ABSTRACT
These are recommended values of thermal neutron activation cross sections and resonance integrals useful for estimation of yields in activation analysis for radioisotope production. Some of the values are monoenergetic (0.025 eV or 2200 m/sec) cross sections, while others are for a Maxwellian distribution of thermalized reactor neutrons. The published literature makes very little distinction between these, but if a major resonance exists at very low energy, the yield will differ from that calculated using the monoenergetic cross section. In the epithermal energy region, the presence of many large resonances contributes to the total yield if a significant flux of epithermal or fast neutrons is present. In many nuclear reactors, in fact, the ports used for activation of samples provide a large flux of epithermal in addition to thermal neutrons. In such cases, the yield of activated product must be estimated by using the sum of the thermal neutron plus the resonance integral contributions. The resonance integrals given in this table are epicadmium values, assuming a Cd cutoff at 0.5 eV. It can be noticed from this table that there is a need for many new and/or more accurate measurements of thermal neutron cross sections. To perform a reasonably accurate calculation of the estimated yield of an activated product in a given reactor irradiation, it is necessary to have a knowledge of the complete spectrum of neutrons that exists at the location in the reactor where the sample is to be placed, since the contribution from the resonance energy neutrons can often equal or exceed that from the purely thermal neutrons. Details of such calculations are given in standard reference books on neutron physics. 1
