Nuclear Fission and Fusion - Part II

2. Forces inside the nucleus
Let us now look at the nucleus a bit more carefully. It has positive protons bound together. Why don’t the positively protons fly apart due to coulomb’s repulsion? Like charges repel each other.

There is very strong force acting within the nucleus, which is able to overcome the Coulomb’s repulsion. The strong forces do not depend on the charge of the particles. Strong forces between proton-proton, neutron-proton and neutron-neutron are found to be identical. Strong forces can be experienced only in the dimension of the nucleus (10^-15m).  Strong force does not differentiate whether the particle is a proton or a neutron, hence these particles together are referred to as nucleons.

It is seen that in nature, for low mass atoms, N and Z are nearly equal. But as Z increases, N becomes more than Z, so as to counter balance the proton-proton coulomb repulsion.

But as mass of atoms becomes very large, they tend to be unstable and break up. The break up results in emission of energetic particles.

Unstable nucleus is called a radioactive nucleus. The instability in the nucleus can be spontaneous or can be induced by a nuclear reaction.

The energetic radiation given off by a radioactive nucleus is called radioactivity.

Radioactivity is categorized as :

Alpha particle : ( - particles)          : these are positively charged nucleus of helium ⁴₂He.
Beta particles : (- particles)           : these are negatively charged electrons or positively
                                                       charged positrons.
Gamma rays  : (- rays )                 : these have no charge and are electromagnetic radiation.

Very rarely a radioactive atom may also emit neutrons.

3. Nuclear fusion and nuclear fission reactions
There are two basic types of nuclear reactions : Nuclear fusion reaction and nuclear fission reaction.

Nuclear fusion reaction, where low mass nuclei fuse to form high mass nucleus and give off large amounts of energy and radiation.

Nuclear fusion reactions are never spontaneous. They have to be induced by bringing the fusing nuclei close together by increasing the temperature of the reactants. These types of reactions happen inside the core of the stars, where temperatures are very high.

Nuclear fission reaction, where a heavy mass nucleus breaks up into low mass stable nuclei. Emission of radioactivity and large amounts of energy accompany this.

This is an example of induced nuclear reaction.

This is an example of spontaneous nuclear reaction.

Some heavy nuclei spontaneously break up. There are many examples of this phenomenon. ²³⁵₉₂U is one such nucleus, but it needs a very slow moving neutron to start the initial reaction. The neutron enters the nucleus and makes it highly unstable. The nucleus breaks up into two medium mass nuclei and gives off some extra neutrons. The mass of the resultant nuclei and neutrons liberated is less than that of the parent nucleus. The missing mass appears as enormous amounts of energy. The conversion of mass into energy occurs. The energy released is according to Einstein’s equation E = mc².  The energy appears in terms of heat and light energy and -radiation.

The fission products are not necessarily barium and krypton. There is a distribution of masses in the fission products. 

There are many nuclei, which undergo fission reaction that is initiated by slow moving neutrons. Some of them are ²²⁷₉₀Th (Thorium-227), ²³⁰₉₀Th (Thorium-230), ²³¹₉₂U (Uranium-231), 
²³⁹₉₄Pu (Plutonium-239), etc.

The diagram below shows how ²³⁸₉₂U spontaneously fissions and stabilizes when ²⁰⁶₈₂Pb nucleus is reached.   

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