Assessment of an automatic TDCR liquid scintillation counter for use in low-level tritium measurement D Hillegonds, L Wassenaar, R Juvonen, T Oikari, S Wisser INTRODUCTION A newly developed, low-cost, triple-to-double coincidence ratio (TDCR) decay counting and analysis system (Hidex 300 SL; Hidex Oy) was compared with a traditional ultra-low-level liquid scintillation counting (LSC) systems (Perkin Elmer Quantulus 1220), in order to the assess applicability of TDCR for low-level tritium ( 3 H) analysis by electrolytic enrichment of mL natural water samples. STUDY DESIGN The primary test samples were six IAEA tritium inter-comparison (TRIC2012) samples, with known low tritium contents from to 7.5 TU, as well as low-level precipitation samples obtained from the IAEA Global Network of Isotopes in Precipitation (GNIP). All water samples were processed using routine mL to 10 mL IAEA electrolytic tritium enrichment and final distillation procedures. All samples, blanks, and standards were measured on a Quantulus 1220 decay counting instrument; thereafter the same vials were placed into the Hidex 300SL TDCR instrument, and counted for an identical time (500 minutes, using 10 cycles of 50 minutes). CONCLUSION The results reveal that the low cost Hidex TDCR-based instrument produced tritium results that were similar to that of the more costly traditional decay counting method, when an advanced statistical spectral fitting algorithm was applied to account for the higher, but exceptionally stable, background count rates. Our tests showed that the Hidex 300 SL was capable of producing accurate 3 H results to at least 0.4 TU when using water samples electrolytically enriched from 500 mL to ~15 mL. Although further work is needed to complete the development of the statistical fitting of Hidex data, these results are very encouraging for the use of low cost TDCR instruments in the context of tritium measurements for hydrological research studies. RESULTS Average values of the triplicate enriched TRIC test samples run are shown in Figure 1. Prior to electrolytic enrichment (factor of 29±1) these six water samples are representative of the low-level environmental tritium concentrations one might encounter in ground water investigations, ranging from 0 TU (T20) to 7.5 TU (T25). Reproducibility between LSC instruments was identical within statistical expectations, although a very small systematic bias for Hidex was apparent from this dataset. Figure 2 shows comparative percentage differences between the Hidex and Quantulus measurements on 128 unknown water samples, including the 18 samples in Figure 1 (depicted individually). The known 3 H values (pre-enrichment) of the TRIC samples are given in the caption to illustrate how these measurements compare to routine samples measured in a typical enrichment/LSC laboratory. The dispersion around perfect agreement (0 %) falls within what is statistically anticipated, as shown by the dotted lines: these dotted lines represent the total propagated uncertainty margins for the Quantulus system results (at one sigma). The overall dataset in Figure 2 does not show any pronounced bias for the Hidex SL300. CALCULATIONS Raw data from the Quantulus 1220 (counts per minute, CPM) were collected with assigned channel optimization for each of the Quantulus instruments used in the test. Unenriched tritium-free lab standards were used for determining blank corrections, and laboratory tritium standards were used to calculate final DPM. The CPM/DPM ratio for the standard measurements were 0.36±0.02 (Hidex) and 0.31±0.03 (Quantulus). Despite that the background count rate for Hidex (5.7±0.2 cpm) was considerably higher than for the Quantulus (0.5±0.2 cpm), an advanced statistical background correction algorithm was developed by Hidex. Algorithm employs several windows over 3 H ROI. Both 3 H amplitude distribution (spectrum) and background spectrum, together with background count rate (CPM) are stored with reference samples. For unknown samples, Poisson weighted least-squares fitting is used to get the most probable value for 3 H CPM.