On the basis of the asymmetric double sigmoid function,a simple method was developed to fit the LSC(liquid scintillation counting)spectra of mixed ~(90)Sr/~(90)Y samples under different quench levels using the software Origin.Because the external standard quench index is usually affected by the statistical fluctuations significantly for a sample with high radioactivity,aquench index is defined,which is based on the characteristics of ~(90)Y spectrum and will not change with the sample radioactivity.The defi...