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Additional recommended knowledge
Scattering versus absorption
When a neutron approaches an atomic nucleus, it will be scattered or absorbed. If absorbed, the atomic nucleus moves up on the table of isotopes by one position; for instance, U-235 becomes U-236* with the * indicating the nucleus is highly energized. This energy has to be released and the release can take place through any of several mechanisms.
Types of scattering cross-section
The scattering cross section can be further subdivided into coherent scattering and incoherent scattering, which is caused by the spin dependence of the scattering cross section and for a natural sample, presence of different isotopes of the same element in the sample.
Since neutrons interact with the nuclear potential, the scattering cross section varies with the atomic number of the element in question. A very prominent example is hydrogen and its isotope deuterium. The total cross section for hydrogen is over 10 times that of deuterium, mostly due to the large incoherent scattering length of hydrogen. Metals tend to be rather transparent to neutrons, aluminum and zirconium being the two best examples of this.
Types of decay
The element uranium decays in the following manner: U-235+n=U-236*. U-236*(-gamma ray)=U-236. U-236+n=U-237*. U-237*(-Beta)=Np-237.
Because a large number of the isotopes of the elements in the actinide series are fissionable via neutron absorption, the higher an element is on the table of isotopes the more rarely it is formed by these reactions. As an example Th-232 has a half life on the order of 14 billion years and is the most common of the actinide series on Earth. Adding neutrons and allowing for beta decay and fission events you can build from Th-232 up to any arbitrary member of the Actinides like Pu-242. This chain moves through one of the following decay sequence:
With elements lower on the periodic table than the actinides the predominant form of emission is gamma or beta decay. As an example, when stable O-18 absorbs a neutron it becomes O-19*, then decays to O-19*(-Beta)=F-19.
A few rare isotopes undergo alpha decay, most notably Li-7+n=Li-8*; Li-8*(-1 Beta, (-2 Alpha))= 2(He-4) AND B-11+n= B-12*; B-12*(-1 Beta,(-3 Alpha))=3(He-4)OR B-12*(-Beta)=C-12. The time for these reactions to occur are under 25 milliseconds.
Because Li-8 and B-12 form natural stopping points on the table of isotopes for hydrogen fusion it is believed that all of the higher elements are formed in very hot stars where higher orders of fusion predominate. A star like the Sun produces energy by the fusion of simple H-1 into He-4 through a series of reactions. It is believed that when the inner core exhausts its H-1 fuel the sun will contract slightly increasing its core temperature until He-4 can fuse and become the main fuel supply. Pure He-4 fusion would lead to Be-8, which decays back to 2(He-4)therefore the He-4 must fuse with isotopes either more or less massive than itself to result in an energy producing reaction. When He-4 fuses with H-2 or H-3 it forms stable isotopes Li-6 and Li-7 respectively. The higher order isotopes between Li-8 and C-12 are synthesized by similar reactions between Hydrogen, Helium and Lithium isotopes.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Neutron_cross-section". A list of authors is available in Wikipedia.|