Neutron Capture Cross Sections

Cross section formulas for neutron capture are developed by means of statistical theory, for intermediate and heavy weight nuclei for energies of the order of 1 kev to a few Mev. The formulas do not exhibit resonances, but are rather averages over resonances in the energy region where these exist. The case is treated where the residual nucleus levels are well separated, although the method is applicable in principle where these levels form a virtual continuum. The processes competing with capture in the energy range considered are elastic and inelastic scattering. The energy dependence of radiation widths is estimated. The ratio of energy level spacing to radiation width at the dissociation energy of a neutron enters as a parameter in the cross-section formulas. This ratio is chosen to make experiment and theory agree as well as possible. Taking the spacing of levels of a given spin and parity at the dissociation energy of a neutron to be 25 ev, the values of the radiation widths are found to be 0.08 ev for ${\mathrm{Ag}}^{108}$ and ${\mathrm{In}}^{116}$, 0.10 ev for ${\mathrm{Ag}}^{110}$, and 0.20 ev for ${\mathrm{Au}}^{198}$.