Question 9: What is meant by nuclear fusion? Discuss how can energy be released in the fusion process? Illustrate with examples.

#### Nuclear Fusion

When two lighter nuclei (A<20) combine to form a heavier nucleus with the release of energy, the process is called nuclear fusion.

Explanation: Fusion is the reverse of nuclear fission process. However, release of huge amount of energy is common in both processes. The rest mass of the heavy nucleus is less than the rest masses of the individual nuclei which are reacting. The mass decreased is, therefore, converted into energy in line with Einstein’s mass energy equation.

Examples: Nuclear fusion takes place under stellar conditions two different processes.

• In sun or other stars, four protons combine to form Helium nucleus. This process is called proton-proton cycle. This happens in a number of steps; the direct collision of protons, which results in the formation of heavy nuclei and again the collision of heavy nuclei, yields Helium nuclei. It is summarized as below.

1H1 + 1H1      ———-→           1H2 + e+ + ν (neutrino)

The deuteron thus produced combines with another proton.

1H1 + 1H2          ———-→          2He3 + γ

Finally, two 2He3 nuclei react to form Helium nuclei and two protons.

2He3 + 2He3          ———-→          2He4 + 1H1 +1H1

Total energy evolved in the process is 24.7 MeV.

(2) The second process that could place in stars is the carbon cycle. It takes place in a series of steps.

1H1 + 6C12          ———-→          7N13

7N13        ———-→           6C13 +e+ + ν

1H1 + 6C13             ———-→           7N14 + γ

1H1 + 7N14             ———-→           8O15 + γ

8O15        ———-→           7N15 + e+ + ν

1H1 + 7N15             ———-→           6C12 + 2He4

Again, an alpha particle is formed; two positrons are released with 24.7 MeV of energy.

This idea was first given by Edington in 1920. He said that in sun and other stars protons are converted into helium nuclei and a tremendous amount of energy is released. Stars have very high interior temperatures (for sun it is 2 x 106 K). Due to high temperature, the atoms are in the ionized form and moving with very high speeds. Due to extremely high speeds, the protons (Hydrogen nuclei) overcome the electrostatic force and are fused together. When they fuse together, relatively larger nuclei are formed and energy is released. The energy liberated in the fusion of light nuclei into heavier ones is called thermonuclear energy.