06 - Lecture 6 The Gadget Thin Man Fat Man Little...

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Lecture 6: The Gadget Thin Man, Fat Man & Little Boy (1943-1945) 1. To Build a Nuclear Bomb: “The Gadget” A. To Use Fusion? Teller pushes it. Two problems in the 1940s, scientists were unsure you could make the bomb. They were more sure the fission bomb was possible. Second, in order to make a fusion (hydrogen) bomb, you needed a fusion bomb already. You needed an atomic bomb to set off a hydrogen bomb. When they started collecting at Los Alamos, they wanted to do fission bomb first. They gave Teller money, but low priority. B. To Use Fission? What isotope to use? That is the first question. Groves answers the question and decides to make machines for both. i. To Use U235? Absorbs neutron, two or more neutrons on average. The energy that is released is the strong force of the nuclear reaction. ii. To Use Pu239? With plutonium, it's more stable. Fissiong of Plutonium spreadsheet. iii. To Use Some Other X??? 2. Nuclear Fission & Bombs A. Fast Neutrons & Geometric Generation Each ounce of uranium gives you something, but if you have billions of it happening every second, you have a huge chain reaction. You have to have enough nuclei packed together that the probability that enough neutrons will find the nucleus. THe odds are that the neturons are going to escape. Los ALamos wants to make sure that neutron generation works (slide). Multiplier effect: each generation doesn't double, there isa factor. One of the things that Los Alamos figures out, is that the standard process goes through 80 generations. Each generation goes through a billionth of a second. B. “Critical” vs. “Supercritical” Chain Reactions: Reactors vs. Bombs If people are just talking about bombs, they are talking about "critical." Nuclear reactors involve critical mass and chain reaction, and a bomb involves a supercritical mass and supercritical chain reaction. Critical mass, is a mass density (amount of mass in matter, in a certain amount of density). If you increase the density, you need more mass. Fermi was trying to discover the critical mass for his pile. C. The Core: The Nature & Problem of a “Supercritical Mass” Supercritical mass is the next step up. It's going to be a mass volume relation so that once you have that and it gets neutron in it, the chain reaction will be uncontrollable. It will explode. One of the things Los Alamos has to do is to figure out how much uranium is standard volume, how much to work with? Same with plutonium. Looking for the minimum amount you need, once you get that, the chain reaction will explode. Suppose the figure out, with a certain dense of uranium, you get a supercritical mass. You find out the supercritical mass of U235. If you bring a supercritical mass to class, once you determine that mass, you can't store uranium and plutonium together. Once

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it's together that the neutrons in the room that it might not explode the minute you walk in, but it will. The core of a nuclear weapon has to be kept apart.
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