IGNOU MPHE 26 2026 - Describe the Fermi-age theory and derive Fermi-age approximation

Question

Describe the Fermi-age theory and derive Fermi-age approximation of slowing down equation for a weak absorbing medium. Discuss the S-G approximation.

19 Jan 2026

Answer :

Word Count : 446

The Fermi-age theory is used to describe the slowing down of neutrons in a moderating medium, particularly in systems where absorption is weak. It provides a statistical description of how fast neutrons lose energy through scattering until they reach thermal energies. The theory assumes isotropic scattering and treats the neutron slowing down as a diffusion-like process in energy space. In a weakly absorbing medium, the neutron balance in energy can be written in terms of the neutron flux ( \phi(E) ) as: [ * \frac{d}{dE} \left[ \xi \Sigma_s(E) \phi(E) \right] + \Sigma_a(E) \phi(E) = S(E) ] Here, ( \Sigma_s(E) ) is the scattering macroscopic cross-section, ( \Sigma_a(E)

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