Effects of 55Fe impurities on activity standardization of 59Fe

During activity standardization of 59Fe by 4πβ-γ coincidence counting and efficiency extrapolation it was noted that at high counting efficiencies (when (1-epsilonsub(β))/epsilonsub(β) approx.< 0.09 from measurements of coincidences between β-rays and 1291 keV γ rays) 55Fe contamination shows up as a sudden change in the slope of the efficiency function. A more definite identification of 55Fe could be made from an analysis of the pulse-height spectrum produced in the 4π counter, especially at high counting gas pressures when it was possible to estimate 55Fe concentration quantitatively. Although accurate standardization of the activity of 59Fe could be carried out avoiding effects due to 55Fe, once these effects have been identified, suppliers of radionuclides should find it possible to keep 55Fe in 59Fe well below 0.50% which is small enough to permit activity standardizations of 59Fe with an accuracy close to 0.10% [68% confidence limits (CL)]. (author)