“…Although the lower background obtained with ultra low level liquid scintillation (> 3 dpm) is greater than for the proportional counter, liquid scintillation is frequently used [2,5,18,19,23,28,36,[42][43][44][45] because it gives a b spectral distribution for the three radionuclides. Furthermore, liquid scintillation is characterized by a high counting efficiency (> 85% for 90 Sr and 90 Y) and avoids counting problems such as self absorption, poor geometry or nonuniformity of sample mounting.…”