External standard method of quench correction: advanced techniques

The counting efficiency for a liquid scintillation sample varies with its composition. Impurities from solubilizers as well as the specimen itself typically quench the available energy, reducing both the pulse heights and the total number of detectable events. Counting efficiency as measured in a pulse height channel is therefore uncertain, particularly when assaying low-energy radionuclides or dual labeled samples. An external source of gamma radiation is frequently utilized to quantitate the degree of quenching. This is done by analyzing the additional pulse height spectrum created from Compton electrons, which are produced in the sample by the radiation from the gamma source. The resulting data may then be used to correct for losses in pulse height and counting efficiency. The counts induced by the gamma source are usually measured in two pulse height channels and the ratio of the counts in these channels is used to quantitate quenching. This is called the external standard ratio (ESR). The fixed windows in these channels, however, limit the useful dynamic range of quench determination or may produce unacceptable statistical uncertainty in the estimated counting efficiencies of quenched samples. A new method of quench determination based upon relative pulse height (RPH) is described. Applying this technique to the external standard Compton spectrum provides a means for quantitating quench in terms of RPH over a 30 to 1 change with less than 11/2% standard deviation. By recognizing the quantum characteristic of weak scintillations, accurate pulse height restoration can be extended to quenched samples containing low energy radionuclides such as 3H and 14C. Isotope energy, window settings, and true disintegration rate can be computed from the RPH quench value of unknown samples without empirical curves