In 1938, Ottohann and Stresemann bombarded slow neutrons on uranium. As the neutron is a neutral particle, so it was expected that the uranium nucleus will absorb the neutron and become a heavy uranium nucleus. But what happened was, that the uranium nucleus itself broke into two equal parts. In this process two or three neutrons were also liberated. This experiment can be indicated by
Here Ba and Kr denotes barium and Krypton respectively. In this process of breaking a heavy element into two nuclei, large amount of energy sis released.
Breaking of a nucleus into two parts with the release of large amount of energy is called fission reaction.
In the fission reaction, when a nucleus breaks, the sum of the masses of the produced nuclei and the neutrons is less than the release of energy according to Einstein’s mass-energy equation.
Fig. 18.2 fission chain reaction
When a neutron reacts with a uranium nucleus, two or three neutrons are released. Every one of these reacts with next nuclei producing two or three more neutrons and hence, the number of available neutrons and the fission goes on increasing the fission reaction thus becomes uncontrollable and hence whole of the matter explodes, which results in great destruction. Such a reaction is used in atomic bomb.
If the fission reaction is controlled, then the desired amount of energy can be produced which may be used in a useful manner. A system in which the fission reaction is controlled is called a controlled fission reaction. In this way the surplus neutrons are absorbed by boron or cadmium rods (fig. 18.3). This is done in a nuclear reactor.
Fig. 18.3 Controlled fission reaction
Fusion Reaction
We have already read that breaking of a heavy nucleus into two comparatively smaller nuclei is called fission. Let us now study its reverser process.
Since neutrons and protons are present inside a nucleus, so the total mass of the nucleus should be equal to the total sum of the masses of the neutrons and protons. But when the mass of a nucleus was determine d it was less than the total mass of the nucleons. For example, if we determine the mass of a helium nucleus, it should be equal to the total mass of two neutrons and two protons but it is little less than the total mass. We can explain this difference in mass by Einstein’s mass-energy equation. If the deficiency of mass is m then energy 
will be liberated on the formation of the nucleus.
If an atom of deuterium is fused with an atom of tritium then a helium nucleus or a particle is formed.
This process is expressed by the following equation:
A reaction in which a few small nuclei form a heavy nucleus is called the fusion reaction.
More energy is released in the fusion reaction as compared to the fission reaction in a fusion reaction if isotopes of Hydrogen form a helium nucleus, nearly 17.6 MeV of energy is liberated. But it is very difficult to produce a fusion reaction. When nuclei are brought near each other for fusion, work has to be done against the electrostatic force which requires great amount of energy or heat for the fusion reaction the required temperature is achieved by exploding a fission bomb i.e., and atom bomb inside a hydrogen bomb. Efforts are being made to achieve energy through fusion controlling the fission reaction, and then we will be able to get an infinite store of energy without radiation hazards.
Fusion Reaction
It is considered that the energy coming from the sun and the stars is due to the fusion of nucleons, for producing a particles i.e., the hydrogen is converted into helium because the temperature inside the sun is very high. For example the temperature at the centre of the sun is nearly 20 million Kelvin whereas it is nearly 5million Kelvin at the surface. The scientist, Bethe had suggested that a fusion chain reaction is taking place inside the sun. According to this reaction when 4 hydrogen nuclei fuse together to form a helium nucl`eus, then along with two positrons and there a rays, nearly 25.7 MeV of energy is also released. This reaction is responsible of the solar and sateller energy.
Do You Know?
Positron is a particle with mass equal to the mass of an electron having opposite and equal charge.