archived as http://www.stealthskater.com/Documents/Nuke_15.doc [pdf]

more of Nuclear Weapons at http://www.stealthskater.com/Nuke.htm

note: because important websites are frequently "here today but gone tomorrow", the following was archived from http://nuclearweaponarchive.org/Usa/Med/Dawnage.html on July 21, 2005. This is NOT an attempt to divert readers from the aforementioned website. Indeed, the reader should only read this back-up copy if it cannot be found at the original author's site.

15 - Dawn of the Atomic AgeLast changed April 1997

A New Century - A New Universe

The turn of the Century marked a profound revolution in the development of Science and our understanding of the fundamental principles of the natural world.

During the 19th Century, Classical Physics (the laws of motion, electromagnetic fields, and thermodynamics) had reached an advanced state of development. Chemistry had also reached a considerable degree of sophistication but on a largely empirical basis. The fundamental basis of chemistry remained mysterious. Much had been learned about the Earth and solar system as well. Estimates of the age of the Earth had risen from about 6,000 years in the late 18 th Century to tens-or-hundreds of millions-of-years. And the view that Life, the Earth, and the rest of the Solar System had arisen in a single great upheaval in recent times had been replaced by the idea of gradual change over eons.

To some, it seemed that Science (especially Physics) was reaching such a state of maturity that few fundamental principles remained to be discovered. But there were problems. Essentially nothing was known about the fundamental structure of matter that gave rise to the Periodic Law and other chemical behaviors. The very existence of atoms was largely conjectural. Geology and Astronomy seemed in serious conflict since the apparent age of the geologic record could not be reconciled with the only power source for the Sun then conceivable -- gravitational contraction -- which would exhaust itself in mere millions of years. An important part of classical thermodynamics was stubbornly resisting resolution -- the properties of blackbody radiation. In fact, by the end of 1900s it had become clear that within the existing framework of physics no solution of the blackbody problem was possible (the untenable prediction made by existing physics was termed the "ultraviolet catastrophe"). Something important was missing.

Advancing experimental technique in the seemingly well-understood field of Electricity and Magnetism gave the first clues to the new universe. In 1895, Wilhelm Konrad Roentgen at the University of Wurzburg discovered X-rays. He had been conducting experiments involving high-voltage currents in evacuated tubes. The penetrating radiation that he discovered was wholly new and unexpected.

The following year (1896) -- by serendipitous accident while investigating X-rays -- Henri Becquerel (at the Museum of Natural History in Paris) discovered radioactivity in a piece of uranium salt. This discovery provided for the first time direct evidence of the fundamental structure of matter

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and also revealed the existence a totally new source of energy independent of the Sun's rays or of chemical fuels. And vastly more concentrated than either.

Discoveries followed rapidly. Marie Skladowska Curie and her husband Pierre Curie immediately began isolating sources of radiation from uranium ore. This led to the discovery of Polonium in 1896 and Radium in 1897. Different types of radioactive emissions were soon identified. In 1899, Becquerel found that at least some of the radiation emissions were electrically-charged. Ernest Rutherford further distinguished 2 types of charged emissions -- alpha and beta rays. Paul Villard identified neutral gamma rays.

During this time, another key discovery was in the making -- the development of Quantum Theory. 2 threads led to the foundation of the theory: one theoretical and one experimental. The theoretical development was by Max Planck at the University of Berlin. In pursuing the perplexing problem of blackbody radiation, he developed a theory announced in 1900 that successfully predicted the observed blackbody spectrum. This theory postulated that matter could only absorb or emit energy in arbitrary units or "quanta".

In 1898, J.J. Thomson detected the emission of electrons when a metal surface is illuminated by ultraviolet light -- the photoelectric effect. The properties of this phenomenon could not be explained -- particularly a metal-dependent frequency threshold for the emissions.

Albert Einstein united these threads with his theory of the photoelectric effect in 1905 which proposed the existence of the photon -- quantized light (for which he received the Nobel Prize). Also in 1905, Einstein formulated his Special Theory of Relativity, one aspect of which (the equivalence of mass and energy) began to give some insight into the origin of the atomic energy that had been revealed by the discovery of radioactive decay.

These developments had also greatly extended the understanding of the Earth and Sun. In 1905, Rutherford and Boltwood used the ratio between radioactive isotopes and their decay products to date a rock to 500 million years old. This great age sharpened the conflict with classical theories of solar development. But radioactivity also offered a resolution. Perhaps some atomic transformation process -- not then understood -- was the source of the Sun's brilliance and longevity.

The New Universe Explored

With the hints given by these new discoveries and the powerful new probes of matter offered by the newly-discovered ionizing radiations, more discoveries followed swiftly.

Rutherford soon demonstrated that alpha particles were, in fact, Helium atoms minus their electrons.

In 1906, Rutherford began a series of experiments at McGill University where he was now professor and continued at the University of Manchester. In these experiments, he studied how alpha rays were scattered by thin layers of mica and gold.

The age of the Earth jumped again in 1907 when Boltwood identified a piece of uraninite as being 1.64 billion years old.

In 1911, Rutherford published his conclusions drawn from the alpha scattering experiments -- that nearly all of the mass of the atom is concentrated in a tiny positively charged region in the center called the nucleus.

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J.J. Thomson discovers isotopes of Neon in 1912 -- showing that the atoms of the same element could have different masses.

Although it was realized late in the 19th Century that the identities of chemical elements were related to the number of electrons that each atom contained (the atomic number), it was difficult to determine this number accurately for most elements. In 1913, H.G.J. Moseley demonstrated that by studying X-ray emissions, the atomic number could be easily measured.

It was now possible to study the relationship between the atomic charge (the atomic number) and the atomic mass. Evidence began to accumulate that there were 2 principal contributors to the mass of the atom and the nucleus -- one that was positively-charged (later called the proton) and one that was neutral (the neutron).

Also in 1913, Niels Bohr made a key theoretical breakthrough. He devised the "Bohr atom" -- a planetary model of the hydrogen atom with the electron orbiting the positively-charged nucleus -- that explained studying the spectrum of light emitted by hydrogen atom. This model was based on the quantum theory and was consistent with the atomic structure observed by Rutherford.

Although Physics and Science continued to advance (Einstein completed the General Theory of Relativity during this period, for example), there was a temporary doldrum in key discoveries about the structure of matter lasting for several years. This is partly explainable by the calamity of the First World War that disrupted all of Europe. Some of the destructive effects of the war on Science were quite direct. The young genius Moseley perished in the trenches of Gallipolli.

Discovery of the Neutron

On June 3, 1920, Ernest Rutherford gave his second Bakerian Lecture in London. In the course of this lecture, he speculated on the possible existence and properties of the neutron. This is apparently the earliest public proposal of the idea of positive and neutral particles composing the atomic nucleus.

In 1921, the American chemist H.D. Harkins coined the term "neutron" in a proposal of nuclear structure. Rutherford published further work on the idea in this same year. Little progress was made on developing the idea or proving its existence for the next several years.

In 1930, two German physicists -- W. Bothe and H. Becker -- observed unusually penetrating radiation being emitted from beryllium metal when it was bombarded by alpha particles. On December 28, 1931, Irene Joliot-Curie (Marie and Pierre's daughter) reported on these same emissions but -- like Bothe and Becker -- believed them to be energetic gamma rays. Joliot-Curie discovered that these emissions produced large numbers of protons when they passed through paraffin or other hydrogen containing materials -- something never observed (and apparently impossible to explain) with gamma rays.

Over a 10-day period from February 7-to-17, 1932, James Chadwick conducted a series of experiments that conclusively demonstrated that these unusual emissions were actually neutrons. Using this new potent new tool, rapid progress on the structure of matter began to be made.

Although radioactive decay releases an enormous amount of energy compared to chemical processes, this energy release is gradual and cannot be modified to any significant degree. The possibility of "atomic energy" as a source of human-controlled power thus came into existence as a concept but without any known means of bringing it about -- even in theory. On September 12, 1933, this changed.

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On that day, the brilliant Hungarian physicist Leo Szilard conceived the idea of using a chain reaction of neutron collisions with atomic nuclei to release energy. He also considered the possibility of using this chain reaction to make bombs. These insights predate the discovery of an actual chain reaction process (fission) by more than 6 years.

Invention and Discovery: Atomic Bombs and FissionLast changed April 1997

Leo Szilard and the Invention of the Atomic Bomb

It would be logical to assume that the discovery of fission preceded the invention of the atomic bomb. It would be normal also to expect that no single individual could really claim to be "the inventor" since the possibility sprang naturally from a physical process and required the efforts of many thousands to bring it into existence. Many descriptions of the origin of atomic bombs can be found that logically and normally say exactly these things. But they are not correct.

The idea of "invention" does not usually require the physical realization of the invented thing. This fact is clearly recognized by patent law which does not require a working model in order to award a patent. It is common for inventions to require additional discoveries and developments before the actual thing can be made. In these cases, an invention may fairly have more than one inventor -- the originator of the principle idea and the individual who actually made the first workable model.

In the case of the atomic bomb, there is clearly one man who is the originator of the idea. He was also the instigator of the project that led ultimately to the successful construction of the atomic bomb and was a principal investigator in the early R&D both before and after the founding of the atomic bomb project -- making a number of the key discoveries himself. By any normal standard, this man is the inventor of the atomic bomb.

This man is Leo Szilard.

On September 12, 1932 -- within 7 months of the discovery of the neutron and more than 6 years before the discovery of fission -- Leo Szilard conceived of the possibility of a controlled release of atomic power through a multiplying neutron chain reaction. He also realized that if such a reaction could be found, then a bomb could be built using it.

On July 4, 1934, Leo Szilard filed a patent application for the atomic bomb. In his application, Szilard described not only the basic concept of using neutron-induced chain reactions to create explosions but also the key concept of the critical mass. The patent was awarded to him, making Leo Szilard the legally-recognized inventor of the atomic bomb.

Szilard did not patent this prescient and tremendously important idea for personal gain. His motive was to protect the idea to prevent its harmful use for he immediately attempted to turn the idea over to the British government for free so that it could be classified and protected under British secrecy laws.

On October 8, 1935, the British War Office rejected Szilard's offer. But a few months later in February 1936, he succeeded in getting the British Admiralty to accept the gift. Szilard's actions in attempting to restrict the availability of the atomic bomb are also the earliest case of nuclear arms control. Later when the possibility of a German atomic bomb had been shown to be nonexistent,

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Szilard campaigned vigorously against the use of the bomb and went on to help found The Bulletin of Atomic Scientists and The Council for a Livable World.

The Discovery of Fission

With the discovery of the neutron by James Chadwick in February 1932, a scientific "gold rush" ensued to discover what effects would be produced by bombarding different materials with this new particle. Over the next several years, teams of researchers in several countries (especially one headed by Enrico Fermi in Rome) bombarded every known element with neutrons and recorded scores (even hundreds) of new radioactive isotopes.

On May 10, 1934, Fermi's research group published a report on experiments with neutron bombardment of Uranium. This was the first such investigation to be reported on. Several radioactive products are detected, but positive identifications were not made. Interpreting the results of neutron bombardment of Uranium became known as the "Uranium Problem" since the large number of different radioactivities produced defied rational explanation. The dominant theory was that a number of trans-Uranic elements never before seen were being produced. But the chemical behavior as well as the nuclear behavior of these substances were unexpected and confusing.

The first statement of the correct resolution of the Uranium Problem was published by German chemist Ida Noddack in September. Her letter in "Zeitshrift fur Angewandte Chemie" argued that the anomalous radioactivities produced by neutron bombardment of uranium may be due to the atom splitting into smaller pieces. No notice of this suggestion was taken.

Fermi discovered the extremely important principle of neutron behavior called "moderation" on October 22, 1934. Moderation is the phenomenon of enhanced capture of low-energy neutrons as when they are slowed down by repeated collisions with light atoms.

December 1935 -- Chadwick won the Nobel Prize for discovery of the neutron.

November-December 1938 -- Otto Hahn and Lise Meitner correctly unravel the Uranium Problem. Hahn determines conclusively that one of the mysterious radioactivities is a previously known isotope of Barium. Working with Meitner, they develop a theoretical interpretation of this demonstrated fact. On December 21, 1938, Hahn submits a paper to "Naturwissenschaften" showing conclusive evidence of the production of radioactive barium from neutron-irradiated Uranium (i.e., evidence of fission).

In the first few weeks of January, word of the discovery traveled quickly in Europe.

January 13, 1939 -- Otto Frisch observed fission directly by detecting fission fragments in an ionization chamber. With the assistance of William Arnold, he coins the term "fission".

By mid-January, Szilard heard about the discovery of fission from Eugene Wigner and immediately realized that the fission fragments -- due to their lower atomic weights -- would have excess neutrons which must be shed. The multiplying neutron chain reaction that he had postulated had finally been discovered.

January 26, 1939 -- Niels Bohr publicly announces the discovery of fission at an annual theoretical physics conference at George Washington University in Washington, DC. This announcement was the principal revelation of fission in the United States.

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January 29, 1939 -- Robert Oppenheimer hears about the discovery of fission. Within a few minutes, he realized that excess neutrons must be emitted and that it might be possible to build a bomb.

February 5, 1939 -- Niels Bohr gained a crucial insight into the principles of fission -- that U 235 and U238

must have different fission properties that U238 could be fissioned by fast neutrons (but not slow ones) and that U235accounted for observed slow fission in uranium.

At this point, there were too many uncertainties about fission to see clearly whether or how self-sustaining chain reactions could arise. Key uncertainties were:

1. The number of neutrons emitted per fission, and2. The cross-sections for fission and absorption at different energies for the uranium

isotopes.

For a chain reaction, there would need to be both a sufficient excess of neutrons produced and the ratio between fission to absorption averaged over the neutron energies present would need to be sufficiently large.

The different properties of U235 and U238 were essential to understand in determining the feasibility of an atomic bomb or of any atomic power at all. The only Uranium available for study was the isotope mixture of natural-Uranium in which U235comprised only 0.71%.

March, 1939 -- Fermi and Herbert Anderson determine that there are about 2 neutrons produced for every one consumed in fission.

June, 1939 -- Fermi and Szilard submit a paper to Physical Review describing sub-critical neutron multiplication in a lattice of Uranium Oxide in water. But it is clear that natural-Uranium and water cannot make a self-sustaining reaction. This paper is the first experimental evidence of neutron multiplication.

July 3, 1939 -- Szilard writes to Fermi describing the idea of using a Uranium lattice in Carbon (graphite) to create a chain reaction. This is the first proposal of the Graphite-moderated reactor concept.

August 31, 1939 -- Bohr and John A. Wheeler publish a theoretical analysis of fission. This theory implies that U235 is more fissile than U238 and that the undiscovered element 94Pu239 is also very fissile. These implications are not immediately recognized.

September 1, 1939 - Germany invades Poland, beginning World War II.

The Manhattan Project (and Before)Last changed 30 March 1999

Prelude to the Manhattan Project

At the same time that the key discoveries in neutron physics and neutron reactions were occurring, the political situation around the World was deteriorating. The 2 key developments were of course:

● the armed expansionism of military dominated Japan (commencing with invasion of Manchuria in September 1931), and

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● the rise and expansion of Nazi Germany (commencing with Adolf Hitler's appointment as Chancellor in January 1933 and his acquisition of dictatorial power in March).

The deterioration was more general than that though. Political and social repression, instability, and military violence were rising throughout Europe (Italy, Spain, Central Europe) and Stalinism was reaching a fevered pitch in the Great Purges (1936-38).

The rush of ominous events is too thick to enumerate in a brief overview but some principal ones are:

● September 1935: Nuremburg Laws begin severe persecution of Jews● March 1936: Occupation of the German Rhineland● July 1937: Japan invades China● November 1937: The Axis Alliance is created by a pact between Germany, Japan, and Italy● March 1938: the Anschluss (occupation of Austria by Germany)● September 1938: German occupation of the Sudetenland in Czechoslavakia

It is against this background that Szilard fretted about the possibility of an atomic bomb. The discovery of fission came just as Germany was girding itself to abandon expansion by intimidation and resort to armed conquest.

World War II erupted at a moment when the promise of atomic energy had progressed from being possible to being probable. It was not clear whether this energy could be released explosively, however.

Szilard, as always, was both a man-of-vision and a man-of-action. Well-known among European physicists, Szilard drafted a letter in consultation with Albert Einstein that was addressed from Einstein to President F.D. Roosevelt and which warned him of the possibility of nuclear weapons (the "Einstein Letter"). This letter was delivered to FDR on October 11, 1939. And 10 days later, the first meeting of the Advisory Committee on Uranium (the "Briggs Uranium Committee") was held in Washington, DC on Pres. Roosevelt's order.

Szilard and Einstein together after the War

Due largely to persistent official lack of interest, the progress on the subject was desultory and inconclusive in the United States. The next key developments occurred in the United Kingdom.

During February 1940, ex-patriot physicists Otto Frisch and Rudolf Peierls -- living in the UK -- prepared a theoretical analysis of the possibility of fast fission in U235. Their report contains the first well-grounded (although rough) estimates of the size of a critical mass ("a pound or two") and probable efficiency, and proposed practical schemes for bomb design and the production of U235. This "roadmap"

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for fission weapon development would be elaborated upon and modified to a spectacular degree in the coming years. But it remains basically sound.

So persuasive is the report by Frisch and Rudolf Peierls' that a study committee is formed at the highest levels of government (eventually code-named the MAUD Committee) on April 10. By December, the MAUD Committee would issue a key report selecting gaseous diffusion as the most promising method of uranium enrichment.

Through 1940 and well into 1941, work accelerated in the U.S. and important discoveries accumulated although official interest and support languished. In February 1941, Philip Abelson began actual development of a practical Uranium enrichment system (liquid thermal diffusion). On February 26, Glenn Seaborg and Arthur Wahl discover Plutonium. During March, the first American measurements of the U235 fission cross-section allow Peierls to calculate the first experimentally-supported estimate of a critical mass for U235 (18 lb as a bare sphere; 9-10 lb when surrounded by a reflector).

By July 1941, Plutonium was demonstrated to be a superior fissile material and the MAUD Committee completed its final report, describing atomic bombs and project proposals for building them in some technical detail.

On September 3, 1941 with PM Winston Churchill's endorsement, the British Chiefs of Staff agree to begin development of an atomic bomb. But it is not until December 18 -- after months of bureaucratic struggling and the U.S. entry into the War -- that a U.S. project to investigate atomic weapons (as opposed to "study fission") finally gets underway.

This Manhattan Project predecessor -- code-named the S-1 project -- was headed by Arthur H. Compton. The core group of scientists that would lead the development of the atomic bomb had coalesced well before this and was already working as hard as resources allowed on the problem.

In January 1942, Enrico Fermi's on-going work with Graphite and Uranium was transferred to a new secret project code-named the Metallurgical Laboratory (Met Lab) at the University of Chicago. In April, Fermi begins design of CP-1 -- the World's first (human-built) nuclear reactor.

Throughout early- and mid-1942, fundamental neutron physics research proceeded as did work on developing industrial scale processes for producing fissile materials. But it became increasingly obvious that since this was to be an industrial-scale project, a proven project manager was called for. Furthermore since it was a weapons project, it needed to be brought under an organization experienced in producing weapons.

On June 18, 1942, Brig. Gen. Steyr ordered Col. James Marshall to organize an Army Corps of Engineers District to take over and consolidate atomic bomb development. During August, Marshall created a new District organization with the intentionally misleading name "Manhattan Engineer District" (MED) -- now commonly called "The Manhattan Project".

The Manhattan Project

Despite its official founding in August, the Manhattan Project really began on September 17, 1942 when Col. Leslie Richard Groves was notified at 10:30 a.m. by Gen. Brehon Somervell that his assignment overseas had been cancelled. Groves -- an experienced manager who had just overseen the colossal construction of the Pentagon

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-- seized immediate and decisive control. In just 2 days, he resolved issues that had dragged on for months under Compton.

On September 18, Groves ordered the purchase of 1,250 tons of high-quality Belgian Congo uranium ore stored on Staten Island and the next day purchased 52,000 acres of land to be the future site of Oak Ridge. Groves was promoted to Brigadier General on September 23. By September 26, Groves had secured access to the highest emergency procurement priority then in existence (AAA). The era of weak, indecisive leadership was over.

Groves' pushy, even overbearing demeanor won him few friends among the scientists on the Manhattan Project (in particular, a special enmity developed between Groves and Szilard). Many detested him at the time, considering him a boor and a buffoon. It was only after the War that many scientists began to appreciate how crucial his organizational and managerial genius was to the MED.

During the fall while Fermi built CP-1 in Chicago, Groves took the fissile material programs out of the hands of the scientists and placed them under the management of industrial corporations like DuPont and the Kellog Corporation. He ordered construction begun immediately on the fissile material production plants (even though designs and plans had not yet been drawn up), realizing that the same basic site preparation work would be required no-matter-what.

On October 15, 1942, Groves asks Dr. J. Robert Oppenheimer to head Project Y -- the new planned central laboratory for weapon physics research and design. The site for which he selected on November 16 at Los Alamos, New Mexico.

Dr. J. Robert Oppenheimer (taken during 1945)

Oppenheimer and Groves inspecting the remains of the Trinity test tower September 9, 1945. Despite his suit, in this picture one can sense how emaciated Oppenheimer had become during the Manhattan Project. Click for big image (532x750, 86K)

Oppenheimer -- a professor of physics at Berkeley -- had demonstrated a special skill at leading groups of scientists during the S-1 program which Groves quickly took notice of. Oppenheimer and Groves developed a good relationship with each recognizing how critical the other was to the project.

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On December 1, 1942 after 17 days of round-the-clock work, Fermi's group completed CP-1 (sooner than planned) when Fermi projected that a critical configuration had been reached. It contained 36.6 metric tons of Uranium Oxide, 5.6 metric tons of Uranium metal, and 350 metric tons of Graphite.

On December 2, 1942 3:49 pm, CP-1 went critical and was allowed to reach a thermal output of 0.5 watts (ultimately it was operated up to a maximum power level of 200 watts).

In January 1943, Groves acquired the Hanford Engineer Works -- 780 square miles of land on the Columbia River in Washington for Plutonium production reactors and separation plants. During March, Los Alamos began operations as the staff arrived.

During the remainder of 1943, work continued on the construction of the Plutonium production facilities (reactors and chemical processing) at Hanford and the Uranium enrichment plants (gaseous diffusion and electromagnetic separation) at Oak Ridge. A large experimental graphite reactor (the X-10) was also constructed at Oak Ridge to provide research quantities of Plutonium and went critical on November 4. Refinement of gun-assembly based weapon designs continued at Los Alamos. Preliminary implosion research also proceeded initially at a low level of effort but -- after promising early results -- at an accelerated rate late in the year.

The first attempt at large scale Uranium enrichment -- the electromagnetic Alpha tracks at Oak Ridge -- went on-line in the fall but failed completely. By the end of the year, complete rebuilding was ordered.

Also in the fall, Project Alberta began. Its purpose was to prepare for the actual combat delivery of atomic weapons by conducting weapons delivery tests, modifying aircraft for carrying the atomic weapons, and organizing and training flight crews and field teams for weapons handling.

In 1944, work proceeded on all fronts:● weapon development● fissile material production● combat delivery preparations

In January, a major problem surfaced with the diffusion barriers intended for the K-25 gaseous diffusion plant at Oak Ridge. The process then being developed for barrier production seemed unpromising and Groves to switched planned production to a new process creating months of delays in equipping K-25 for operation. Abelson -- then in the process of constructing a thermal diffusion Uranium enrichment plant -- learned about the problems with the Manhattan Project's gaseous diffusion plant and leaked information about his technology to Oppenheimer.

On April 5, the first sample of X-10 reactor produced plutonium arrived from Oak Ridge. Emilio Segre immediately began monitoring its spontaneous fission rate. By April 15, his preliminary estimate of a spontaneous fission rate indicated that it was far too high for gun assembly. The report was kept quiet due to limited statistics and observations continued.

By mid-May 1944 (6 months after the start of accelerated implosion research), little progress towards successful implosion had been made. The experimental and theoretical work on the problem had been reorganized a number of times and resources devoted to it kept expanding. New IBM calculating equipment was now being put to use.

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At this point, 2 British scientists joined Los Alamos who had important impacts on the implosion program. Geoffrey Taylor (arrived May 24) pointed out implosion instability problems (especially the Rayleigh-Taylor instability) which ultimately led to a very conservative design to minimize possible instability problems. James Tuck brought the critical idea of explosive lenses for detonation wave shaping.

On June 3 after visiting the thermal diffusion uranium enrichment pilot plan at the Naval Research Laboratory, a team of Manhattan Project experts recommended that a plant be built to feed enriched material to the electromagnetic enrichment plant at Oak Ridge. On June 18, Groves contracted to have S-50 (a liquid thermal diffusion Uranium enrichment plant) built at Oak Ridge in no more than 3 months.

On July 4, 1944, Oppenheimer revealed Segre's spontaneous fission measurements to the Los Alamos staff. The neutron emission for reactor-produced Plutonium was too high for gun assembly to work. The measured rate was 50 fissions/kg-sec; the fission rate in Hanford Plutonium is expected to be over 100 times higher still.

This discovery was a turning point for Los Alamos, the Manhattan Project, and eventually for the practice of large-scale science after the War. The planned Plutonium gun had to be abandoned. Oppenheimer was forced to make implosion research a top priority, using all available resources to attack it. A complete reorganization of Los Alamos Laboratory was required.

With just 12 months to go before expected weapon delivery, a new fundamental technology -- explosive wave shaping -- had to be invented, made reliable, and an enormous array of engineering problems had to be solved. During this crisis, many foundations for post-War science were laid. Scientist administrators (as opposed to academic or research scientists) came to the forefront for running large-scale research efforts. Automated numerical techniques (as opposed to manual analytical ones) were applied to solve important scientific problems, not just engineering applications. The dispersal of key individuals after the end of the War later carried these insights as well as the earlier organizational principles developed at Los Alamos throughout American academia and industry.

July 1, 1944 -- The Manhattan Project was granted the highest project-wide procurement priority (AA-1).

July 20, 1944 -- The Los Alamos Administrative Board decided on a reorganization plan to direct the laboratory's full resources on implosion. Instead of being organized around scientific and engineering areas of expertise, all work was organized around whether it applied to implosion or the Uranium gun weapon with the former receiving most of the resources. The reorganization was completed in less than 2 weeks.

During August, Groves made his first estimate of bomb availability since the beginning of the Manhattan Project (the estimate was mid-Spring, 1945). Also this month, the Air Force began modifying 17 B-29s for combat delivery of atomic weapons.

Alpha Tracks at Y-12 The Alpha Track Control Room Beta Tracks at Y-12

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September 1944 marked a difficult period:● K-25 was half-built, but no usable diffusion barriers had been produced. The Y-12

electromagnetic enrichment plant was operating at only 0.05% efficiency. S-50 enrichment plant began partial operation at Oak Ridge. But leaks prevented substantial output. The total production of highly-enriched uranium to date was only a few grams. The only workable bomb design at hand -- the gun-type weapon -- required U235 which has no proven practical production methods available.

● Plutonium production had not yet begun, but the production techniques appeared to have a high probability of success. However, plausible approaches to building a Plutonium bomb did not yet exist.

● Project Alberta on the other hand has moved into a new phase as Air Force Lt. Col. Paul Tibbets began organizing the 509th Composite Group -- which would deliver the atomic bombs in combat -- at Wendover Field, Utah. [Interestingly, the 509 th stayed together after the War and exists to this day (1999) as a U.S. Air Force Strategic Command bomber force.]

Then a new crisis struck the Plutonium production effort. On September 26, the first full-scale Plutonium reactor -- the 'B pile' at Hanford -- was completed and loaded with Uranium. This reactor contained 200 tons of Uranium metal, 1200 tons of Graphite, and was cooled by 5 cubic meters of water/sec. It was designed to operate at 250 megawatts, producing some 6 kg of Plutonium a month.

On this day, Fermi supervised reactor's first start-up. After several hours of operation at 100 megawatts, the B pile inexplicably shut down … then started up again by itself the next day. Within a few days, this was determined to be due to poisoning by the highly-efficient neutron absorber Xenon135

-- a radioactive fission product. The 'B reactor' and others under construction had to be modified to add extra reactivity to overcome this effect before production could begin.

October 27, 1944 - Oppenheimer approved plans for a bomb test in the Jornada del Muerto valley at the Alamagordo Bombing Range. Groves approved the plan 5 days later, provided that the test be conducted in Jumbo.

By the end of the year, things start looking up:● Y-12 output had reached 40 grams of highly-enriched Uranium a day in November, then to 90

grams/day in December.● In mid-December, the first successful explosive lens tests established the feasibility of making

an implosion bomb.● December 17, 1944 - The 'D pile' went critical with sufficient reactivity to overcome fission

product poisoning effects. Large-scale plutonium production begins.

the Y-12 Plant the D-Reactor at Hanford

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By the start of 1945, the Manhattan Project had "turned the corner". The Uranium bombs seemed assured of success in a matter of months. The prospects for the Plutonium bomb were looking up although meeting an August 1 deadline imposed by Groves was far from certain. However, Allied military successes against Germany and Japan made it a horse race to see whether it would matter to the war effort.

January, 1945:● Y-12 output reached an average of 204 grams of 80% U235 a day; projected production of

sufficient material for a bomb (~40 kg) was July 1.● Usable barrier tubes began arriving at the K-25 plant. The first stage of the K-25 plant was

charged with Uranium Hexafluoride and began operation.● The Dragon experiment (January 18) conducted by Frisch created the World's first assembly

critical through prompt neutrons alone (i.e., it reached prompt critical). The largest energy production for a drop was 20 megawatts for 3 milliseconds (the temperature rose 6 oC in that time).

The K-25 Plant

February, 1945:

● The 'F reactor' went on-line at Hanford, raising theoretical production capacity to 21 kg/month.● Uranium gun design was completed and frozen. Only planning for deployment and combat

use once the U235 was delivered was now required.● Plutonium began arriving from Hanford.● Tinian Island was selected as the base of operations for atomic attack.● A meeting between Oppenheimer, Groves, and Los Alamos division leaders (February 28)

fixed the design approach for the plutonium bomb. The next day, the powerful Cowpuncher Committee was organized to "ride herd" on implosion bomb development.

March, 1945:● S-50 thermal diffusion plant finally began enriching uranium in quantity.● Oppenheimer officially froze explosive lens design (March 5).● By mid-month, the first evidence of solid compression from implosion was observed (5%).

April, 1945:● April 3 -- Preparations began at Tinian Island to support the 509 th Composite Group and to

assemble the atomic bombs.● April 11 -- Oppenheimer reported optimal performance with implosion compression in sub-

scale tests.● April 12 -- President Roosevelt died of a brain hemorrhage.

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● April 13 -- Pres. Truman learned for the first time of the existence of atomic bomb development from Secretary-of-War Henry Stimson.

● April 25 -- Truman received first in-depth briefing on the Manhattan Project from Stimson and Groves.

● April 27 -- The first meeting of the Target Committee was held to select targets for atomic bombing. 17 targets are selected for study: Tokyo Bay (for a non-lethal demonstration), Yokohama, Nagoya, Osaka, Kobe, Hiroshima, Kokura, Fukuoka, Nagasaki, and Sasebo (some of these were soon dropped because they had already been burned down).

the 100-Ton Test

May, 1945:● May 7 -- The 100-ton test was conducted. 108 tons of 'Composition B' laced with 1,000 curies

of reactor fission products were exploded 800 yards from Trinity ground zero to test instrumentation for Trinity. This was the largest instrumented explosion conducted up to this date.

● May 8 -- V-E Day. Germany formally capitulated to the allies.● May 9 -- The draft of general procedures for atomic bombing were completed.● May 10 -- Target Committee reconvened. The target list was shortened to Kyoto, Hiroshima,

Yokohama, and Kokura Arsenal.● Mid-May -- Little Boy was ready for combat use except for the U235 core. It was estimated

that sufficient material would be available by August 1.● May 25 -- Operation OLYMPIC -- the invasion of Kyushu (the southern Japanese island) --

was set for November 1.● May 28 -- Target Committee met with Lt. Col. Tibbets in attendance. Tibbets estimated that

by Jan. 1, 1946, all major cities of Japan will have been destroyed by fire-bombing. The target list was now Kyoto, Hiroshima, and Niigata.

● May 30 -- Sec.-of-War Stimson ruled out Kyoto (the ancient capital of Japan) as a target for atomic attack.

June, 1945:● June 10 -- 509th Composite Group crews began arriving on Tinian with their modified B-29s.● June 24 -- Frisch confirmed that the implosion core design is satisfactory after criticality tests.● Late-June -- LeMay estimated that the 20th Air Force would finish destroying the 60 most

important cities in Japan by Oct. 1.

July, 1945: Final preparations began at the New Mexico test site -- the Jornada del Muerto at the Alamagordo Bombing Range -- for the first atomic bomb test code-named Trinity. The date was set for July 16.

● July 3 -- Casting of the U235 projectile for 'Little Boy' was completed.● July 7 -- Explosives lens casting for Trinity was completed.● July 10 -- The best available lens castings were selected for Trinity.

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● July 11 - Assembly of Gadget -- the first atomic bomb -- began.● July 12-13 -- The Plutonium core and the Gadget components left Los Alamos for the test site

separately. Assembly of Gadget began at 1300 hours on July 13. Assembly of Gadget's explosive lens, Uranium reflector, and Plutonium core was completed at Ground Zero at 1745 hours.

● July 14Gadget was hoisted to the top of the 100-foot test tower and the detonators were

installed and connected. Final test preparations began.

Little Boy bomb units -- accompanied by the U235 projectile -- were shipped out of San Francisco on the USS Indianapolis for Tinian.

The only full-scale test of the implosion lens system (before Gadget) was conducted. Initial analysis indicated failure. But Bethe later corrected mistaken calculations and found that the measurements were consistent with optimum performance (he also discovered that the test instrumentation was incapable of distinguishing success from failure).

● July 16 - At 5:29:45 am, Gadget was detonated in the first atomic explosion in History. The explosive yield was 20-22 kt (initially estimated at 18.9-kt), vaporizing the steel tower.

[note: a thorough history and insight into all the major personalities involved in the "Bomb" has been chronicled in Brotherhood of the Bomb by Gregg Herken, Henry Holt & Co., 2002, ISBN 0-8050-6588-1 . "The tangled lives and loyalties of Robert Oppenheimer, Ernest Lawrence, and Edward Teller"]

(continued at "Gallery of U.S. Nuclear Tests" => doc pdf URL )

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