A comparative study on the influence of the type of acid used for 55Fe and 63Ni determination in steel samples through Liquid Scintillation Counting

Abstract

In decommissioning of nuclear facilities the presence of activated steel requires the determination, along with other radionuclides, of 55Fe and 63Ni. These can be considered as hard-to-measure-radionuclides because of their low energy beta particles or X radiation emitted. Furthermore, those radionuclides are usually mixed with a relevant fraction of non-radioactive Fe and Ni. This implies that, in those cases involving very low radioactive samples close to background activity level, a significant mass of steel sample must be physically and chemically treated and analyzed to achieve the Minimum Detectable activity Concentration (MDC) required by the Authority for clearance purposes. Due to colour quenching the acquisition time is particularly long. Minimizing the colour effect is therefore important for laboratories that must perform many determinations in a reasonable time, achieving the MDC in accordance with the Authority requests. For this reason, the choice of solvent can be an important information to save analyses time. The intensity of the colour quenching is, in fact, strongly dependent on the type of acid used to solubilize the sample before the mixing with the liquid scintillation cocktail. Some types of acid can even increase the phenomenon while others can drastically reduce such an effect. The aim of this work is to offer, through a correlation study between sample concentration in the LSC vial and type of solvent used in dilution, a reliable method for quantifying 63Ni and 55Fe in steel samples with very low activity, close to the background level.