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THE ATOMIC BOMB Seven months after the discovery of the neutron, on September 12, 1932, Leo Szilard inaugurated the possibility of a controlled release of atomic power through a multiplying neutron chain reaction. Leo was legally recognized as the inventor of the atomic bomb on July 4, 1934. He described not only the basic concept of using neutron induced chain reactions to create explosions, but also the key concept of the critical mass. There are 2 types of atomic explosions, one in which two fragments of comparable mass, with the evolution of approximately 100 million to several hundred million volts of energy. This energy is expelled explosively and violently in the atomic bomb. The second type of atomic explosion derives its power from the fusing of nuclei of various hydrogen isotopes in the formation of helium nuclei.
The massive power behind the reaction in an atomic bomb arises from the forces that hold the atom together. These forces are akin to, but not quite the same as, magnetism. Atoms are comprised of three sub-atomic particles. Protons and neutrons cluster together to form the nucleus (central mass) of the atom while the electrons orbit the nucleus much like planets around a sun.
It is these particles that determine the stability of the atom. Most natural elements have very stable atoms which are impossible to split except by bombardment by particle accelerators. For all practical purposes, the one true element whose atoms can be split comparatively easily is the metal Uranium. Uranium's atoms are unusually large, henceforth, it is hard for them to hold together firmly.
It is also a heavy metal, heavier than gold, and not only does it have the largest atoms of any natural element, the atoms that comprise Uranium have far more neutrons than protons. This does not enhance their capacity to split, but it does have an important bearing on their capacity to facilitate an explosion. There are two isotopes of Uranium. Natural Uranium consists mostly of isotope U- 238, which has 92 protons and 146 neutrons (92 + 146 = 238).
Mixed with this isotope, one will find a 0. 6 % accumulation of U- 235, which has only 143 neutrons. This isotope, unlike U- 238, has atoms that can be split, thus it is extremely useful in making atomic bombs. Being that U- 238 is neutron-heavy, it reflects neutrons, rather than absorbing them like its brother isotope, U- 235. (U- 238 serves no function in an atomic reaction, but its properties provide an excellent shield for the U- 235 in a constructed bomb as a neutron reflector. This helps prevent an accidental chain reaction between the larger U- 235 mass and its 'bullet counterpart within the bomb.
Also note that while U- 238 cannot facilitate a chain-reaction, it can be neutron-saturated to produce Plutonium (Pu- 239). Plutonium can be used in place of Uranium- 235 {albeit, with a different model of detonator} in an atomic bomb. Both isotopes of Uranium are naturally radioactive. Their bulky atoms disintegrate over a period of time. Given enough time, (over 100, 000 years or more) Uranium will eventually lose so many particles that it will turn into the metal lead. However, this process can be accelerated.
This process is known as the chain reaction. Instead of disintegrating slowly, the atoms are forcibly split by neutrons forcing their way into the nucleus. A U- 235 atom is so unstable that a blow from a single neutron is enough to split it and henceforth bring on a chain reaction. This can happen even when a critical mass is present.
When this chain reaction occurs, the Uranium atom splits into two smaller atoms of different elements, such as Barium and Krypton. When a U- 235 atom splits, it gives off energy in the form of heat and Gamma radiation, which is the most powerful form of radioactivity and the most lethal. When this reaction occurs, the split atom will also give off two or three of its 'spare neutrons, which are not needed to make either Barium or Krypton. These spare neutrons fly out with sufficient force to split other atoms they come in contact with. In theory, it is necessary to split only one U- 235 atom, and the neutrons from this will split other atoms, which will split more and so forth.
This progression does not take place arithmetically, but geometrically. All of this will happen within a millionth of a second. The minimum amount to start a chain reaction as described above is known as SuperCritical Mass. The actual mass needed to facilitate this chain reaction depends upon the purity of the material, but for pure U- 235, it is 110 pounds (50 kilograms), but no Uranium is never quite pure, so in reality more will be needed. Uranium is not the only material used for making atomic bombs. Another material is the element Plutonium, in its isotope Pu- 239.
Plutonium is not found naturally (except in minute traces) and is always made from Uranium. The only way to produce Plutonium from Uranium is to process U- 238 through a nuclear reactor. After a period of time, the intense radioactivity causes the metal to pick up extra particles, so that more and more of its atoms turn into Plutonium. Plutonium will not start a fast chain reaction by itself, but this difficulty is overcome by having a neutron source, a highly radioactive material that gives off neutrons faster than the Plutonium itself.
In certain types of bombs, a mixture of the elements Beryllium and Polonium is used to bring about this reaction. Only a small piece is needed, because the material merely acts as a catalyst to the greater reaction.
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