Dear All,
The neutron cross section of element generally falls with the increase in neutron energy. But in U-238 the fission cross section at Neutron energy of 1MeV is 0.3 barns whereas it increases to 1 barn at neutron energy of 10MeV. Why does the neutron cross section increases with increase in neutron energy for U-238?
I will be very much thankful for the reply.
This is not specific to U-238, but to fission. Beyond the fission threshold, the reason is the opening of the second-chance fission (n,nf) at about 6 MeV, followed by the third-chance fission (n,2nf) at about 12 MeV, etc... Actually, the (n,f) cross section is the sum of the fission cross sections of the compound nuclei A+1, A, A-1, etc... Each of these partial fission cross sections reaches a plateau (or decreases), but the sum increases with neutron energy.
This is not specific to U-238, but to fission. Beyond the fission threshold, the reason is the opening of the second-chance fission (n,nf) at about 6 MeV, followed by the third-chance fission (n,2nf) at about 12 MeV, etc... Actually, the (n,f) cross section is the sum of the fission cross sections of the compound nuclei A+1, A, A-1, etc... Each of these partial fission cross sections reaches a plateau (or decreases), but the sum increases with neutron energy.
Another reason might be resonance enhancement. At energies around and above 10 MeV. The newly formed compound nucleus has the Giant Dipole Resonance, which acts as doorway state. The GDR is a quasicontinuum of close lying level. All of them have sizable neutron components. So 10 MeV kinetic energy + neutron binding energy one should end up nicely in the GDR region.
The question is not quite correct.Because the thermal neutrons(0.25 eV) which has the required cross section of interaction with U(235) which causes its fission,while its interaction cross ection withU(238) is very small, it does not cause the fission of
U(238),which is fissioned by a fast neutrons(1MeV and more). This depends on the potential barrier of each element.So we say thermal neutrons which is well defined by its very low energy,not slow neutrons,because they might be with0.45 MeV which is slow relative to those with 0.64MeV.The thermal neutron is the right physical term.
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