Particle Distributions in Energy and Angle


In order to follow the transport of nuclear radiation through a material, it is important to know which secondary products are produced, the yield of each product, and how each product is distributed in energy and angle. The capabilities of the ENDF-format to represent this information about the products has evolved from fairly simple representations using Files 4 and 5 to the current rather complete capabilities found in File 6. In general, the cross section (in barns/steradian) for producing a particle can be written

mf6 formula

in terms of a cross section σ(E), a yield y(E), and a normalized distribution in initial energy E, final energy E', and cosine μ.

The cross section is always given in File 3.

The yield may be implicit as determined by the MT number, or in File 6, it may be given explicitly as integers for simple reactions or in noninteger form for the complex summation reaction MT=5.

The distributions may be represented using three different approaches:

  • For simple two-body reactions, E' can always be computed from kinematics, and it is only necessary to give f(E,μ). The function is given using File 4 or a special "law" in File 6. This option is used for elastic scattering (MT=2), neutron discrete-levels (MT=51-90), or neutron and particle discrete reactions (MT=50-90, 600-648, 650-698, etc.).

  • For older evaluations (incident neutrons), it is often assumed that the secondary-neutron distribution can be represented as a product of an angular distribution and an energy distribution, i.e., f(E,μ)*g(E,E'). The angular distribution is given in File 4 and the secondary-energy distribution is given in File 5. This approach is often used for MT=16, 18, or 91.

  • For newer evaluations, the distributions are often given as fully coupled energy-angle distributions in File 6 using one of the several "laws" offered there. The use of File 6 is required for incident charged particles.

The File 5 representation is always used for fission in ENDF files. The neutrons are assumed to be emitted isotropically in the laboratory reference frame.


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15 December 2012 T-2 Nuclear Information Service ryxm@lanl.gov