Nuclear fusion: Hydrogen bomb and Stellar energy

Nuclear fusion

Nuclear fusion is a process in which two or more lighter nuclei combine to form a heavier nucleus. The mass of the product nucleus is always less than the sum of the masses of the individual lighter nuclei. The difference in mass is converted into energy. The fusion process can be carried out only at a extremely high temperature of the order of 10⁷ K because, only at these very high temperatures the nuclei are able to overcome their mutual repulsion. Therefore before fusion, the lighter nuclei must have their temperature raised by several million degrees. The nuclear fusion reactions are known as thermo-nuclear reactions.

Hydrogen bomb

The principle of nuclear fusion is used in hydrogen bomb. It is an explosive device to release a very large amount of energy by the fusion of light nuclei. The temperature required for the purpose of fusion is produced by fission reactions. The explosion of an atom bomb produces temperature of the order of 50 million degree celcius. A suitable assembly of deuteron and triton is arranged at the sight of the explosion of the atom bomb. Favourable temperature initiates the fusion of light nuclei in an uncontrolled manner. This releases enormous amount of heat energy.

The fusion reaction in hydrogen bomb is

₁H³ + ₁H²  -- -- > ₂He⁴ + ₀n¹ + energy

Stellar energy

Fusion is the source of stellar energy. The temperature of the interior of the sun is about 1.4 × 10⁷ K and the temperature of some stars is of the order 10⁸ K. It has been estimated that the total energy radiated by sun is about 3.8 × 10²⁶ joule per second. The origin of such a tremendous amount of energy is neither chemical nor gravitational. The fusion of protons into helium is supposed to release energy in sun and stars. All the elements like hydrogen and helium are in a highly ionised state called plasma at such a high temperature. The energy produced in fusion is responsible for the maintenance of high temperature of the sun and stars and also for the emission of energy in the form of heat and light.

Proton−proton cycle and carbon−nitrogen cycle are the two important types in which nuclear fusion takes place in sun and stars.

Proton - Proton cycle

₁H¹ + ₁H ¹ → ₁H² +₁e ⁰ + ν (emission of positron and neutrino) ₁H¹ + ₁H² → ₂He³ + γ (emission of gamma rays)

2₂He³  →  ₂He⁴   +          2₁H¹

The reaction cycle is written as

4₁H¹ →  ₂He⁴ + 2₁e⁰ + 2ν + energy (26.7 MeV)

Thus four protons fuse together to form an alpha particle and two positrons with a release of large amount of energy.

Carbon - Nitrogen Cycle

The following cycle of reactions take place in carbon - nitrogen cycle in which carbon acts as a catalyst.

₁H¹ + ₆C¹² → ₇N^13* + γ (emission of gamma rays)

₇N^13* → ₆C ¹³ + ₁e⁰ + ν (emission of positron and neutrino) ₁H¹ + ₆C¹³ → ₇N¹⁴ + γ (emission of gamma rays)

₁H¹ + ₇N¹⁴ → ₈0^15* + γ (emission of gamma rays)

₈0^15* → ₇N¹⁵ + ₁e⁰ + ν (emission of positron and neutrino)

₁H¹ + ₇N¹⁵ → ₆C¹² + ₂He⁴

The overall reaction of the above cycle is given as 4 ₁H¹ → ₂ He⁴ + 2₁e⁰ + 2ν + energy (26.7 MeV)