Fifteen Manhattan Project Myths and Misconceptions
By B. Cameron Reed, Department of Physics (Emeritus), Alma College
Over many years of studying the Manhattan Project, I have come across a number of distorted or outright erroneous statements regarding either the physics involved in it or of how various aspects of it unfolded. These statements usually turn up in semi-popular treatments of the Project, where the authors are not physicists and likely just reiterated something claimed in another source without bothering to check an authoritative record. In some cases one has to wonder if claims are made to bring notoriety to the claimant or for advancing some agenda. Like most myths and misconceptions, I have no idea where most of them started, nor can I cite a specific source for some of them beyond saying that I know I have heard them over the years. Some do contain germs of truth, but have grown into perversions of the facts.
These misconceptions will likely never be stamped out. My motivation in preparing this article is the hope that a physicist who does hear them can try to set the record straight. What follows are listed in roughly chronological order. Details on many of these issues can be found in my book The History and Science of the Manhattan Project; several other supporting references are also cited . When I cite a source, it is by no means to imply that this is where something started, but rather where I happened to come across it recently.
I would be pleased to hear from readers who have information on how any of these misconceptions originated or have others they would like to share.
(1) Albert Einstein was “The Father of the Bomb”
The idea of the work of an avowed pacifist being involved with the most destructive weapon in history has a powerful appeal, but is largely incorrect. Einstein’s most significant involvement in the Project was that he signed a letter to President Roosevelt warning him of the potential dangers of nuclear energy. The letter, however, was written by Leo Szilard and Eugene Wigner, who recruited Einstein on the basis that his name would be recognized by Roosevelt. Einstein actually dispatched four letters to FDR between 1939 and 1945; copies and transcriptions can be found at various sites . I have read that Einstein was consulted on problems involving diffusion at Oak Ridge, but this does not seem to have amounted to much. Nuclear physics was never Einstein’s research area, and the idea of a chain reaction apparently came as a revelation to him when he was approached by Szilard and Wigner. Einstein would surely have been regarded by Manhattan Commander General Leslie Groves as a dangerous security threat.
Along the same line, E = mc2 had little direct connection to the Manhattan Project. To be sure, the energy released in fission arises from a minute loss in mass during that process, but this famous equation was never used to predict fission, was not directly involved in its interpretation (which came through the liquid-drop model), played no role in the experiments which led to the understanding of the roles of different isotopes in the fission process, was not involved in uranium enrichment or plutonium synthesis, and had nothing to do with establishing the values of critical masses or in designing the bombs themselves. The use of fission as a weapon can be treated as a largely empirical affair. Quantum physics played no role at all.
Left: In this 1946 photo, Einstein and Szilard re-enact the preparation of the letter to President Roosevelt. Source: Courtesy Atomic Heritage Foundation
Right: President Roosevelt signs the declaration of war against Japan, December 8, 1941 Source: Wiki Franklin Roosevelt
(2) The energy of fission can make a grain of sand visibly jump*
I analyzed the physics of this claim in a paper published in the December 2018 edition of The Physics Teacher . The idea of a grain of sand being propelled off your desk by the fission of a single uranium nucleus is an appealing image, but the physics doesn’t hold up. This myth was recently repeated in Gerard DeGroot’s book The Bomb: A Life, where he mistakenly attributes it to having been calculated by Lise Meitner and Otto Frisch when they were preparing their paper on the physics of fission .
Some numbers: The most common form of sand is silicon dioxide, usually in the form of quartz, which has a density of 2.65 g cm-3. A grain of diameter 1 mm will have a mass of about 1.4 milligrams. The energy released in fission averages about 170 million electron-volts (MeV) per event, or about 2.7 x 10-11 J. If all of this energy is directed into projecting the grain upwards, then the usual mgh formula for potential energy shows that we can expect to reach a maximum height of about 0.002 millimeters, or a mere 1/250 of the radius of the grain itself. Unless you have super-power eyes, you are unlikely to be able to discern this. The visual acuity of the eye is about one minute of arc. If we optimistically assume half a minute, we can use a standard arc-length calculation to estimate the distance from which we would have to view the jump in order to resolve it. For a jump of 0.002 mm we get about 14 mm, or a little over a half-inch. The near point of the eye (the closest distance at which one can still focus) is about 25 cm, so we are out of luck. If you want a more accurate figure, you are safe in saying that the complete fission of a single kilogram of uranium-235 releases as much energy as does exploding some 17 million kg of dynamite.
(3) American scientists and government officials were relatively inactive on atomic matters before Pearl Harbor*
The period before Pearl harbor could be considered a sort of “quiet phase” of the Manhattan Project. The Szilard-Wigner-Einstein letter reached President Roosevelt in October, 1939, and resulted in the establishment of the “Advisory Committee on Uranium” headed by the Director of the National Bureau of Standards, Lyman Briggs. One of the group’s first actions was to contribute
$300,000 had been let for contracts for fission and isotope separation research to various universities, industries, government agencies, and private research institutions; details can be found in Chapter 4 of Ref. 1. Of particular importance during this time were three reports prepared by a committee under the chairmanship of Arthur Compton, which examined the possibilities for reactors and bombs. The second of these (June, 1941) related that plutonium had been synthesized and tested for fissility by Glenn Seaborg, and the third, dated November 6, examined the physics of a putative fission bomb in considerable detail . This latter report was heavily (although unofficially) influenced by a British report. Bush had been briefing FDR periodically since inheriting the project, and during an October 9 meeting, the President, clearly recognizing the possible implications, ordered that any considerations of atomic policy were to be restricted to a group comprising himself, the Vice President, the Secretary of War, the Chief of Staff of the Army, and Bush and Conant: the “Top Policy Group”. At the same meeting, the possibility of having the Army take over the project was also discussed. A theme in Bush’s reports was that while he had no information on what might be happening in the way of nuclear research in Germany, this was certainly a concern. Bush briefed FDR on the third Compton report on November 27, about the time the Japanese fleet was setting sail to Pearl Harbor. The President authorized further research and development, and more detailed plans were developed at a meeting between Bush and his scientific advisors in Washington on December 6, the day before the attack. The rest, as is said, is history.
Some of the key figures of wartime research, April, 1940: Left to right: Ernest Lawrence, Arthur Compton, Vannevar Bush, James Conant, Karl Compton, Alfred Loomis. Source: Wiki Image
(4) The headquarters of the Manhattan Project were in Manhattan*
This one is partially true, although many people know little of how this came to be. When control of the uranium project was handed over to the Army in June, 1942, the first officer to whom it was assigned, Colonel James C. Marshall of the Corps of Engineers, set up his headquarters in an office building located at 270 Broadway in New York City, the location of the Corps’ North Atlantic Division. One of the contractors for the project, Stone and Webster Engineering, had offices in the same building, and it was also convenient to Columbia University, where research on uranium enrichment was underway.
Marshall’s assignment was unusual. The Army divided the country into eleven geographical divisions, each under the authority of a Division Engineer. Within these divisions, smaller areas within which individual projects were sited (camps, airfields, ordnance plants, depots, ports, etc.) fell under the authority of “District Engineers”. Marshall’s new “Manhattan Engineer District” had no geographical restrictions; in effect, he had all of the authority of a Division Engineer.
In September, 1942, Marshall was replaced by then-Colonel Leslie R. Groves, although he did remain as District Engineer until July, 1943, when Groves eased him out in favor of Marshall’s own deputy, Colonel Kenneth D. Nichols; at the same time, the District headquarters were shifted to Oak Ridge. Groves’ appointment came with promotion to Brigadier General. His personal headquarters was a small suite of offices on the fifth floor of the New War Building at the intersection of Twenty-First street and Virginia Avenue NW in Washington. This building is now part of the Department of State.
Groves graduated from West Point in November 1918, and also trained at the Army Engineer School, the Command and General Staff School, and the Army War College. His career in the Corps of Engineers was marked by steady advancement, and by 1942 he was responsible for overseeing all Army construction within the United States as well as at off-shore bases. His intimate knowledge of how the War Department and Washington bureaucracies functioned and of contractors who could be depended upon to undertake the design, construction, and operation of large plants and housing projects made him the perfect candidate to oversee Manhattan. In the spring of 1942, one of the projects on his plate was the construction of the Pentagon, which was completed within sixteen months of ground being broken.
While the terms Manhattan Project and Manhattan Engineer District are often used interchangeably, the legal Army term was the latter; “Manhattan Project” only came into general use after the war.
(5) A “suicide squad” manned Enrico Fermi’s CP-1 reactor in case it ran out of control*
A group of three young physicists were perched atop CP-1 at its startup, armed with jugs of neutron-absorbing cadmium-sulfate solution to dump into the pile in case it threatened to go into an uncontrollable divergent reaction. Richard Rhodes characterizes the group a suicide squad . A Department of Energy publication on CP-1 lists the group as Harold Lichtenberger, Warren Nyer, and Alvin Graves . In a reminiscence published in 1982, Albert Wattenberg attributed the idea to Samuel Allison, and recalled that several people were upset with it: If an accidental breakage had occurred, the pile would have been ruined . But the term “suicide squad” is literary license; “Chicago pile hit squad” might be more apt. Fermi had designed the pile to be barely critical, and provided for redundant over-control. Cadmium-sheathed wooden rods were used as control rods; inserting any one of them into the pile would bring it to below criticality, but several were used (the exact number now seems unknown; Fermi himself later described it as several). In addition, two safety rods (known as “zip” rods) and one automatic control rod were also incorporated into the design. During operation, all but one of the rods would be withdrawn from the pile. If neutron detectors signaled too great a level of activity, the vertically-arranged zip rods would be automatically released, accelerated by 100-pound weights. The automatic control rod could be operated manually, but was also normally under the control of a circuit which would drive it into the pile if the level of reactivity increased above a desired level, but withdraw it if the intensity fell below the desired level. During the historic startup, one of the safety rods was tied off to a balcony railing, with Norman Hilberry standing by with an axe in order to cut the rope in case the automatic system failed. According to some sources, the phrase “to scram” a reactor – execute an emergency shutdown – is an acronym for “safety control rod axe man.”
(6) The Clinton Engineer Works at Oak Ridge was using 1/7 of the electricity being generated in the United States*
This claim seems to have originated in the autobiography of Colonel Nichols . As described in an article published in the Spring 2015 FHP newsletter, this one has an element of numerical truth .
Data on electrical facilities at Oak Ridge can be found in Vincent Jones meticulously researched book on the Army’s role in the Manhattan Project . By mid-1945, transmission facilities there could provide electrical power up to 310 megawatts (MW), of which 200 MW were for the Ernest Lawrence’s electromagnetic separation plant. In August, 1945, electricity used by all facilities totaled about 200 million kilowatt-hours (MkWh). Statistics on national generating capacity can be obtained from back issues of the Statistical Abstracts of the United States . Figures published in the 1949 edition indicate that national generating capacity remained fairly steady between 1943 and 1945 at an average of about 272.8 billion kWh per year. One month’s worth would be about 22,700 MkWh, of which the Oak Ridge fraction cited above would have been about 0.9%.
Oak Ridge got its power from the Tennessee Valley Authority (TVA), and Nichols’ figure corresponds to roughly 1/7 of that agency’s generating capacity. Details are given in the newsletter article cited above, but the short version is that by mid-1945, TVA capacity stood at about 2,500 MW. The full CEW capacity of 310 MW would represent just over 12% of the latter figure, or about one-eighth.
(7) Robert Oppenheimer viewed the Trinity test from* Campañia Hill
This assertion appears in David Schwartz’s otherwise excellent biography of Enrico Fermi, and is included here as an example of how errors can slip into reputable sources . Campañia Hill was located some 20 miles to the northwest of the test site, and served as a viewing location for personnel who were not needed at the control station during the countdown. The Campañia group included Hans Bethe, James Chadwick, Richard Feynman, Ernest Lawrence, Edward Teller, and Robert Serber. Oppenheimer viewed the test from the control station 10,000 yards to the South of ground zero; Groves was at Base Camp at about 17,000 yards.
(8) The light from the Trinity test could have been seen reflected from the Moon
This appears in Richard Rhodes’ The Making of the Atomic Bomb; I analyzed it in some detail in a paper published in 2006, but a refined estimate is given here [14,15]. The issue boils down to what change in astronomical magnitude would have been involved. This would involve the phase of the Moon at the time, so numbers here will necessarily be back-of-the envelope. The radiant energy at 10,000 yards from the explosion was estimated at about 12,000 Joules per square meter. If this was emitted over a time of one microsecond (probably too short, which will make the numbers overly optimistic), this would correspond to a power of ~ 7 Watts per square meter at the Moon. The solar flux at the Moon will be essentially the same as that as at the Earth, ~ 1400 Watts per square meter. A change of one part per 200 corresponds to about 0.005 magnitudes. A variation of this size is certainly detectable now with a large telescope and modern detectors, but would have been very iffy in 1945. At best, this assertion is a stretch. At the Trinity site, the Moon had set about 4.5 hours before the test. Had an observer on the Moon been looking toward New Mexico at the time, however, the explosion would have momentarily appeared thirty times brighter than Venus.
(9) The Enola Gay and Bockscar B-29 bombers had fighter escorts
I have heard occasionally of stories of fighter pilots describing with great emotion how they participated in the bombing missions at Hiroshima (Enola Gay) and Nagasaki (Bockscar) by valiantly protecting the bombers against possible Japanese action; that there were escorts is claimed in James Kunetka’s 2015 book on Oppenheimer and Groves.[i] Copies of the mission orders can be found on a Time magazine website, and indicate no fighter escorts; a copy of the Nagasaki order also appears in John Coster-Mullen’s fantastic book on the bombs, as does a field order indicating that no friendly aircraft were to be within a 50 mile area of any of the targets from four hours prior to six hours after planned strikes times [17,18]. Fighters would have been too fragile to withstand the shock waves from the bombs, and General Groves wanted nothing that would draw extra attention to the bombers; the Japanese were used to seeing lone or small formations of bombers on reconnaissance and weather missions. Weather-report bombers had preceded the strike bombers to each possible target, and the bombers were accompanied by observation bombers which dropped instrument packages.
(10) The Little Boy and Fat Man bombs were dropped by parachute
The bombs each weighed about five tons. Parachute drops would not only have been impractical and complicated the designs of the bombs, but would have been undesirable, giving Japanese anti-aircraft crews time to take aim at them. However, instrument packages were dropped by parachute, so it is entirely plausible to imagine that survivors recalled seeing parachutes.
(11) The crews of the Enola Gay and Bockscar all suffered from cancers, radiation poisoning, sterility, and mental problems after the war
A total of 24 men flew on the Enola Gay and Bockscar; one of these, radar officer Jacob Beser, flew both missions. A few years ago I undertook an extensive online search for obituary data, and was able to find information for all but four. Ages at death ranged from 46 (acute leukemia in 1967) to nearly 94; three survived to over 90. The average age at death was 76. Causes of death did include five cancers (including the leukemia), but this is certainly not out of line with the fact that about 20% of the population overall succumbs to cancers. Other causes included the menu of woes one would expect for an aging population: pneumonia, emphysema, heart failure, heart attacks, strokes, cardiac arrest following prostate surgery, and an automobile accident. Between them, these men fathered at least 50 children (surely an incomplete count), including 10 by Bockscar pilot Charles Sweeney and 5 by Enola Gay Co-Pilot Robert Lewis. Not unlike any group that served in any war, some wrote memoires of their experiences while others said little to family and friends.
Left: Partial crew of the Enola Gay: Standing (l-r): John Porter (ground maintenance officer), Theodore Van Kirk, Thomas Ferebee, Paul Tibbets, Robert Lewis, Jacob Beser; kneeling (l-r): Joseph Stiborik, George Robert Caron, Richard Nelson, Robert Shumard, Wyatt Duzenbury. Right: Partial Bockscar crew. Standing (l-r): Kermit Beahan, James Van Pelt, Don Albury, Fred Olivi, Charles Sweeney; kneeling (l-r): Edward Buckley, John Kuharek, Ray Gallagher, Albert Dehart, Abe Spitzer. Photos courtesy John Coster-Mullen.
(12) Only a few bombs were available; the Japanese could have kept fighting and not suffered further nuclear attacks
This one is just plain wrong. To be able to enrich enough uranium or synthesize enough plutonium to make a bomb in a short enough time to affect the war – two years, say – required building facilities which, once operating, turned out material continuously. To get a sense of this, we can do no better than to quote from memos from General Groves to Army Chief of Staff General George C. Marshall. On August 10, 1945, the day after the bombing of Nagasaki, Groves informed Marshall that 
The next bomb of the implosion type had been scheduled to be ready for delivery on the target on the first good weather after 24 August 1945. We have gained 4 days in manufacture and expect to ship from New Mexico on 12 or 13 August the final components. Providing there are no unforeseen difficulties in manufacture, in transportation to the theatre or after arrival in the theatre, the bomb should be ready for delivery on the first suitable weather after 17 or 18 August.
A couple weeks earlier, on July 30, Groves had outlined anticipated bomb production figures:
In September, we should have three or four bombs. One of these will be made from 235 material and will have a smaller effectiveness, about two-thirds that of the test type, but by November, we should be able to bring this up to full power. There should be either four or three bombs in October, one of the lesser size. In November there should be at least five bombs and the rate will rise to seven in December and increase decidedly in early 1946. By some time in November, we should have the effectiveness of the 235 implosion type bomb equal to that of the tested plutonium implosion type.
Thus, some 18-20 bombs were expected to be available from September through the end of the year, or about one every six to seven days. This rapid pace testifies to the fact that all fissile-material production plants were reaching full capacity at that time.
(13) Most of the victims of the bombings died by radiation poisoning
Various estimates of the fraction of victims who died of radiation exposure have been published, but to claim that even the majority of victims perished in this way is an exaggeration. Surveys of the effects of the bombs were carried out by the Manhattan Engineer District, the United States Strategic Bombing Survey, and the Atomic Bomb Casualty Commission . If you are close enough to a nuclear explosion to receive an injurious dose of radiation, you are more likely to have been blasted or burnt to death already. The Manhattan Project’s medical director, Dr. Stafford Warren, estimated that some 7% of deaths resulted primarily from radiation, although some estimates ran as high as 15-20%. This said, radiation effects were unpleasant to say the least. Radiation effects included depressed blood counts, loss of hair, bleeding into the skin, inflammations of the mouth and throat, vomiting, diarrhea, and fever. Deaths from radiation began about a week after exposure, peaked in about 3-4 weeks, and ceased by 7-8 weeks. A person who survived but remained continuously in a bombed city for six weeks afterwards could expect to receive a dosage estimated at 6-25 rems (Hiroshima) or 30-110 rems (Nagasaki), with the latter figure referring to a localized area; the usual benchmark for a lethal single-shot dose is ~ 500 rems. The USSBS report states that of women in Hiroshima in various stages of pregnancy who were known to be within 3,000 feet of ground zero, all suffered miscarriages, and some miscarriages and premature births where the infant died shortly after birth were recorded up to 6,500 feet. Two months after the bombing, the city’s total incidence of miscarriages, abortions and premature births ran to 27%, as opposed to a normal rate of 6%.
(14) President Truman knew little of the Manhattan Project before the bombs were dropped
While Truman had only an inkling of the project before President Roosevelt’s sudden death on April 12, 1945 [the Vice President of the Top Policy Group in item (3) above was Truman’s predecessor, Henry Wallace], he was brought up to speed on the new weapon very quickly. After a brief Cabinet meeting following his swearing-in, Truman was approached by Secretary of War Henry Stimson, who related that he wished to inform the new President “about … a project looking to the development of a new explosive of almost unbelievable destructive power.” The next afternoon, James Byrnes, head of the Office of War Mobilization (and soon to be Truman’s Secretary of State), dramatically told Truman that “we are perfecting an explosive great enough to destroy the whole world. It might well put us in a position to dictate our own terms at the end of the war.”
At noon on April 25, Stimson and Groves briefed Truman on the project. Two days earlier, Groves had submitted to Stimson a background memorandum to be given to the President. Essentially a primer on the entire Manhattan Project, this memorandum, titled “Atomic Fission Bombs,” ran to only 24 double-spaced pages, but managed to cover every aspect of the work from the idea of uranium fission up to the prospects for fusion weapons . The report opened by relating that “Within four months we shall in all probability have completed the most terrible weapon even known in human history, one bomb of which could destroy a whole city,” and went on to state that “The successful development of the Atomic Fission Bomb will provide the United States with a weapon of tremendous power which should be a decisive factor in winning the present war more quickly with a saving in American lives and treasure. … Each bomb is estimated to have the equivalent effect of from 5,000 to 20,000 tons of TNT now, and ultimately, possibly as much as 100,000 tons.”
At the time of the Trinity test on July 16, Truman was in Germany for the Potsdam conference, but he received a report that evening that the test was successful. In an entry in his personal diary for July 25, Truman indicates that he clearly understood the power of the new weapon (excerpted):
We have discovered the most terrible bomb in the history of the world … we think we have found the way to cause a disintegration of the atom. An experiment in the New Mexico desert was startling - to put it mildly. Thirteen pounds of the explosive caused the complete disintegration of a steel tower 60 feet high, created a crater 6 feet deep and 1,200 feet in diameter, knocked over a steel tower 1/2 mile away and knocked men down 10,000 yards away. The explosion was visible for more than 200 miles and audible for 40 miles and more. This weapon is to be used against Japan between now and August 10th. I have told the Sec. of War, Mr. Stimson, to use it so that military objectives and soldiers and sailors are the target and not women and children. … It is certainly a good thing for the world that Hitler’s crowd or Stalin’s did not discover this atomic bomb. It seems to be the most terrible thing ever discovered, but it can be made the most useful…
The notion that the effects of the bomb could be limited to purely military objectives was illusory, but it cannot be said that Truman did not appreciate the implications of the new weapon.
(15) The Germans were close to having an atomic bomb
That German scientists maintained an active nuclear research program during the war, particularly devoted to experimental piles, is unquestionably true: They constructed some 20 piles (if not more) of various designs, mostly involving heavy water. The last of these, pile B-VIII, came close to achieving a chain reaction in the closing weeks of the war . However, the German effort was always at a much smaller scale of funding, priority, and personnel than was the Allied effort, and was hobbled by personality clashes and bureaucratic turf wars. In 2005, historian Rainer Karlsch published a book titled Hitlers Bombe in which he made the remarkable assertion, based on German documents captured by the Russians and returned to the Max Planck Society in 2004, that that a group of scientists under Kurt Diebner and Walther Gerlach achieved a chain reaction and detonated two hybrid fission/fusion bombs before the end of the war. An English-language summary of the work was published by Karlsch and Mark Walker, a noted historian of wartime German nuclear efforts . However, historian of science Dieter Hoffmann has concluded that, while Karlsch produced a valuable work that brought to light much previously unknown archival material, the bomb assertion is not borne out by the book’s content: The reaction rates and pressures of the purported design are too small by at least two orders of magnitude to initiate a fusion reaction, it is not made clear how the Germans obtained plutonium or enriched uranium, there is no physically plausible description of the bomb’s design, and there is no reliable analysis of the purported test regions to show that nuclear reactions really occurred . There may yet be more to the German program to be revealed, but these sort of claims are examples of how tantalizing but unverifiable information can be extrapolated to sensationalized, premature conclusions.
 Reed. B. C. The History and Science of the Manhattan Project (Springer, Berlin, 2019).
 Reed, B. C. “Can the Energy of Fission Make a Grain of Sand Visibly Jump?,” The Physics Teacher 56, 583 (2018).
 DeGroot, G. The Bomb: A Life (Harvard University Press, Cambridge, MA, 2004), p. 16.
 B. C. Reed, “Arthur Compton’s 1941 report on explosive fission of U-235: A look at the physics,” Am. J. Phys. 75(12), 1065-1072 (2007).
 R. Rhodes, The Making of the Atomic Bomb (New York: Simon and Schuster, 1986), p. 438.
 A. Wattenberg, “December 2, 1942: the event and the people,” Bull. Atom. Sci. 38(10) 22-32 (1982).
 Nichols, K. D, The Road to Trinity (New York: William Morrow and Company, 1987), p. 146.
 Reed, B. C. “Kilowatts to Kilotons: Wartime Electricity Use at Oak Ridge,” History of Physics Newsletter XII(6), 5-6 (2015).
 V. C. Jones, Manhattan: The Army and the Atomic Bomb (Washington: Center of Military History, United States Army, 1985), p. 391.
 D. N. Schwartz, The Last Man Who Knew Everything: The Life and Times of Enrico Fermi, Father of the Nuclear Age (New York: Basic Books, New York, 2017), p. 257.
 Ref. 5, p. 672.
 Reed, B. C., “Seeing the Light: Visibility of the July ’45 Trinity Atomic Bomb Test from the inner solar system,” The Physics Teacher 44, 604-606 (2006).
 J. Kunetka, The General and the Genius: Groves and Oppenheimer – The Unlikely Partnership That Built the Atomic Bomb (Washington, DC: Regnery History, 2016), p. 357.
 J. Coster-Mullen, Atom Bombs: The Top Secret Inside Story of Fat Man and Little Boy (Coster-Mullen, Waukesha, WI, 2016). See pp. 326 and 331.
 Groves’ July 30 memo can be found at https://nsarchive2.gwu.edu//NSAEBB/NSAEBB162/45.pdf. The August 10 memo can be found at National Archives and Records Administration microfilm record M1109, reel 3, images 0653-0654. This microfilm set is the Correspondence (“Top Secret”) of the Manhattan Engineer District, 1942-1946 (Records of the Office of the Chief of Engineers; Record Group 77; 5 rolls).
 MED: http://www.atomicarchive.com/Docs/MED/index.shtml
 Groves’ April 23 memo can be found at http://www.gwu.edu/%7Ensarchiv/NSAEBB/NSAEBB162/3a.pdf
 Excerpts from Truman’s diary can be found in R. H. Ferrell, Harry S. Truman and the Bomb (High Plains Publishing Co., Worland, WY, 1996)
 B. C. Reed, “Piles of piles: In inter-country comparison of nuclear pile development during World War II,” https://arxiv.org/abs/2001.09971.
 R. Karlsch and M. Walker, “New light on Hitler’s bomb,” Physics World 18(6), 15-18 (2005).
 D. Hoffmann, “The race for the bomb: How close was Nazi Germany to developing atomic weapons?” Nature 436 (7047), 25-26 (2005).