war, the notes were passed around physics departments all around the U.S. Finally, they were declassified in 1963.

The notes came into a publicly releasable form in 1985, according to the declassification stamp on the cover page. Robert Serber’s lectures, as folks recalled postwar, were a tour de force of nuclear physics, comprehensive of all that was known in April 1943. The Los Alamos Primer, later annotated by Robert Serber in 1992 for the University of California Press publication, is still required reading for all nuclear engineers and people interested in the science and design of nuclear weapons and nuclear power today. Richard Rhodes, who wrote the Pulitzer Prize winning book The Making of the Atomic Bomb, wrote a new foreword in 2020.

Top of Form

MakM Everything that was known about the physics and engineering of an atomic bomb in 1943 - "The Los Alamos Primer" – was presented here.

Robert Serber was simple and direct from the first instance he started talking.

“The object of the project 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.”

Robert Serber's "The Los Alamos Primer" elucidates the core physics governing atomic bombs, providing the theoretical framework for the Manhattan Project.

• Core Concept: Fission Chain Reaction: The fundamental principle is nuclear fission, where "when a neutron strikes a uranium nucleus, it’s temporarily absorbed to make a nucleus with one extra neutron." This combined nucleus can then "fission, with a corresponding release of energy and ejection of secondary neutrons." The goal is to sustain a chain reaction.

• Fissile Materials: Uranium-235: Referred to as "25," it is the rarer isotope of uranium. A "Little Boy" (gun-design) bomb used "about fifty kilograms of uranium enriched to above 90 percent uranium-235." For uranium-235, the average number of neutrons created per fission (ν) is 2.6, the fission mean free path (λfiss) is 17 cm, and the transport mean free path (λtrans) is 3.5 cm.

• Plutonium-239 (Pu239): Referred to as "49," this isotope of plutonium was also of primary interest. Its radioactivity posed predetonation challenges.

• Energy Release: The energy of fission of 1 kg of uranium was calculated to be "20,000 tons of exploding TNT" (short tons). The energy released goes into the kinetic energy of the expanding core.

• Critical Mass: The minimum amount of fissile material needed to sustain a nuclear chain reaction.

• Factors Affecting Critical Mass:Density: "The critical radius can thus be decreased by compressing the fissile material, as is done in implosion weapons." Critical mass is "inversely proportional to the square of the density."

• Shape: For a given mass, a sphere provides the most favorable geometry. A cubical shape requires 1.24 times greater critical mass than a sphere.

• Tamper: A surrounding material (e.g., uranium, gold, tungsten) reflects neutrons back into the core, reducing the critical mass and increasing efficiency. For example, the critical mass of bare uranium-235 is 56 kg, reduced to 15 kg with a thick uranium tamper. For plutonium-239, bare is 11 kg, reduced to 5 kg with a thick uranium tamper. The tamper's weight is "about a ton" with an effective thickness of approximately 13-17 cm.

• Neutron Behavior: Neutron Density (N): Describes the concentration of neutrons at a given point. The rate of increase of neutron density is crucial for a chain reaction.

• Mean Free Path (l): The average distance a neutron travels before colliding with a nucleus. For metallic uranium, the mean free path is approximately 2.6 cm.

• Cross Section (σ): Represents the effective area of a nucleus for a neutron interaction (fission, capture, scattering). The geometric cross section is πR², roughly 3 x 10^-24 cm² for a nucleus with a radius of 10^-12 cm.

• Weapon Designs: Gun Design (Little Boy): Utilized a "gun" mechanism to fire one subcritical piece of fissile material into another to achieve a supercritical state. The gun used in Little Boy weighed 1,000 pounds and was six feet long.

• Implosion Design (Fat Man): Involved compressing a subcritical sphere of fissile material using conventional explosives to increase its density and achieve supercriticality. In this method, an initial critical mass could become "four critical masses in the compressed state" by doubling the density.

• Predetonation: The risk of the weapon firing prematurely due to unavoidable neutron sources. This necessitates rapid assembly mechanisms. For a bomb with 2-3 critical masses and a 0.3 final ν' value, with 10^4 neutrons/sec from unavoidable sources, and a component moving 10 cm at 10^5 cm/sec, there is approximately a "15% chance of predetonation."

Key findings and recommendations in the Los Alamos Primer

Top of Form

Key Findings:

• Fission Potential: The direct energy release in the fission process is substantial, approximately 170 MeV per atom, vastly exceeding ordinary combustion processes. 1 kg of uranium-235 can theoretically release the energy of about 20,000 tons of TNT .

• Chain Reaction: A large-scale energy release is feasible due to each fission process releasing multiple neutrons, enabling a chain reaction.

• Material Requirements: The materials of interest are uranium-235, uranium-238, and plutonium-239. Natural uranium contains only about 1/140 of uranium-235, or 0.71%.

• Neutron Spectrum: Neutrons released in fission have a mean energy of about 2 MeV.

• Neutron Multiplication: The average number of neutrons produced per fission is approximately 2.2.

• Critical Mass: There exists a critical radius and mass, dependent on material density, below which a chain reaction cannot be sustained due to neutron losses. The critical mass for Material 49 (Plutonium-239) might be less than that of Material 25 (uranium-235) by a factor of about 3 .

• Tamper Effect: Surrounding the active material with a tamper can reduce the critical mass by reflecting neutrons back into the core and slowing down the expansion of the active material.

• Predetonation Risk: There is a risk of predetonation due to background neutrons, which can initiate a chain reaction before the optimal configuration (the ‘critical assembly’) is achieved.

• Material 49 Challenges: Material 49 (plutonium-239) is extremely difficult to work with due to neutron background from (α, n) reactions with impurities, necessitating high-velocity firing. [Note: This problem was later solved by the implosion method. The gun method was never used for plutonium-239.]

Key Recommendations:

• Neutron Property Measurement: Prioritize measuring the neutron properties of various materials.

• Ordnance Development: Address ordnance problems related to bomb assembly and firing mechanisms.

• Critical Size Determination: Conduct studies to directly determine critical size and time scale, working with large but sub-critical amounts of active material.

• Tamper Use: Utilize tamper materials to enhance efficiency by reflecting neutrons and slowing expansion.

• Impurity Control: Minimize light element impurities in active materials, especially Plutonium-239, to reduce neutron background.

• High-Velocity Firing: Implement high-velocity firing methods to reduce the risk of predetonation, particularly when using Material 49 or uranium tampers.

• Detonating Source: Consider using a strong neutron source that becomes active upon achieving the correct configuration to ensure detonation.

• Isotope Separation: Improve the enrichment process so that the fraction of 25 is increased at least 10-fold to make an explosive reaction possible.

The "Primer" emphasizes the need for ongoing research and experimentation to refine these findings and address the numerous technical challenges associated with creating a functional atomic bomb.

Richard Rhodes' foreword in "The Los Alamos Primer" highlights several key aspects of the Los Alamos Project and the historical significance of the Primer itself.

Key points made by Richard Rhodes in the foreword:

• Context of the Los Alamos Project: Scientists gathered at a top-secret laboratory in Los Alamos, New Mexico, in April 1943 to design and build the world's first atomic bombs, with the unofficial understanding that their work could end World War II. The broader context was a global war, fear of Nazi Germany developing an atomic bomb, and the hope that a new weapon could bring about a "miracle of deliverance".

• The Site and its Challenges: The Los Alamos site, a high, pine-forested plateau, was initially a construction mess, but core log buildings provided sanctuary. The project faced immense challenges, including the need to learn more about fast-neutron fission physics, determine critical masses, master metallurgy of new elements, invent initiators, treat explosives as precision instruments, address radiation medicine, and modify bombers for weapon delivery.

• Key Personnel: The team included distinguished scientists like J. Robert Oppenheimer, Enrico Fermi, I.I. Rabi, Edward Teller, and Hans Bethe, though the average age of the group was only twenty-four.

• Robert Serber's Lectures: Robert Serber, an Oppenheimer protégé, initiated the work at the laboratory with a series of five lectures, an "indoctrination course," in April 1943. These lectures brought new recruits up to speed on the known physics of nuclear weapons, drawing on research from late 1938 until Los Alamos's opening.

• Purpose of the Project: Serber's opening statement about the project’s aim was clear and direct: to produce a "practical military weapon in the form of a bomb in which the energy is released by a fast neutron chain reaction". Oppenheimer advocated for scientific freedom and open discussion, which was deemed essential for the project's success despite military secrecy.

• The "Primer" Document: Edward Condon, the associate director, compiled Serber's lecture notes into "twenty-four mimeographed pages dense with formulas, graphs, and crude drawings," which became known as "The Los Alamos Primer". New recruits were given a copy upon arrival, and the population of the Hill grew significantly.

• Fission and Plutonium Discoveries: Rhodes recounts the discovery of nuclear fission by Lise Meitner, Otto Hahn, and Fritz Strassmann as a "complete surprise," revealing an "unprecedented and extraordinary" energy source. The Manhattan Project, established in August 1942 and led by Leslie Groves, involved massive industrial efforts to isolate uranium-235 and synthesize plutonium-239.

• The Gun vs. Implosion Methods: The simpler "shooting method" (gun assembly) was suitable for the uranium-based Hiroshima bomb ("Little Boy"). However, the discovery of plutonium-240 contamination in reactor-bred plutonium made the gun method unfeasible for plutonium due to unacceptable spontaneous fission rates, which would cause predetonation. This led to a "crisis" at Los Alamos and the development of the "implosion method," a new technology proposed by Richard Tolman and significantly advanced by John von Neumann and Robert Christy.

• The Trinity Test and its Aftermath: The implosion mechanism was tested in the "Trinity" bomb, detonated on July 16, 1945, in southern New Mexico, marking the first full-scale manmade nuclear explosion on Earth with a yield of 18,600 tons of TNT. The subsequent use of bombs on Hiroshima and Nagasaki was justified by military advisors to save lives, deter the Soviet Union, and validate the immense cost of the project.

• Legacy of the Nuclear Age: The bombings revealed that science had "drilled a well into an essentially inexhaustible source of energy," creating a "completely new situation" in international politics. The "Los Alamos Primer" is viewed as a "historic marker" of science becoming the most influential institutional force for change, including political change.

Historical and Scientific Significance of The Los Alamos Primer:

• Foundational Document: It is considered "mandatory reading for anyone interested in the origins of nuclear weapons". Robert Serber, its author, stated that it is "essentially a summary of everything we knew in April 1943 about how to make an atomic bomb".

• Comprehensive Overview: The Primer "adroitly summarized the state of existing knowledge and laid out a prescient road map for the work ahead and the challenges that might arise". It covers characteristics of fission reactions, estimates of critical mass, efficiency, damage, and triggering issues.

• Blueprint for the Project: It served as an "indoctrination course" for scientists new to Los Alamos, ensuring a shared understanding of the project's goals and underlying physics. Its density of formulas, graphs, and drawings made it the "essence of what anyone in the world knew at that point about a secret new technology that would change forever the way nations thought about war".

• Predictive Power: Serber's ability to anticipate many unknowns and challenges speaks to his "extraordinary command of the experimental, theoretical, and engineering issues of the project". His analyses were "elegant and compact yet sensibly accurate".

• Historical Import: Rhodes posits that the Primer, alongside the Frisch-Peierls memorandum, carries a "greater freight of historic import than perhaps any other documents in the history of technology". It's not a "recipe" for building a bomb but rather a detailed account of the physics and engineering challenges.

• Enduring Legacy: Despite being a "Top Secret; Limited Distribution" document for many years, it was declassified in 1965 and has since been used in college courses, standing as a "historic marker" of science's influential role in global change and a "clear and concise exposition of what was known at the time and what problems there were to be solved". It offers "great deal of insight into the thinking of the scientists of the Manhattan Project".

Physics, chemistry, metallurgy, and engineering challenges from 1943-1945 at Los Alamos

Project Y, also known as the Los Alamos Project, faced a multitude of overarching scientific and engineering challenges in its mission to develop the atomic bomb. These challenges were compounded by the project's wartime constraints, including an urgent timeline, unprecedented secrecy, and the remote location of the laboratory.

The project's primary objective was to produce a practical military weapon based on a fast-neutron chain reaction. This required addressing fundamental unknowns in nuclear physics and developing entirely new engineering methods.

I. Scientific and Theoretical Challenges:

A significant initial hurdle was the lack of absolute experimental confirmation of the bomb's feasibility in terms of its basic nuclear processes. Key uncertainties included:

• Neutron Number: The average number of neutrons produced per fission had not been directly measured for fast neutrons, only for slow fission. This was a critical unknown, especially for plutonium. The first experiment at Los Alamos in July 1943 confirmed that plutonium-239 yielded more neutrons than uranium-235, justifying plutonium production efforts.

• Fission Timescale: The time between fissions in a fast chain reaction was not definitively known, though theoretically predicted to be negligible. Experiments quickly confirmed that neutron emission delays were minimal.

• Cross Sections: Accurate measurements of fission, capture, and scattering cross sections for various materials (like uranium-235 and plutonium-239) were crucial. Their dependence on neutron energy was particularly important. The data from these measurements influenced decisions, such as the eventual abandonment of the uranium hydride bomb program.

• Neutron Diffusion Theory: Developing an accurate integral diffusion theory was essential for predicting critical masses, as ordinary diffusion theory was inadequate for the bomb's small, dense core. This involved refining assumptions like isotropic scattering and uniform mean free paths.

• Efficiency Calculations: This was one of the most complex theoretical problems, requiring calculations of neutron chain reactions in rapidly changing materials, considering the effects of radiation.

• Thermonuclear Reaction (The Super): Investigations into a thermonuclear "Super" bomb, though secondary, presented significant theoretical challenges, including understanding ignition temperatures, energy dissipation, and the potential need for tritium. Early concerns about igniting the Earth's atmosphere were theoretically ruled out.

• Plutonium-240 Discovery: The unexpected discovery of plutonium-240, an isotope with a high spontaneous fission rate, was a critical scientific development. This significantly increased the neutron background in reactor-produced plutonium, rendering the gun method for plutonium assembly impractical. This fundamental scientific finding directly necessitated the success of the implosion program.

II. Engineering Challenges:

The development of the bomb required extensive engineering innovation, as much of the work involved exploring new fields of engineering:

• Assembly Mechanism (Gun vs. Implosion): The "most difficult of all problems" was assembling critical masses fast enough to produce a high-order explosion. Gun Method: While appearing more practical initially, the gun method for plutonium required extremely high assembly velocities (around 3,000 feet per second) near the upper limits of standard gun design, and strict chemical purity to minimize neutron background and prevent predetonation. Designing a lightweight, short, and reliable gun with specific ballistic properties was a unique challenge. Ultimately, the high spontaneous fission rate of plutonium-240 made the gun method unfeasible for plutonium. Implosion Method: Initially considered a fallback, implosion became an "absolute necessity" for plutonium after the Pu-240 discovery. Its challenges were far greater: Simultaneity of Detonation: Achieving perfectly simultaneous detonation over the surface of a high-explosive sphere was crucial and presented unknown difficulties. This led to the development of precise electric detonators and detonation systems. Hydrodynamics of Compression: Understanding the behavior of solid matter under extreme pressures, where kinetic energy is converted into potential energy of compression, was unprecedented. This required extensive theoretical and computational work, initially with IBM machines. Symmetry and Instability: Ensuring a perfectly symmetrical imploding shock wave was extremely difficult. Early experiments revealed serious asymmetries and jet formation, which could significantly reduce efficiency. The "Taylor instability" principle highlighted the problem of light high-explosives pushing against heavier tamper material. This drove the development of complex "explosive lenses" designed to convert multiple-point detonations into a converging spherical wave. Initiator Design: Developing a strong neutron source that could be "turned on" precisely at the moment of complete assembly was a critical component of the implosion bomb. This involved sophisticated radiochemistry, especially with polonium.

• Chemistry and Metallurgy: Extreme Purity: Maintaining extremely low levels of light-element impurities (down to a few parts per million) was essential, especially for plutonium, due to (α,n) reactions contributing to neutron background. This required developing supersensitive analytical techniques. Material Processing: Developing methods for reducing, casting, and shaping uranium and plutonium metals and their compounds (like uranium hydride) was complex due to their unique properties. Refractories: Identifying and producing crucible and liner materials that would not contaminate the highly purified active materials at high temperatures was a significant challenge. Handling Hazardous Materials: The severe toxicity and radioactivity of plutonium and polonium necessitated the development of enclosed apparatus, remote control systems, and elaborate health monitoring programs to protect personnel [3.94, 156, 327, 393, 498].

• Arming and Fusing: Designing reliable triggering mechanisms was crucial given the immense value of a single bomb. This involved developing devices for high-altitude detonation (e.g., using radio altimeters) and ensuring extremely low failure rates.

• Bomb Delivery: Integrating the bomb's components into a practical airborne weapon required significant design and testing, including ballistic performance, aircraft modification (B-29), and ensuring safe in-flight assembly.

III. Overarching Operational and Administrative Challenges:

Beyond the specific scientific and engineering problems, Project Y contended with unique organizational and logistical difficulties:

• Secrecy vs. Collaboration: Maintaining absolute secrecy while fostering intense internal collaboration among diverse scientific disciplines was a delicate balance. This led to unique security policies, such as geographically enforced isolation of personnel.

• Site Isolation: The remote Los Alamos location exacerbated problems with personnel recruitment and retention, procurement of materials, and establishing reliable communication channels.

• Rapid Growth and Staffing: The laboratory experienced exponential growth, doubling its population approximately every nine months. Recruiting and training a large, specialized workforce (scientists, technicians, engineers, explosives experts, military personnel) in wartime conditions was a continuous struggle.

• Coordination and Administration: Bridging the gap between military authority and scientific autonomy, and coordinating diverse research and engineering groups (often with differing priorities) was a constant challenge for the Director and various boards and committees.

• Time Pressure and Limited Testing: The urgent wartime schedule meant that information had to be obtained quickly, often through theoretical anticipation, as extensive experimental verification or multiple full-scale tests were impossible. This inherent "all-or-none character" of the weapon demanded high reliability based on theoretical predictions and component testing.

• Lack of Established Industrial Base: Many of the required processes, such as precision high-explosives casting and machining, were entirely new and had no commercial or military precedent. This required the project to develop its own production methods and train personnel from scratch.

• Health and Safety: Managing the severe and largely unknown hazards of working with radioactive materials like plutonium and intense radiation sources was a constant concern, leading to the development of new monitoring and protective measures.

Despite these formidable challenges, Project Y successfully developed and tested the first atomic bomb at Trinity.

The Trinity Test: A Brief Overview

The Trinity test, conducted on July 16, 1945, at Los Alamos, New Mexico, was the world's first man-made nuclear explosion. This event was the culmination of extensive scientific and engineering efforts under the "Project Y" code name for Los Alamos.

• Objective: To test the first atomic bomb, releasing explosively the "enormous energy confined within the nuclei of atoms."

• Site and Setup: The base camp and test site plans were approved in October 1944. The weapon, referred to as the "Jumbo," was set up on a 50-foot tower. Photographs show the "Main instrumentation and firing bunker," "Typical blast wave gauge," "Jumbo being delivered," "Jumbo on trailer," and a "Special tank with lead lining and air bottles mounted on side for air supply for crew. Trap door underneath permitted earth samples to be scooped up while tank was in crater."

• Personnel and Organization: The project involved a vast network of personnel and facilities, including Oak Ridge, Tennessee; the University of Chicago; and Hanford, Washington, which were instrumental in producing the necessary fissile materials. Key individuals and their roles included:

• K. T. Bainbridge: Responsible for checking the arming routine.

• J. L. McKibben: In charge of timing and control circuits.

• C. B. Kistiakowsky: Checked arming operations and aided in focusing cameras/spectrographs.

• Lt. H. C. Bush: Commanding Officer of the Trinity Base Camp, credited for "wise and efficient running" contributing "greatly to the success."

• H. L. Anderson: Responsible for the collection of fission products and plutonium on filters from planes at high altitude.

• M. G. Holloway: Responsible for meteorological problems, particularly the dilution of active gases.

• R. R. Wilson: Led all nuclear physics measurements.

• P. B. Moon, Sgt. W. J. Breiter, Ens. I. Halpern, T/4 J. A. Hofmann, J. Hughes, T/5 M. J. Pincus: Involved in delayed gamma rays, electrical parts, ionization chambers, and shelter design.

• Distinguished Visitors: Included J. R. Oppenheimer, V. Bush, J. B. Conant, Brig. Gen. T. F. Farrell, Maj. Gen. L. R. Groves, I. I. Rabi, Sir Geoffrey I. Taylor, and Sir James Chadwick, present from July 10-16.

• Measurements and Observations: A wide array of instruments were deployed to capture data, including:

• Blast and Shock: Piezoelectric gauges, paper diaphragm gauges, condenser blast gauges, Barnes’ boxes, condenser gauge blast measurements from planes, geophones, and seismographs. The project noted "excellent meteorological service" and successful forecasts leading up to the test.

• Radiation: Gold foil and fast-ion chambers for neutron measurements; recording in planes, dropped "gauges," and gamma-ray sentinels for gamma rays. Equipment was installed for measuring delayed neutrons (Williams) and delayed gamma rays (Moon).

• Efficiency: Nuclear efficiency measurements were taken.

• Photographic Studies: Fastaxes at 800 yd, spectrographic studies, photometric measurements, and "Ball of fire studies" using various cameras, including color cameras and SCR-584 radars.

• Immediate Hazards and Effects:Blast: Calculations for a 100,000-ton equivalent explosion suggested that "bodily injuries will not occur" at 10,000 m, with ear injury possible from 1 to 5 p.s.i. "This was one of the most successful blast measuring methods (a)."

• Radiation: At 10,000 m, the "peak neutron flux would be less than 1 n/cm2, which is far below tolerance." Gamma radiation was estimated at "10^–4 at 10000 m." Observers within 10 mi were to be "specially protected by smoked glasses."

• Post-detonation Environment: Twenty-four hours after the test, the site revealed a "radioactive crater of green, glassy, fused desert sand," with a smaller crater from an earlier 100-ton high explosives test. The report details the conversion of 49 (Plutonium-239) to fission products, noting only 2% was found in the soil within a 300-ft radius, indicating "simple scaling laws do not properly allow for the increase in updraft with increased charge."

Connecting the Theory to the Test:

The Trinity test served as the practical validation of the complex theoretical calculations outlined in the Primer. The extensive instrumentation used at Trinity directly measured the physical phenomena predicted by Serber and other physicists, such as blast pressure, radiation levels, and the dynamics of the fireball and cloud formation. The data collected was essential for refining models and understanding the full destructive potential of nuclear weapons.

A detailed timeline of challenges and solutions

The scientists at Los Alamos faced and overcame numerous challenges throughout the Manhattan Project, ranging from fundamental scientific unknowns to complex engineering feats, logistical hurdles, and administrative complexities. These challenges evolved as the project progressed, often dictated by new discoveries and the escalating urgency of wartime development.

Here is a detailed timeline of these challenges and their resolutions:

Early Period: Inception and Initial Planning (Mid-1942 to Early 1943)

• Scientific Problem: The most urgent requirement of the Development of Substitute Materials (DSM) project was the large-scale production of nuclear explosives, not detailed theoretical or experimental work on the bomb mechanism. Work on fast neutron chain reactions was fragmented. Solution: J. Robert Oppenheimer was appointed Director of the work in June 1942, starting with a small group of theoretical physicists at the University of California, Berkeley. Solution: A conference in Berkeley in June 1942 thoroughly reviewed theoretical and experimental work, clarifying basic ideas and defining problems. It became clear that fission bomb development required a major scientific and technical effort.

• Organizational Problem: By October 1942, it was evident that the magnitude of difficulties necessitated the formation of a new, unified project, as initial work was hampered by a lack of organization in one locality. Solution: Project Y's site was selected in November 1942 at the Los Alamos Ranch School, chosen for its large proving ground, suitable climate for outdoor work, remoteness from coasts, and inaccessibility for security.

• Security vs. Collaboration: Normal military procedure for secrecy (subdivision of information) conflicted with the scientific staff's requirement for free communication within the laboratory. Solution: A policy was adopted to allow internal freedom but impose severe external restrictions, which reinforced the choice of an isolated location for the project.

• Fundamental Scientific Unknowns (as of April 1943): Challenge: No absolute experimental confirmation of the bomb's feasibility regarding basic nuclear processes. The neutron number had not been measured for fissions induced by fast neutrons, only for "slow" fission. The time between fissions in a fast chain might be longer than assumed. Plutonium fissioning had been studied, but no proof that its neutron number was the same as U235. Solution (July 1943): The first physical experiment completed at Los Alamos observed neutrons from Pu239 fissioning, finding it somewhat greater than U235, justifying the decision to construct the plutonium production pile at Hanford. Solution: Measurements showed delayed neutron emission was negligible.

• Theoretical Problem: Earlier calculations for critical mass contained rough assumptions, such as isotropic scattering, same mean free path for core and tamper, single neutron velocity, and no energy loss through inelastic collisions. Solution: The theoretical program focused on refining these assumptions, requiring more precise experimental knowledge. Fission cross sections for Pu239 and U235 were plotted as a function of energy. Solution: This work helped lead to the abandonment of the uranium hydride program because it showed the energy-dependence needed for it to be an efficient weapon did not occur.

First Period of Operation: Foundation and Initial Development (April 1943 – August 1944)

• Logistical Challenges Upon Arrival (March-May 1943): Challenge: Construction was incomplete when staff arrived (March 15, 1943); laboratory buildings and housing were still in the hands of contractors. Solution: The first project office opened in Santa Fe, and staff were temporarily housed in nearby guest ranches. Challenge: Living conditions in guest ranches were difficult (crowding, inadequate facilities), transportation was haphazard, roads were poor, and eating facilities at the site were not yet operational. Solution: Project offices and most staff moved to Los Alamos by mid-April as laboratory space and housing became available. Challenge: Frictions developed with the U.S. Engineer staff (Army Corps of Engineers, Albuquerque District Office), due to slow construction (labor, union issues, slow procurement of basic equipment); technical supervisors were forbidden to enter buildings until formal acceptance. Solution: Oppenheimer assumed a greater administrative burden, and a temporary organization of scientific staff and technicians took over project operations to prevent delays in research work.

• Personnel and Administration: Challenge: Difficulty obtaining adequate scientific and technical staff, as most skilled personnel were already mobilized for other war work. Solution: Existing groups working under Oppenheimer transferred to Los Alamos, and assistance was obtained from the National Research Council (NDRC) chairman, J.B. Conant, to release other individuals and groups. Later, Dean Samuel T. Arnold of Brown University and M.H. Trytten of the National Roster of Scientific and Technical Personnel assisted in recruiting. Challenge: Salary inconsistencies (e.g., academics paid less than industrial hires or technicians) and lack of a system for merit increases created low morale. Solution: A salary policy was developed by Hughes in June 1943 and approved on February 2, 1944, establishing a scale and increase plan, though some restrictions remained. Challenge: Draft deferment for young, draft-vulnerable scientific and technical employees was complex due to the project's secrecy. Solution: The Laboratory secured cooperation from New Mexico Selective Service and Manhattan District agencies. When War Department policy in February 1944 forbade deferment of men under 22, essential personnel were reassigned to the Special Engineer Detachment (SED). Challenge: Administrative responsibility for scientific research was typically unorganized, falling to the scientists themselves, which led to some inefficiency. Solution: While administrative efficiency was sometimes lost, unity in the Laboratory was gained by keeping the center of gravity in the scientific staff. Challenge: The extreme isolation policy created liaison difficulties with other Manhattan District branches and external agencies. Solution: Procedures for information exchange were established in June 1943 with the Metallurgical Laboratory, and Dr. Tolman's office was instrumental in obtaining external reports (e.g., gun design, explosives).

• Procurement: Challenge: Supplying a large, new research laboratory secretly in wartime, far from markets and transportation hubs, was a monumental task. Solution: The Procurement Office, led by D.P. Mitchell, prioritized speed and was authorized to place urgent orders, often obtaining high War Production Board (WPB) priorities. Challenge: Equipping labs from scratch with a vast variety of materials, many of which were scarce due to war production. Solution: Project Y was assigned AA-1 priority by the WPB, and the Procurement and Purchasing Offices ensured equipment was on hand quickly. Stockrooms for chemical, general lab, and electronic supplies, as well as shops, were established. Challenge: Security regulations prevented direct contact between purchasing offices (in Los Angeles, New York, Chicago) and the Los Alamos using groups, leading to misunderstandings about critical specifications. Solution: The local Procurement Office made a serious effort to have using groups prepare accurate and complete specifications without revealing the nature of their work, checking them closely before transmission.

• Health and Safety: Challenge: The sudden appearance of plutonium in early 1944 introduced a serious health hazard due to its toxicity and radioactivity. Solution: Dr. Hempelmann's Health Group investigated radium handling, and three committees were established in the Chemistry and Metallurgy Division to develop plutonium hazard control methods (instrumentation, apparatus design, safety rules). Solution: Enclosed apparatus was developed wherever possible to prevent contamination. Procedures were enforced by a section of the Service Group, providing protective equipment, laundering, monitoring, and decontamination. Challenge: Lack of adequate monitoring equipment for plutonium; instruments from Chicago did not meet local specifications. Solution: Development of monitoring equipment began in the Electronics Group of the Physics Division in May 1944. Solution: An extensive educational campaign on plutonium toxicology was carried out for working groups.

Second Period: Implosion Focus and Final Development (August 1944 – August 1945)

• Shift in Focus to Implosion: Challenge: The discovery of Pu240 in July 1944 (from spontaneous fission measurements) meant Hanford plutonium would have a neutron background too high for gun assembly. The implosion became an "absolute necessity". Solution: The Laboratory underwent a complete reorganization in August 1944, creating two new divisions, G (Weapon Physics) and X (Explosives), to intensely study implosion dynamics. The U235 gun program continued separately. Challenge: The gun method's high velocity (3000 ft/sec) needed for plutonium required light, expendable, short guns operating at high peak pressure. Solution: By August 1944, the high velocity gun had been thoroughly proved and techniques well developed, allowing the U235 gun to proceed without new basic difficulties.

• Implosion Technical Challenges: Challenge: The hydrodynamics of implosion were difficult to calculate, with initial hand-calculations yielding uninterpretable results. Solution: The problem was set up for IBM machine calculation (machines arrived April 1944), which proved "extremely satisfactory". Challenge: Achieving a spherically converging shock wave from multipoint detonations was impossible, leading to "jets" and irregularities. Solution: Theoretical attention focused on the interference of detonation waves. Calculations determined tolerable asymmetry (5% velocity variation) and were undertaken to design explosive "lenses" to convert divergent waves into convergent spherical ones. Challenge: The Taylor instability (light material pushing heavy material, or vice-versa, creating an unstable interface) posed a serious problem for implosion symmetry. Solution: Extensive calculations on the development of asymmetry allowed for preliminary statements on tolerable asymmetry.

• Chemistry and Metallurgy for Implosion: Challenge: Plutonium's unique physical properties, including five allotropic forms and brittleness, made fabrication difficult; it was also highly corrosive. Solution: Extensive studies were conducted on plutonium's physical properties, and research began on alloys to stabilize a high-temperature phase at room temperature. Fabrication, surface cleaning, and protection methods were developed. Challenge: Producing high-quality large castings for high explosives was unprecedented, as military techniques were crude. Solution: The machining of explosives (e.g., using risers and overcasting techniques) was developed at S Site, becoming a "revolutionary development" in manufacturing, with virtually no hazards. Designs for lens molds were "frozen" late 1944, which, despite deficiencies, allowed schedules to be met. Challenge: Handling and processing highly radioactive materials like radio-lanthanum (RaLa) for implosion studies. Solution: A "mechanical chemist" (remote control plant) was designed and built at Bayo Canyon for handling RaLa. A new hydroxide-oxalate process for RaLa separation was developed, allowing for greater radiation protection.

• Testing and Integration (Project Trinity and Alberta): Challenge: Planning a full-scale nuclear explosion (Trinity) was extremely difficult, as integral experiments could not fully duplicate bomb conditions. Solution: Project TR (Trinity) was established in March 1945 under K.T. Bainbridge, integrating personnel from various divisions for test preparation. Challenge: Scheduling for Trinity was stringent (e.g., full-scale lens molds by April 2, plutonium sphere fabrication by July 4) and faced procurement delays. Solution: The "Cowpuncher Committee" was formed March 1, 1945, to "ride herd" on the implosion program, providing executive direction and publishing detailed progress reports. Challenge: The Fat Man firing unit ("X-units") experienced production delays from external manufacturers. Solution: While delays prevented extensive testing, 155 test units were dropped at Wendover or Inyokern between October 1944 and August 1945, providing crucial data for bomb design improvements. Challenge: "Safing" the Fat Man (leaving assembly incomplete during takeoff) was not possible like the Little Boy, posing a risk of widespread contamination or a high-order nuclear explosion in case of a crash. Solution: While special precautions were requested, the Air Force commander decided they were unnecessary, and the takeoff was made without incident. Challenge: Information exchange between the Tinian island base and Los Alamos was "extremely unsatisfactory" due to tight security rules and indirect communication channels. Solution: Teletype messages were relayed through the Washington Liaison Office using elaborate codes, and personnel like J.H. Manley were sent to Washington to ensure information flow. Trinity: The Trinity test on July 16, 1945, was a resounding success, with a yield exceeding expectations (15,000 to 40,000 tons of TNT equivalent), confirming the "major technological victory" achieved by the Laboratory.

U.S. Army’s significant roles at Los Alamos

The Army Corps of Engineers, specifically through the Manhattan District, played critical and extensive roles in the establishment, administration, security, and logistical support of the Los Alamos Project (Project Y).

Their key responsibilities and involvement included:

• Overall Executive Responsibility and Project Control: Major General Leslie R. Groves of the Corps of Engineers was given over-all executive responsibility for the Manhattan Project, under the direction of a Military Policy Committee. The Los Alamos Project, originally under the Office of Scientific Research and Development (OSRD), was transferred to the Manhattan District in 1942 to ensure the highest degree of secrecy and priority.

• Site Selection and Establishment: The Los Alamos site was selected in November 1942 by the military authorities. The reasons for its selection, such as the need for a large proving ground, a climate suitable for outdoor winter work, and remoteness from potential attack, were heavily influenced by military security policy. The site, including a large surrounding area, was established as a military reservation, and the community became an army post, with the laboratory located within a "Technical Area".

• Administrative and Personnel Management: The Commanding Officer of Los Alamos reported directly to General Groves and was responsible for military personnel, maintaining suitable living conditions for civilians, preventing trespass, and overseeing security guards. The military administration was also involved in the assignment of enlisted men and women, particularly through the Special Engineer Detachment (SED), who filled crucial scientific and technical roles. They also handled draft deferments for Laboratory employees.

• Contractual and Financial Oversight: The University of California operated Project Y as the prime contractor under a formal contract with the Manhattan Engineer District of the War Department. The Contracting Officer, a representative of the War Department, exercised supervision over financial matters, including salary policy and procurement.

• Construction and Infrastructure: The U.S. Engineers (Albuquerque District Office) were initially in charge of constructing the original laboratory buildings and housing. Later, General Groves decided that all construction would be handled by the Army Engineers, establishing separate organizations for the Technical Area and the general post, housing, and administrative areas. They were responsible for alterations, additions, repairs to buildings, and installation of utilities.

• Security Measures: The Army Corps of Engineers was instrumental in enforcing stringent security measures, including restricting travel and social contact outside the project area, and implementing mail censorship to prevent the inadvertent spread of information. The Military Intelligence Officer advised the Scientific Director on security matters.

• Procurement and Logistics: While the University of California handled financial and procurement operations as the prime contractor, the Army contributed directly by supplying certain materials (e.g., electronic components, guns, explosives) from Army/Navy stores. The Post Supply Section, under Major Edward A. White, also provided various items for the Technical Area.

• Involvement in the Trinity Test and Combat Delivery (Project Alberta): The Army played a critical role in the Trinity test, with a Military Police detachment taking up residence at Base Camp. General Groves was present at the conference that froze the implosion bomb design, and Brigadier General T. F. Farrell, Groves' deputy, signed for the active material at Trinity. The 509th Composite Group, an Army Air Forces unit, was specifically formed and trained at Wendover Army Air Base, Utah, for the combat delivery of the bombs Project Alberta, the overseas mission for combat delivery, was led by Captain Parsons, USN, with military and semi-military organizations like the 20th Air Force and Bureau of Ordnance involved.

Despite initial frictions between the Laboratory's scientific staff and the military organization, often due to differing backgrounds and perspectives, the common purpose of the project ultimately led to effective cooperation.

The Making of the Atomic Bomb was a compelling read.

I have read World War II histories as a kid, sometimes hearing a few stories too from my grandfather who fought the Japanese from 1937-1945.

While toiling away at my ultrahigh vacuum laboratory set up to shoot various beams of molecules at a single crystalline surface of iridium, I found time to read The Making of the Atomic Bomb by Richard Rhodes forty years ago. I found the book to be such a compelling narrative that I took a couple of weekends to write a book review for The California Tech, the student-run newspaper of the California Institute of Technology (Caltech).

At the Massachusetts Institute of Technology, I enjoyed covering and writing sports stories for The Tech, the student-run newspaper, especially in covering the MIT men’s track and field team in both indoor and outdoor meets. I must have missed that regular rhythm of writing sports stories, which I later stepped up my game to write opinion columns, that I felt I needed to write a book review. Indeed, in my graduate school career, I wrote a second book review about a new book on computation (long forgotten, both the book and my review).

I recently found a pdf copy of the 1 May 1987 newspaper issue of The California Tech from the Caltech archives with the book review I wrote about The Making of the Atomic Bomb. I reproduced the pages below.

Arthur Lee

Chevron Fellow Emeritus