Application of the Triple-Photomultiplier Liquid Spectrometer Hidex 300 SL in Radiocarbon Dating

Abstract

The Hidex 300 SL is a liquid scintillation analyzer with an automatic sample changer and a triple-photomultiplier tube detection assembly that registers triple- as well as double-coincidence spectra. In the triple mode, the background in the 14C window is 13.7 cpm (14C standard 30.8 cpm; =154.3 cpm/5.01), so the factor of merit equals 8.7. The triple-to-double coincidence ratio (TDCR) allows for determining the 14C counting efficiency, the quench level, and quench correction. However, in the case of very low-activity samples, which is the case even for modern 14C samples, the TDCR is not the best method for the correction of benzene impurities. We propose using the position (channel) of the right slope of the sample (14C) logarithmic pulse-height spectrum. In the case of near-background samples, the cosmic muon peak can be used instead. The Monte Carlo modeling of spectra gave the 14C level below which the muon peak is a better quench correction parameter than the position of the 14C spectrum. The spectrometer, with the proposed quench correction method, was tested with wood samples dated dendrochronologically. For 21 samples, there is no systematic bias observed, and the standard deviation of the age differences scaled by the Poisson errors is 1.24 +/- 0.15, which means that the counting statistics account for 80% of the total variability (including sample preparation).