THERMAL NEUTRON CROSS SECTIONS AND DEFINITION OF THE f-FACTOR IN BNL-325 Page: 4 of 10
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THERMAL NEUTRON CROSS SECTIONS AND DEFINITION
OF THE f-FACTOR IN BNL-325
J. A. Harvey
Oak Ridge National Laboratory
The absorption and .fission cross sections of the fissionable nuclides
U23, U235, and Pu239 have been measured with monoenergetic neutrons to'an
accuracy of a few percent in the thermal neutron energy range. The cross
sections are not 1/v and, hence, when a distribution of neutron velocities is
used, such a Maxwellian velocity distribution, -the cross section must be
averaged.over the distribution. There has been considerable misunderstanding
with regard to the definition of the f-factor in the Neutron Cross Section
Compilation BNL-325. The f-factor was introduced as the correction factor
needed when using a Maxwellian velocity neutron distribution instead of mono-
energetic neutrons. The effective cross section for a Maxwellian, velocity
distribution at a temperature, To, or at. an energy, Eo, (corresponding to the
energy of the neutron with the most probable velocity, vo) is related to the
cross section for monoenergetic neutrons of energy Eo by the expression
eff (Eo) = f (Eo) C (o).' (1)
For a 1/v law, f(Eo) = 1 and deff(Eo) = c~~ (Eo).
For a Maxwellian spectrum the f-factor for a particular process x is
defined by the following equation:
6'x(E) (nv) dv
fx(Eo) = (2)
'x(Eo) -.. (nv) dv
-1-394 002
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Harvey, J.A. THERMAL NEUTRON CROSS SECTIONS AND DEFINITION OF THE f-FACTOR IN BNL-325, report, June 1, 1956; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc1025291/m1/4/: accessed April 26, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.