Retrospective dosimetry of fast neutrons focused on the reactions Nb-93(n,n ')Nb-93(m) and Fe-54(n,p)Mn-54

An overview is given of practices for "retrospective" dosimetry based on extraction and counting of niobium and manganese present in typical pressure vessel materials. The main problem of counting structural materials is the overwhelming presence of the nuclide Co-60 in the gamma-ray spectra of irradiated materials. Chemical separation of niobium and manganese is needed to determine the induced Nb-93(m) and Mn-54 activities. Procedures have been developed for retrospective dosimetry applicable at normally equipped radiochemical laboratories. Chemical separation procedures have been applied followed by determination of the amount of niobium in dissolution after separation. Special attention has been given to the accurate counting of the very weak X rays of Nb-93(m). Different techniques have been applied. Advantages, disadvantages, and the accuracy that can be achieved are discussed. Special attention has been given to the maximum accuracy that can be reached and the minimum amount of niobium that has to be collected to reach this accuracy.