Systematic study of direct neutron capture

The direct neutron capture reaction is investigated within the potential model. All allowed electric dipole ($E1$), electric quadrupole ($E2$), and magnetic dipole ($M1$) transitions are taken into account. The nuclear structure ingredients involved in the calculation are determined from experimental data whenever available, and if not, from global microscopic nuclear models. A special emphasis is put on the excitation spectrum deduced from a mean field plus combinatorial model of nuclear level densities. It is shown that considering either a total intrinsic nuclear level density or one-particle, one-hole neutron excitations give rise to similar predictions provided the corresponding average spectroscopic factor is renormalized. The potential model is shown to provide a fair agreement between the predicted radiative neutron capture cross section and experimental data for light targets as well as with previous calculations. A systematic study for about 6400 nuclei with $8\ensuremath{\le}Z\ensuremath{\le}102$ lying between the proton and neutron drip lines shows that the direct capture cross sections (and consequently reaction rates of astrophysical interest) are proportional to the number of levels available below the neutron threshold and decreases with decreasing neutron separation energies. It is found that the $E2$ and $M1$ components are usually negligible with respect to the $E1$ contribution, but that they can dominate the direct capture rate for about several hundred nuclei.