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Charged-particle elastic scattering uses MF=3, MT=2 and MF=6, MT=2, but the
cross section in File 3 is set equal to 1.0 at all energies represented using
LTP=1 and LTP=2. For LTP=12 and 14, File 3 must contain the "nuclear plus
interference" cross section in barns. The elastic File 6 uses LAW=5. Whenever possible, the nuclear amplitude expansion should be used. Note that the a and b coefficients are not independent, being related by their mutual dependence on the nuclear scattering amplitudes, which are themselves constrained by unitarity and various conservation conditions. Thus, any attempt to fit data directly with the equations given would underdetermine the a's and b's, giving spurious values for them. The only feasible procedure is to fit the experimental data in terms of a direct parameterization of the nuclear scattering amplitudes (phase shift, R-matrix, etc.) and extract the a and b coefficient from the fit. The residual cross section expansion (LTP=2) can be used when an approximate direct fit to the experimental data is desired. The simple pole approximation for the Coulomb amplitude implied by this representation becomes increasingly poor at lower energies and smaller angles. Since the deficiencies of the approximation are masked by the dominance of the Rutherford cross section in the same region, however, one could expect a reasonable representation of the net scattering cross section at all energies and angles, provided that the coefficients Cl are determined by fitting data excluding the angular regions where the Rutherford cross section is dominant. Tabulated distributions (LTP=12 or 14) using the nuclear plus interference representation are also useful for direct fits to experimental data. In this case, the choice of the cutoff cosine is used to indicate the angular region where Rutherford scattering is dominant. The following two plots illustrate a typical cross section computed with amplitudes and the corresponding residual cross-section representation: |
See ENDF102, Section 6.3.1