Direct-semidirect neutron capture calculations applied to R-matrix data evaluations in the resolved resonance region

We present a method for computing direct-semidirect neutron radiative capture and its applications to R-Matrix evaluations of ¹⁹F, ²⁷Al, ^28,29,30Si, ^35,37Cl, and ^39,41K performed by the Nuclear Science and Technology Division's Nuclear Data group at ORNL. The method includes both direct and semidirect capture; the latter is a core-polarization term in which the giant dipole resonance is formed. We compare our method with that employed in the TEDCA code, which has been commonly used in computations related to astrophysics, as well as in nuclear data evaluations. For nuclear data evaluations in the resolved resonance region, where R-Matrix evaluations are commonly performed using the Sammy code, direct capture is significant between resonant capture peaks, as well as at very low neutron incident energy below the lowest resonance (e.g., thermal capture). The experimental data for which the R-Matrix analysis are being performed is subject to background effects that at energies below 10 keV could be sufficiently large to obscure the direct capture cross section. Consequently, an accurate evaluation of TOF data requires a theoretical model of the direct capture cross section, to which end we apply our method.