As the neutron energy increases the material cross section decreases. Why these effect happens?
The neutron cross-section is variable and depends on:
Target nucleus (hydrogen, boron, uranium, etc.) Each isotope has its own set of cross-sections.
Type of the reaction (capture, fission, etc.). Cross-sections are different for each nuclear reaction.
Neutron energy (thermal neutron, resonance neutron, fast neutron). For a given target and reaction type, the cross-section is strongly dependent on the neutron energy. In the common case, the cross section is usually much larger at low energies than at high energies. This is why most nuclear reactors use a neutron moderator to reduce the energy of the neutron and thus increase the probability of fission, essential to produce energy and sustain the chain reaction.
Target energy (temperature of target material – Doppler broadening) this dependency is not so significant, but the target energy strongly influences inherent safety of nuclear reactors due to a Doppler broadening of resonances.
Energy dependent cross sections for neutrons:
http://physics.stackexchange.com/questions/106532/energy-dependent-cross-sections-for-neutrons
For the basics of neutron interaction with matter::
https://www.fas.org/sgp/othergov/doe /lanl/lib-www/la-pubs/00326407.pdf
http://physics.stackexchange.com/questions/106532/energy-dependent-cross-sections-for-neutrons
And to add to F. Glodeanu:
- at least the absorption cross section roughly scales by sqrt{E}, which means it scales by velocity. The slower the neutron, the higher the probability of interaction when passing a nearby nucleus.
- there is a difference between a material cross section and an elemental cross section for thermal neutrons. This especially refers to hydrogen, which when bound for example in water has a different cross section than when free or gaseous.
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