14C and 3H Measurement by the Use of a Liquid Scintillation Spectrometer (IV) : Determination of the Counting Efficiency by the Discriminator Ratio Method

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A Method for the determination of the counting efficiency in liquid scintillation counting was studied. The ratio of the count rate in two windows which observe different parts of out put pulse spectrum each other vary with quenching degree of scintillator and can be used as an index of the counting efficiency. Spectrometer used in this work was TRICARB Model 314 A (Packard Instrument Co.) equipped with three discriminators and two scalers. A number of samples containing same amount of activity of C-14 were quenched to various degree by adding several kinds of solvent as diluter or chemical quencher. The lower discriminator: A was set to 10V, middle and upper discriminators: B, C were adjusted, by trial and error, so that the count ratio of one sealer to the another seemed to show linear relationship to the counting efficiency in window A-C. The same procedure was applied to tritium. These plots are shown in Fig. 2-Fig.6. The linearlity of curve was slightly impaired when either setting of B was not optimum or photomultiplier voltage was higher than the balance point of A-C window. A conclusion was obtained that several types of quencher, i, e. diluter, mild quencher and strong quencher, gave the same effect on discriminator ratio-efficiency relation. Recent application of this method to the measurement of natural C-14 level in essential oil: p-cymene, was also reported.
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