Use of nuclear reaction models in cross section calculations Page: 3 of 26
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Use of Nuclear Reaction Iodels in Cross Section Calculations
Because of the difficulty of cross section measurements involving
neutrons, most studies of nuclear reaction mechanisms have utilized
charged particles. It is normally argued that because of the similarity
between the nuclear interactions of protons and neutrons with nuclei and
because of the poorer quality of neutron data that it is more productive
to concentrate on the charged particle studies. On the other hand, from
a technological standpoint, the neutron cross sections are often the ones
which are of greatest interest. I would like to summarize some procedures
which are of value in estimating neutron induced cross sections based in
large Part on reaction systematics deduced from charged particle studies.
Since neutron cross sections c- 14 MeV are of particular interest to the
CTR program, the emphasis will be on cross sections at that energy, although
the same procedures would be useful both above and below that energy.
The most widely used model in estimating neutron cross sections is the
statistical model (compound nucleus). This reaction mechanism yields (in
general) spectra which are smooth in angular dependence and which are
dominated by low energy neutrons (il MeV) and charged particles just above
the Coulomb barrier. Extensive studies of the reaction mechanisms of
nucleon induced reactions in the energy range below 20 MeV (and for A Z 20)
have indicated that a large fraction (>80%)of the reactions proceed through
formation of a compound nucleus. Thus, this contribution must be included
in calculating neutron cross sections. The parameters needed for such a
calculation include transmission coefficients and level density parameters.
Compilations listing average values for these quantities are available and
many such calculations have shown that the averaged parameters produce
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Grimes, S.M. Use of nuclear reaction models in cross section calculations, article, March 1975; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc1015937/m1/3/: accessed June 17, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.