Applications and misapplications of the channel-capture formalism of direct neutron capture

We discuss the channel-capture approximation of slow neutron direct-capture theory. We show that this approximation gives a generally good representation of the neutron capture cross sections for several electric dipole transitions in a broad range of nuclides from A = 9 to A = 136; these are mostly near-spherical nuclei. Despite this body of agreement, we examine the accuracy we can expect from the simple channel-capture theory. Comparison with calculations of the potential-capture cross section from physically more realistic optical model calculations show that, in general, the channel-capture cross section can be up to approx. =40% in error. In cases where the expected channel-capture cross section is much smaller than the ''hard-sphere'' capture cross-section estimate, the disagreement with potential capture can be much worse than this. Also, in these cases, compound-nucleus capture can be of comparable or greater magnitude. These effects have been shown to completely undermine recent attempts to determine nuclear interaction radii for targets, such as 12C and 9Be, by application of the channel-capture formula to capture cross-section data. 20 refs