Standardization of Alpha Emitters by Liquid Scintillation Counting

Internal liquid scintillation counting enables high precision to be attained in the standardization of solutions of alpha emitting nuclides. The method has a number of advantages: under appropriate experimental conditions, a 4π geometry is obtained; the difficulties with sample self-absorption and backscattering often encountered in methods necessitating mounting of the samples on plates are eliminated. One condition of particular importance for standardization is the stability of the liquid scintillation sources over long periods of time. Measurements with actinide nuclides from thorium to curium showed that the ions of the actinides can be stabilized by means means of tri-n-octyl-phosphineoxide (TOPO), which simultaneously acts as complexing agent and counteracts the quenching effects of acids. A technique for the comparison of liquid scintillation counting with other absolute counting methods, which avoids the possible sources of errors from weighing or pipetting of aliquots, has been developed and verified. The method was applied to new determinations of the specific activity of 239Pu and 233U with an accuracy of ± 0.3% and ± 0.2%, respectively. In the liquid scintillation counting a precision of ± 0.05% was obtained. Because of the high time resolution of liquid scintillation, further improvement in precision can be expected when fast electronic systems are used. (author)