Question

Which is a good absorber of neutrons ? Uranium 235 or Uranium 238 ?

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U235 and U238 are two member of Uranium isotope. Atomic number of both is 92, while neutron numbers of U238 and U235 are 146 and 143, respectively. Therefore, Formed compound nuclei by absorption of one neutron in 238U and 235U are 239U* and 236U*. It is clear that the excitation energy of 236U* is greater than due to pairing energy compared to the excitation energy of 239U* if kinetic energy of absorbed neutrons are the same . As a result, 235U undergoes fission by the absorption of neutron with very low energy while 238U undergoes fission by the absorption of fast neutron(E_n>0.6 MeV). Atomic percentage of U235 and U238 in natural uranium are about 0.72% and 99.28% respectively. U238 also undergoes spontaneous fission with very low rate, so that the spontaneous fission half life of this nucleus is about 10^15years. The half life of U235 is smaller than the half life of U238 by a 10 factor, approximately. Due to this fact, in the future the atomic percentage of U235 becomes smaller than 0.72%.