Fe-55 activity measurements at the NMISA revisited

Twenty years ago South Africa's Radioactivity Standards Laboratory (now operated by the NMISA) was the first to measure the activity of 55Fe by combining the triple-to-double coincidence ratio (TDCR) liquid scintillation method with theoretical efficiency formulae. The extracted activity was however found to be consistently low by about 6%. Due to improvements in both the counting system and analysis technique, it was decided to re-examine the method as applied at the NMISA. The latest results are presented and discussed, particularly with regard to a quantitative study into which vial type is better suited to obtaining accurate 55Fe activity measurements.