Angular Distributions

 The angular distributions for two-body scattering reactions are given in either File 4 or a special "law" of File 6. The most common way of representing them uses Legendre coefficients:

 At low energies for elastic scattering, the angular distribution will be isotropic in the center of mass reference frame. As the energy increases, the scattering typically becomes more and more forward peaked, and higher and higher Legendre orders will be needed. If the energy goes through a resonance, dramatic swings from forward scattering to backward scattering may be seen. The following figure is typical:

 At the highest energies, the scattering is basically the diffraction of a wave around a hard sphere, and the results are similar to diffraction patterns for light. The forward peak is like the bright central spot in a diffraction pattern, and the oscillations at larger angles are analogous to the fringes seen for light. The following is an example of a section of File 4 for elastic scattering (63Cu from ENDF/B-VI). The "1" in the fourth position of the first card indicates that the data are in the center-of-mass system. Cards 5 and 6 indicate that the scattering is isotropic at 1e-5 eV (the P1 coefficient is zero). Cards 9 and 10 show anisotropy beginning to show up at 10 keV. The anisotropy gradually increases with energy until a Legendre order of 14 is needed to represent the angular distribution at 20 MeV.

2.906300+4 6.238900+1          0          1          0          02925 4  2
0.000000+0 6.238900+1          0          2          0          02925 4  2
0.000000+0 0.000000+0          0          0          1         222925 4  2
22          2                                            2925 4  2
0.000000+0 1.000000-5          0          0          1          02925 4  2
0.000000+0                                                       2925 4  2
0.000000+0 2.530000-2          0          0          1          02925 4  2
0.000000+0                                                       2925 4  2
0.000000+0 1.000000+4          0          0          2          02925 4  2
3.214700-3 1.190800-4                                            2925 4  2
0.000000+0 1.000000+5          0          0          4          02925 4  2
3.619500-2 3.845600-3 3.661300-5 0.000000+0                      2925 4  2
0.000000+0 3.000000+5          0          0          4          02925 4  2
7.500000-2 1.800000-2 4.000000-4 0.000000+0                      2925 4  2
0.000000+0 5.000000+5          0          0          4          02925 4  2
1.200000-1 5.500000-2 2.550000-3 1.200000-4                      2925 4  2
0.000000+0 7.500000+5          0          0          4          02925 4  2
1.730000-1 1.070000-1 1.300000-2 2.730000-3                      2925 4  2
0.000000+0 1.000000+6          0          0          6          02925 4  2
2.258400-1 1.602700-1 3.980500-2 1.286300-2 1.560800-5 0.000000+02925 4  2
0.000000+0 1.500000+6          0          0          6          02925 4  2
2.738500-1 2.188700-1 9.602200-2 3.370000-2 1.499300-4 0.000000+02925 4  2
...
0.000000+0 2.000000+7          0          0         14          02925 4  2
8.105400-1 6.500300-1 5.507300-1 4.828500-1 4.177800-1 3.523300-12925 4  2
2.899200-1 2.378900-1 1.840200-1 1.207600-1 6.096100-2 2.102800-22925 4  2
4.210300-3 0.000000+0                                            2925 4  2
2925 4  0

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