Rapid determination of 90Sr Optimum use of a limited total analysis time

A method giving the lowest detection limit when a total analysis time has to be split between ingrowth and counting of 90Y in 90Sr determination is described. The detection limit is a function of background count-rate, total analysis time and the fraction of the total analysis time spent on counting. The optimum fraction used for counting was found to be ∼ 33%, if the total time was less than or equal to 5.4 half-lives of 90Y. An example illustrates the detection limits for different total analysis times using a typical liquid scintillation counter for the Cherenkov measurements of the fast β− particles from 90Y. For a total analysis time of 24 h and a background count-rate of 25 cpm, the instrumental detection limit is lowered by ∼ 15%, if the optimum counting time is used compared to a counting time a factor of two shorter than the optimum one.