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THE SCIENTIFIC AND technical road that led to Los Alamos, the atomic bomb, the destruction of Hiroshima and Nagasaki, and the creation of the New World originated with changes in the science of physics as it moved from the study of visible phenomena, such as thermodynamics, to the study of the invisible world of radiation and the atom. The discovery in the 1880s of radiation, the energy that originates in the atom, by Antoine Becquerel, and the discovery of X-rays by Wilhelm Roentgen initiated this change in focus. Marie and Pierre Curie built on the discoveries of Becquerel and Roentgen and discovered the radioactive elements radium and polonium. Marie Curie received the first of her two Nobel Prizes for this work and also died from complications caused by exposure to radiation. In 1905, Albert Einstein published the first of his three seminal papers on relativity, including his famous equation, E=mc 2. Einstein's work relating mass to energy provided the theoretical basis for understanding the atomic world. In 1911, Ernest Rutherford, working at the University of Manchester, developed his planetary atom model, likening an atom to a miniature universe where electrons (planets) orbit the nucleus (sun), and in 1917 discovered protons.
Danish physicist Niels Bohr expanded on Rutherford's work, publishing articles on the structure of the atom and his theory that electrons were the major determinate of an element's chemical properties. Bohr's research became the basis for quantum theory (quantum mechanics), the dual particle-like and wave-like behavior of matter and energy. In 1930, Wolfgang Pauli predicted the existence of neutrinos, subatomic particles that would not be physically confirmed until the 1960s. In 1931, Harold Urey discovered deuterium, an isotope of hydrogen that would become the primary material of thermonuclear bombs in the 1950s. However, as the body of knowledge about the atom grew, there remained a disturbing fact. An atom weighs more than the combined weights of its two known particles, electrons and protons. What and where was this unidentified missing mass? British physicist James Chadwick postulated and then, in 1932, proved the existence of a third subatomic particle having no electrical charge - the neutron. This particle, part of an atom's nucleus, solved the puzzle of atomic weight. The discovery of the neutron completed the structure of the atom - a nucleus of positively charged protons in combination with neutrons having no electrical charge, surrounded by a negatively charged shell, or cloud, of electrons.
The discovery of the neutron was vitally important to physics for yet another reason. The neutron was an ideal experimental tool for exploring the atomic world because its neutral charge allowed it to easily penetrate an atom's negatively-charged electron shell and its positively-charged nucleus. Using a stream of neutrons, physicists bombarded elements with neutrons and then analyzed the chemical and physical properties of the resulting debris. Physicists, however, did not have all the requisite chemistry skills to correctly analyze the debris. Working at his laboratory in Rome in 1934, Enrico Fermi, a future Nobel Laureate, bombarded a number of elements, including uranium, with neutrons. When Fermi and his team bombarded uranium, the resulting debris consisted of two particles of nearly equal mass that they could not identify. Unknowingly, Fermi and his team became the first scientists to achieve fission. In 1938, at Berlin's Kaiser Wilhelm Institute, Otto Hahn and Fritz Strassman also bombarded uranium with neutrons. Like Fermi, they also split uranium atoms into two almost equal pieces, which to them appeared to be krypton and barium. Hahn and Strassman, like Fermi, could not explain why the new particles appeared to be two distinct elements or why the combined mass of these new particles was slightly less than that of the original atom. Perplexed by their findings, Hahn wrote to a former colleague, Lise Meitner, describing the experimental findings.
Meitner, a chemist, discussed the results with her nephew, Otto Frisch. Together, they deduced what had happened – a uranium atom had been split into two nearly equal pieces: krypton and barium. The "missing mass" had been converted into energy. Fission, the splitting of the atom, had been discovered. A short time later, Frisch shared this revelation with Niels Bohr, who was going to Washington, D.C., to attend the Fifth International Theoretical Physics Conference. Bohr announced the discovery of fission at the conference, creating great excitement throughout the physics community. Other physicists quickly replicated the Hahn and Strassman experiment, achieving the same results. Just as quickly, it became obvious that if a billion or so atoms could be split in a microsecond, the cumulative energy release would create an explosion of extraordinary power. J. Robert Oppenheimer, a young theoretical physicist at the University of California at Berkeley, was among those physicists who realized this. A few months after the 1939 conference, Robert Serber entered Oppenheimer's university office only to find a crude sketch of an atomic bomb on the blackboard. Six years later, under the leadership of Oppenheimer, Los Alamos scientists would engineer fission into the atomic bombs used against Hiroshima and Nagasaki.
Once discovered, fission was a disarmingly simple construct. However, this simple construct was difficult to fashion into a nuclear weapon, both politically and technically. The political problem was a lack of awareness by the President about the possibility of an atomic bomb. Three Hungarian physicists, all refugees from Nazi-controlled Europe, Leo Szilard, Edward Teller, and Eugene Wigner, sought to alert President Franklin Roosevelt about the possibility. Although prominent within the physics community, none had any political standing and could not hope to attract Roosevelt's attention. Knowing this, Szilard drafted a letter that he hoped another European refugee, the very famous Albert Einstein, would sign and forward to the President. Einstein did so because Roosevelt authorized the creation of the Advisory Committee on Uranium to investigate existing research on atomic matters. When the committee found that this research strongly suggested that an atomic bomb was possible, Roosevelt assigned responsibility for the bomb's development to the War Department, which created the Manhattan Engineer District (aka Manhattan Project) to organize and manage laboratories and production plants that produced the first atomic bombs.
Designing and building the first atomic bombs required solving three technical problems. First, large quantities of uranium and plutonium had to be amassed. Only these two metals, specifically their isotopes 235U and 239Pu, could be made to fission. Amassing 235U was very difficult because it is chemically identical to natural uranium, 238U, and also very scarce, constituting only seven percent of all uranium in nature. The second problem in turning fission into a weapon was proving that a self-sustaining chain reaction was possible. A self-sustaining chain reaction occurs when an atom is split, releasing one or more neutrons that split other atoms. This process continues exponentially until the reaction runs out of atoms or, as in the case of a bomb, blows itself apart.
In December 1942, Nobel Laureate Enrico Fermi, who had fled Europe with his Jewish wife and was now working at the University of Chicago, successfully proved that a self-sustaining nuclear chain reaction was possible. His experiment was a rudimentary reactor into which a number of slugs of uranium were placed until enough material was accumulated to reach criticality. Fermi then manipulated the reactor to produce the world's first self-staining chain reaction. Before the experiment could blow itself apart, Fermi engaged his safety mechanism and shut the reactor down. Although Fermi was confident he could control his experiment, he nonetheless stationed three graduate students, known as the suicide squad, on top of the reactor to pour buckets of a cadmium solution over the experiment if the safety mechanism failed. The cadmium solution would soak up neutrons and quash the fission process.
The third problem was to design and build an actual nuclear weapon. The trick was to amass, in a compact form, a slightly subcritical amount of nuclear material. Then, by some manipulation, turn the subcritical piece of material into a supercritical mass. If too little metal was used, the bomb would not yield a nuclear explosion. If too much metal was amassed, the assembly would self-destruct. This task became the job of the Los Alamos Laboratory.
