Analysis of ¹⁶¹Tb by radiochemical separation and liquid scintillation counting

The determination of ¹⁶¹Tb activity is problematic due to its very low fission yield, short half-life, and the complication of its gamma spectrum. At AWE, radiochemically purified ¹⁶¹Tb solution was measured on a PerkinElmer 1220 Quantulus^TM Liquid Scintillation Spectrometer. Since there was no ¹⁶¹Tb certified standard solution available commercially, the counting efficiency was determined by the CIEMAT/NIST Efficiency Tracing method. The method was validated during a recent inter-laboratory comparison exercise involving the analysis of a uranium sample irradiated with thermal neutrons. The measured ¹⁶¹Tb result was in excellent agreement with the result using gamma spectrometry and the result obtained by Pacific Northwest National Laboratory.