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The elastic scattering cross section is generally not known to the same
accuracy as the total cross section. Frequently, the elastic scattering
cross section is obtained by subtracting the nonelastic cross section from
the total cross section. This procedure can cause problems. The result
is an elastic scattering cross section that contains unreal structure.
There may be several causes. First, the nonelastic cross section, or any
part thereof, is not generally measured with the same energy resolution as
the total cross section. When the somewhat poorer resolution nonelastic
data are subtracted from the total, the resolution effects appear in the
elastic cross section. Second, if the evaluated structure in the
nonelastic cross section is incorrect or improperly correlated with the
structure in the total cross section (energy-scale errors), an unrealistic
structure is generated in the elastic scattering cross section. The experimental elastic cross section is obtained by integrating measured angular distributions. These data may not cover the entire angular range or may contain contributions from nonelastic neutrons. Such contamination is generally due to contributions from inelastic scattering to low-lying levels that were not resolved in the experiment. Care must be taken in evaluating such results to obtain integrated cross sections. Similarly, experimental angular distribution data can also cause problems when used to prepare File 4. |
See ENDF102, Section 3.4.2