Capture Cross Sections for Fast Neutrons

The neutron capture cross sections at an effective energy of one Mev have been measured for 32 isotopes, including 4 isomeric pairs. Unmoderated fission neutrons were used as the source, and the high intensities available by this method have made it possible to measure fast cross sections as low as ${10}^{-4\mathrm{}}$ barn. The cross sections (except those for a few "magic number" nuclei) show a smooth variation with atomic weight, ranging from ${10}^{-1\mathrm{}}$ b at $A=100\mathrm{}$ to ${10}^{-4\mathrm{}}$ b at $A=20\mathrm{}$. The rapid change in cross section is a measure of the change in level spacing of the compound nucleus formed after neutron capture. A survey is made of all the existing data on absorption cross sections as functions of $A$ and neutron energy.