Threshold reaction interference in neutron activation analysis

The high energy (>0.5 MeV) component of a reactor flux can produce, via threshold reactions on the major constituents of a sample, identical activities as those produced by the thermal flux, through (n,γ) reactions, on the impurities being determined. In order to predict these interferences in thermal neutron activation analysis, it is necessary to know: 1. The cross section of the threshold reaction in a pure fission neutron flux; 2. The fraction of the reactor thermal flux, nvo, above about 0.5 MeV which has the properties of a pure fission flux. The latter has been determined in an empty fuel rod position in the NRX reactor using the reaction U-238 (n,f) as a monitor. Based on this value, fission neutron cross sections have been measured for several (n,p), (n,α), (n,2n) and (n,n') reactions. The interferences caused by the (n,p) and (n,α) reactions in this particular irradiation position are given. An example of activation by (n,2n) and (n,n') reactions, when the (n,γ) reaction is not suitable, is also discussed. (author)