  To complete the computation of Displacements per Atom (DPA) cross sections,
the present work shows the methods of calculating DPA cross sections with the
nuclear data of energy-angular distribution in both the laboratory and the
Center-of-Mass (CM) frames. The method of direct calculation with data in the
CM frame is proposed and recommended to decrease the computation burden and
keep all information. Theoretical analyses reveal that more than 7-point
Gauss-Legendre Quadrature (GLQ) should be used to ensure the convergence of the
angular integration for DPA computations. Numerical results show that 8-point
GLQ is sufficient for the continuum inelastic neutron scattering, while
64-point GLQ is implemented in NJOY. Because the integrand over secondary
energy is not derivable in the whole domain of the secondary energy, the
trapezoidal integration is used to perform the numerical integration. The
numerical calculations show that the trapezoidal integration is suitable to
perform the integration over the secondary energy on the fine grid given by
nuclear data files at least for 56Fe. The present work reveals that the direct
interpolation of energy-angular-integrated damage can give the same results
computed with standard interpolated energy-angular distributions. The DPA cross
sections will be overestimated if isotropic angular distributions are assumed.
However, the first-order Legendre polynomial can give DPA cross sections within
0.4% deviation, while 12 orders are required to describe the anisotropic
angular distribution.
