Energy Dependence of Fast-Neutron Activation Cross Sections

Fast-neutron capture cross sections of 24 nuclides ranging from $A=51$ to $A=197$ have been measured by an activation method, in the neutron energy region from 0.15 to 6.2 Mev. The neutron energy spreads were of the order of 0.1 Mev so that cross sections averaged over many energy levels of the compound nucleus were measured. Activities induced in samples by fast and thermal neutrons were compared. The relative neutron flux in the fast- and thermal-neutron activations was determined with a U²³⁵ fission counter. A knowledge of the energy dependence of the U²³⁵ fission cross section and of the thermal-neutron activation cross sections allows calculation of the fast-neutron activation cross sections.