RAPID-DETERMINATION OF SR-90 - 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 Y-90 in Sr-90 determi nation is described. The detection limit is a function of background c ount-rate, total analysis time and the fraction of the total analysis time spent on counting. The optimum fraction used for counting was fou nd to be similar to 33%, if the total time was less than or equal to 5 .4 half-lives of Y-90. An example illustrates the detection limits for different total analysis times using a typical liquid scintillation c ounter for the Cherenkov measurements of the fast beta(-) particles fr om Y-90. For a total analysis time of 24 h and a background count-rate of 25 cpm, the instrumental detection limit is lowered by similar to 15%, if the optimum counting time is used compared to a counting time a factor of two shorter than the optimum one.