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3-3: Nuclear Fission

On the previous page: 3-1:The Origin of the Nuclear Energy, the fission of uranium-235 ( ) was taken up as an example of nuclear exothermic reaction, in which a huge amount of energy is released when neutrons being bomberded against uranium-235 nuclei to split them into several fragments. One of the equations for these fission processes is written as

Here Q is the released energy which is more than 200 MeV per one process. "2n" on the right-hand side means that two neutrons are simultaneously emitted. A schematic sketch of this fission process is drawn as the following figure.

[Schematic Drawing of the Fission Process of Uranium-235]
When neutrons are bombarded against uanium-235, the uranium-235 nucleus absorbs a neutron to be a uranium-236 and it splits into two fragments of almost equal masses with emitting some neutrons simultaneously and evolving a huge amount of energy larger than 200 MeV.
This is nothing but one of the examples of fission of uranium-235. It does not always split into Ba and Kr but usually into two fragments with almost equal masses. The number of emitted neutrons is also not always constant but it distributes over one to several. Then the emitted energy is not always constant but is almost 200 MeV.
The above equation is one of the examples of fission of uranium-235. There are many other processes of which examples are as follows:

[The Discovery of Nuclear Fission]
Rutherford carried out alpha-particle scattering experiments by bombarding the alpha rays emerging from radioactive Polonium (Po) or Radium (Ra) elements onto various nuclei. Thereby he succeeded in the first artificial transmutation of elements (1919).
Since then, experiments of alpha-particle scattering on nuclei had been repeated extensively. Chadwick investigated the mysterious penetrating radiation emitted from a beryllium (Be) target which is bombarded by high-energy alpha particles from a Po source. Finally he concluded that this mysterious radiation is a neutral particle with protonic mass. This was the discovery of neutron (1932).
O. Hahn (Germany, 1879 - 1968) and F. Strassmann (Germany, 1902 - 68) carried out the experiments to bombard neutrons from (Po-Be) source against uranium. They observed an important phenomenon in which the uranium nucleus bombarded by neutrons splits into two large fragments. Among the reaction products, they found Barium (Z = 56) and Lanthanum (Z = 57).
L. Meitner (Germany, 1878 - 1968), a former collaborator of Hahn, and her nephew O. R. Frisch, who had fled to Sweden from Nazi Germany, were reported these experimental results by a letter. They analyzed these results and immediately recognized the significance. They were convinced that the experimental results are due to the neutron-induced nuclear fission and they published this idea.
This was the first discovery of nuclear fission.

[The Mechanism of Nuclear Fission]
As explained on the page: 3-1: The Origin of the Nuclear Energy, if such a heavy nucleus as uranium-235 splits into two fragments of almost equal mass, each of them would be bound more tightly than the original nucleus. In that case, an energy excess is brought about and the state after the reaction becomes more favorable than before the reaction. Accordingly, a heavy nucleus has always a possibility of nuclear fission.
Using the idea of Liquid Drop Model, let us explain in what mechanism such a heavy nucleus as uranium-235 fission.
The nucleus is a many-nucleon system consisting of protons and neutrons. It can be assumed to be something like a drop of water or a raindrop. A heavy nucleus is therefore considered to be very "soft" and "flabby" and easy to deform. However, since a strong nuclear force works between nucleons and it plays a role of surface tension, the ground state of a nucleus is rather stable and does not directly fission. If some amount of energy is given to a nucleus to be in an exited state, then it easily deforms and fissions. You would be able to understand the fissioning mechanism by the following two figures.

[Fission Mechanism by Liquid Drop Model]
If such a heavy nucleus as uranium-235 absorbs a neutron, it is in an excited state. The soft nucleus is easy to deform. Sometimes it becomes dumbbells shape and splits into two droplets. At that time the energy excess is released.



[Schematic Graph of Energy in Nuclear Fission]
This diagram schematically shows the potential energy for nuclear fission as a function of the degree of nuclear deformation or the average separation of the two fission fragments. The ground state of the nucleus is confined within the valley of potential barrier and is stable. When it is excited to be close to the top of the barrier, it penetrates the barrier with the tunnel effect and run down along the potential slope to fission.

[The Chain Reaction]
As shown in the equation at the top of this page, the fission of a uranium-235 is accompanied by the emission of neutrons. There are several types of fission processes in the uranium-235 fission, in each of which one, two or three neutrons are emitted; on average, 2.5 neutrons are released. These neutrons are capable to cause further fission of uranium-235. If each fission causes an average of more than one further fission, an avalanche of fission reactions will result. This is called the chain reaction.
As soon as Hahn and Strassmann discovered nuclear fission, many physicists noticed the possibility of the chain reaction. If the chain reaction occurs very rapidly in an uncontrolled way, a violent explosion follows. Then it can be used for a military purpose; i.e. an atomic or strictly speaking a nuclear bomb. If the chain reaction is well controlled, then it can be used as a power source.
The first experiment of nuclear chain reaction was carried out in 1942 by E. Fermi (Italy, USA, 1901 - 54), who had fled to US from Fascists Italy, and his colleagues.

[Nuclear Reactor]
A device in which controlled nuclear fission takes place to produce heat or energy is called a nuclear reactor. It is the main facility of a nuclear power station. The first nuclear reactor was constructed by Fermi and coworkers at the University of Chicago in 1942.

[Other Kinds of Fissionable Materials]
An elemental isotope that undergoes induced fission when being struck by a free neutron is usually called "fissionable". Of course, the most popular fissionable material is uranium-235 ().
One of the other well-known fissionable materials is pultonium-239 (), which is usually produced in a reactor from uranium-238. (Uranium-238 is not fissionable, so that it does not "burn" in a reactor.) While almost all pultonium is artificially produced, extremely tiny amount is found naturally.
Another kind of element that can be used as a nuclear fuel in reactors is thorium, which occurs naturally.
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