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Energy-Angle Distributions
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The old-fashioned approach of representing an energy-angle
distribution as a product of an angular distribution (MF=4) and
a secondary energy distribution (MF=5) is no longer adequate
for modern evaluations. For incident energies larger than a
few MeV, preequilibrium effects begin to appear, which lead
to a coupling between the emission angle and the emission
energy. In addition, it is sometimes necessary to have
detailed information about the emission of other particles than
neutrons, for example, the protons from an (n,n'p) reaction.
The combination of File 4 and File 5 only allows one particle
to be represented. Most modern evaluations are made with the
help of a nuclear model code, such as GNASH, to complete
the ranges of energy and angle that are not available from
experiment. These codes follow all reaction channels and
emitted particles, and it is important to have a way to
include all the code results into the ENDF format. It was
these kinds of arguments that led to the development of File 6
as introduced for the ENDF-6 format.
A section of File 6 is divided into subsections, one for each
product. The products can include neutrons and light charged
particles ordered by ZA, followed by residual nuclei ordered
by ZA, followed by photons, followed by electrons:
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[MAT, 6, MT/ ZA, AWR, 0, LCT, NK, 0] HEAD
[MAT, 6, MT/ ZAP, AWP, LIP, LAW, NR, NP/ Eint/ Y(E)] TAB1
law-dependent data for product 1
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repeat TAB1 and law-dependent data
for the rest of the NK products
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[MAT, 6, 0/ 0.0, 0.0, 0, 0, 0, 0] SEND
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The individual products are identified by ZAP, AWP, and LIP (to
allow for isomers). The different representations allowed in
a subsection are defined by the value of LAW:
- LAW=0, Unknown distribution.;
- LAW=1, Energy-angle distribution.;
- LAW=2, Two-body angular distribution.;
- LAW=3, Isotropic two-body distribution;
- LAW=4, Recoil distribution for a two-body reaction;
- LAW=5, Charged-particle elastic scattering;
- LAW=6, N-body phase-space distribution; and
- LAW=7, Laboratory angle-energy law.
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