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Technical Services Division, Tracerlab j West Richmond, Calif. 94804 Liquid scintillation counting of heterogeneous systems has greatly increased the ability to analyze weak beta emitting substances which are insoluble or of limited solubility in a counting solution. The counting methods described by Hayes, Rogers, and Langham (1) depend on the determination of the settling rate of the suspension or shaking the samples frequently between 1-minute counts. White and Helf (2) and Ott et al., (3) have investigated thixotropic gelling agents which reduce the settling rate of suspended particles. Regardless of the method used, the radioactive samples must be suspended in a finely dispersed state to prevent the self-absorption effects described by Schram (4). Much time and effort are required to reduce the sample to a small aggregate size to minimize self-absorption. Recently, Allred (5) described the use of a "sonication" technique for suspending 14BaC03 samples which reduced settling rate and time needed for sample preparation. This method offers the advantage of rapid sample dispersion which minimizes self-absorption effects and the time consuming sample grinding previously required to effect small particle sizes is eliminated. However, sensitivity is reduced in that only small samples can be prepared. The sample preparation method described here utilizes ultrasonic vibration to disperse the 14BaC03 aggregate and Cab-O-Sil to act as a gelling agent. This method increases the amount of sample which can be suspended and eliminates settling effects. The advantage of preparing samples stable to settling effects is in counting of low level samples requiring long counting times and when counting multiple samples in automatic counters.

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