Word Count : 389

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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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