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The Discovery of the Neutron in 1932 Gave Chemists and Physicists Something New to Throw at Atoms. Neutron Has Mass but No Charg

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The Discovery of the Neutron in 1932 Gave Chemists and Physicists Something New to Throw at Atoms. Neutron Has Mass but No Charg The discovery of the neutron in 1932 gave Chain Reaction chemists and physicists something new to throw at atoms. Neutron has mass but no charge so would not be deflected by protons or electrons. In September 1933 Leo Szilard comes up with the idea of bombarding one element with one neutron. If you could get that element to emit more than one neutron then you could start a chain reaction. After Meitner fled Germany, she and Hahn continued their collaboration. Hahn's lab was smacking uranium with neutrons and instead of getting a heavier uranium atom (as expected) they were getting strange contamination such as barium and krypton, elements much lighter than uranium (artificial fission). Meitner and Hahn's In December 1938 Hahn submits his Lab: Desktop results, but does not provide an science explanation of what is happening, only that he has found barium and krypton and that some of the uranium seems to be disappearing. Bohr compared the nucleus to a water drop - as you add protons you get the next heavier element, and so Liquid drop model of on. Eventually you reach a limit at 92 protons fission (uranium). Beyond this there is so much positive charge that it makes the nucleus unstable. Adding more and more water to a drop will cause it to split into two or more drops. Same with the nucleus. This approach explained why atoms beyond uranium tended to be unstable – they would quickly decay. Using Bohr's idea, in February 1939 Meitner provides the physical explanation as to what was happening: the nucleus of an atom could be split into smaller parts: uranium nuclei had split to form barium and krypton, accompanied by the ejection of extra neutrons and a large amount of energy (the latter two products Artificial fission of accounting for the loss in mass). uranium nucleus Meitner's nephew, Otto Frisch, tells Niels Bohr about artificial uranium fission just as Bohr was leaving for a trip to the US. There Bohr announces uranium fission to an American audience. Very quickly Szilard's chain reaction concept is connected to the concept of artificial fission and the possibility of an atomic bomb is born, where energy is released catastrophically. In America, many scientists were divided about the possibilities of exploiting the power of the atom. Some regarded it as impossible in practice, others only a matter of time. The one group that kept pushing for the development of a nuclear program were the European scientists forced out of Europe and with first-hand experience of Nazi behavior (and of how good German physics was). The assumption was that Germany would jump at the chance to create a nuclear weapon and was almost certainly already at work on such a project. But nothing happened. Why? Because of the structure of American science at this period. (Also America not yet at war.)‏ Before World War II very little public money in the US was available for scientific research – within universities or outside universities. What there was, was for agriculture and medical projects. Projects were small, without expensive equipment. Large projects such as cyclotrons, which accelerate charged particles to high speeds, were funded by private foundation such as the Rockefeller Foundation. World War II changed the way science was funded. There seemed little reason for the government to get involved until Szilard thought to use Einstein's fame and prestige. Szilard convinced Einstein to sign a letter to President Roosevelt that described the possibilities of nuclear fission. The letter pointed out that the Germans had already suspended export of uranium ores and warned of the consequences should Germany create a nuclear weapon before the United States did. In the spring of 1940 Vannevar Bush proposed a scheme to President Franklin Delano Roosevelt that would organize and direct science for the upcoming war effort (though America still not at war at this point). Vannevar Bush pioneered the approach by which the government contracted for research and development from university scientists and private industry. A new relationship was created between science and the government. During the war research funded by the government included the development of radar, sonar, , penicillin, and new insecticides – DDT (which was a powerful weapon against malaria – it killed the mosquitoes which carry malaria). The Manhattan Project, which developed the American atomic bomb, originally came under Bush's scheme, but was soon turned over to the Army Corps of Engineers, because of the amount of technical work that would be required (it would also be easier to hide the huge amounts of money going into the project from Congress). The Manhattan Project became the prototype of Big Science. After the war, the approach pioneered by Bush continues into the Cold War and government support for science even increased. The National Science Foundation was created as a grant-giving agency for researchers at universities. In 1961 Alvin Weinberg, director of research at Oak Ridge National Laboratory, coined the term “Big Science”. “... many of the activities of modern science – nuclear physics, or elementary particle physics, or space research – require extremely elaborate equipment and staffs of large teams and professionals...” Inevitably this approach creates conflict, said Weinberg. How should scarce resources be allocated, who is going to decide between specific projects. The big question of what gets funded and why. (Really) Big Science CERN Particle Accelerator in Europe (26 kilometres long)‏ Manhattan Project Based on the assumption that Germany was at least as far advanced as America, if not more so, in developing a nuclear bomb. After all the original results had come out of Germany, and Germany had Heisenberg, one of the greatest theoretical physicists living. Now just because something is theoretically possible does not mean it is practically possible. What became the Manhattan Project was a huge effort that involved spending well over 20 billion dollars in today's money. At its peak it employed over 160,000 people all over the country. The beginning of big science. What kind of problems did people have to solve? They had to figure out which kind of uranium, 235 or 238, is responsible for fission. In the middle of 1940, scientists discovered that it was U235 that was fissile, and that a chain reaction would be possible with this isotope of uranium. Another problem, how to get large amounts of sufficiently pure U235 from ordinary uranium. U235 makes up only 1% of the uranium dug out of mines. It looked the same and had the same characteristics. Chemical separation was impossible. And how do you manage to slow down the fast moving neutrons expelled from the uranium nucleus. If you didn't slow them down, the chain reaction might go haywire in a nuclear reactor (bad if you're trying to study nuclear reactions) and in a bomb the bomb might blow itself apart before releasing its atomic energy. Also, how much U235 would be needed for a bomb. Too small an amount of uranium and the chain reaction would never get off the ground – too many neutrons would escape. The amount of uranium needed to create an explosion was called the “critical mass” and opinions varied as to how much was needed – from ten to several hundred pounds. In Spring 1940, Otto Frisch (by then in England) calculated that if you had very pure U235 and packed it tightly then you didn't need to slow the neutrons down for a chain reaction to release massive amounts of energy all at once. Frisch was one of the first to realize that an atomic bomb was feasible, and that it would take only a few kilograms of pure U235 to produce a bomb. Frisch had direct experience of the Nazis – they had put his father in a concentration camp after Austria was annexed (Frisch managed to get him out before the war). He quickly convinced other British scientists and in 1941 British Prime Minister Winston Churchill gave the go ahead for a program to build an atomic bomb. Churchill said: “Although personally I am quite content with the existing explosives, I feel we must not stand in the path of improvement.” Britain set up factories to separate U235. But soon the British effort merged with the American effort in the US. In 1941 theoreticians predicted that if a neutron was absorbed by U238 (the other isotope of uranium) it would over a few days produce a completely new element, which they called plutonium (partly after Pluto, the Roman god of the dead). Plutonium was easier to deal with than U235 - no problems with separating out the fissile material - and it appeared as though plutonium would make a successful bomb. The end result was a two pronged project – producing a uranium bomb and a plutonium bomb. In the meantime costs in the US had escalated and for not much in the way of practical results. Bush was under pressure to focus on projects with a more immediate benefit – such as radar. In April 1941 he decided it was time to make a decision – to suspend the nuclear work or go all out. He decided to go all out and reorganized the way work was done. He created three research centers, at Columbia University, at the University of Chicago, and at the University of California, Berkeley. Columbia took up the problem of how to separate the two uraniums. Chicago focused on designing nuclear reactors and and designing the weapon itself. (Later weapon design was transferred to Los Alamos in New Mexico). Berkley ended up supporting the Los Alamos work. For a project of this size and organization, Bush needed a lot of money, and he needed security, (that is, not asking Congress for the money).
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