Effective energy window of the E1 photon strength function for astrophysical neutron-capture reaction rates

The electric-dipole (E 1) photon strength function (PSF), which is a key input in the statistical model calculations for astrophysical neutron-capture reaction rates, has been studied with a focus on the important energy ranges of the emitting gamma rays. Comparing the available experimental data of the E1 PSF with the strength predicted by microscopic models, we find that the uncertainties in the E1 PSF below the neutron separation energy have a great impact on the neutron-capture reaction rates. A further study on the sensitivities of the reaction rate to the variations of the E1 PSF at different gamma -ray energies indicates that there is an effective energy window in which the E1 PSF contributes greatly to the neutron-capture reaction rates. We propose two functions to describe this window, which can be applied to the nuclei with dominant transitions to the continuum levels and is applicable to the general astrophysical environments of neutron captures. Employing the Gaussian approximation function of the effective window, we can conveniently get several specific effective energy ranges according to the contributions to the astrophysical rate. It is found that currently available experimental data of the E1 PSF do not completely cover these important energy ranges. Further theoretical and experimental studies are needed in the proposed energy ranges to improve the descriptions of the E1 PSFs.