Counting quenched liquid scintillation samples using an outside-the-instrument gamma source and an external standard channels ratio method

Abstract

An external standard channels ratio method which can use a gamma source outside the spectrometer has been developed. The calibration procedure for a 2 mCi Cs 137 source is described. The relationship between counting efficiency and the external standard channels ratio is represented by a cubic or quadratic equation fitted by least squares. Once the calibration for a source is established, the same source settings can be used for counting any isotope. Calibration curves which cover a wide quenching range have obtained for H 3, C 14, and Cl 36 samples quenched with chloroform and methyl red. The average dpm determined by this method for some miscellaneous quenched carbon-14 labeled samples had standard deviations less than ±2.5% for replicate samples. The differences between the calculated and supplied dpm of these samples were also less than ±2.5%.