Activity measurement of 55Fe by an efficiency calculation method

A non-extrapolation method for 55Fe activity measurement based on liquid scintillation coincidence counting, is described. The figure-of-merit, P, describing the counting system is established by tracing internally with the γ-emitting radionuclide 54Mn, from which the detection efficiency corresponding to 55Fe is calculated. The technique is similar in concept to the CIEMAT/NIST method for pure β-emitters. The advantage of replacing the external 3H standard with 54Mn is that it has very similar decay energetics to 55Fe. The practical application of the method is appraised particularly with regard to systematic effects on the extracted 55Fe activity for different 54Mn/55Fe activity ratios.