IGNOU MPHE 26 2026 - Draw and explain the energy variation of fission cross-sections for&nb

Question

Draw and explain the energy variation of fission cross-sections for $^{238}_{92}\text{U}$ and $^{232}_{90}\text{Th}$ .

19 Jan 2026

Answer :

Word Count : 389

The fission cross-section, ( \sigma_f ), represents the probability of a nucleus undergoing fission when struck by a neutron. Its variation with neutron energy depends on whether the nucleus is fissile or fertile and on resonance effects. For ( ^{238}*{92}\mathrm{U} ) and ( ^{232}*{90}\mathrm{Th} ), both are fertile nuclei, meaning they are not readily fissile with thermal (slow) neutrons but can undergo fission with fast neutrons or after breeding into fissile isotopes. At very low neutron energies (thermal region, ( E_n \sim 0.025,\mathrm{eV} )): * ( ^{238}\mathrm{U} ) has a very low fission cross-section, practically zero, because

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