Thermal Quantities and Resonance Integrals 

In the thermal reactor business, quite a bit of insight into the
behavior of a particular evaluation can be obtained by examining
certain thermal cross sections and resonance integrals. In a
reactor, most fission neutrons are born from fission reactions
at thermal energies. The quantities of interest there are the
fission cross section itself, the competing capture cross section,
and the number of fission neutrons produced per reaction, nubar.
The deviation of the shape of the cross sections from a 1/v
shape is also of interest. The fission neutrons are produced at
energies up around 1 MeV, and they have to slow down to thermal
energies before producing more neutrons to continue the chain
reaction. The quantities of interest here are the fission and
capture resonance integrals, which help determine the probability
that a neutron will disappear before reaching thermal energies.
For these reasons, the following simple parameters have come into
wide use:
The conventional thermal energy is 0.0253 eV. If the cross section has a 1/v shape, its integral weighted against a Maxwellian spectrum for 0.0253 eV is given by
where the factor G provides a measure of the deviation from a 1/v shape. It will be unity for a pure 1/v. Similarly, the alpha, eta, and K1 integrals are Maxwellian averages of the following quantities:
The quantity K1 is known to be an especially good indicator for k_{eff}. The following is an example for U235 from Release 3 of ENDF/BVI: 
thermal quantities at 300.0 K = 0.0259 eV  fission xsec at 0.0253: 5.8472E+02 fission nubar at 0.0253: 2.4338E+00 capture xsec at 0.0253: 9.8575E+01 fission xsec at tev: 5.7830E+02 fission nubar at tev: 2.4338E+00 capture xsec at tev: 9.7349E+01 thermal capture integral: 8.5429E+01 thermal capture gfactor: 9.9022E01 capture resonance integral: 1.4337E+02 thermal fission integral: 4.9323E+02 thermal fission gfactor: 9.6239E01 thermal alpha integral: 1.6117E01 thermal eta integral: 2.0059E+00 thermal k1 integral: 6.2301E+02 fission resonance integral: 2.7750E+02 
A good reference for exerimental values of these quantities
is S. F. Mughabghab, Neutron Cross Sections, Volume I,
Neutron Resonance Parameters and Thermal Cross Sections, Part B,
Z=61100, Academic Press, 1984. Its values for the quantities
in the table above are

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23 January 2013  T2 Nuclear Information Service  ryxm@lanl.gov 