ベンゼン−液体シンチレーション法における14C計数効率の経時変化

概要

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To assess the reliability of the Liquid Scintillation Counting method, we monitored the weight and counting ratio of a benzene sample (STD059) prepared in 2000. Both benzene weight and the counting ratio generally decreased during the 2 years of analysis, at rates of 7 mg/yr and 0.073 cpm/gC/yr, respectively. The total decrease in the counting ratio was 97% of the initial value, as calculated using a linear approximation. The counting efficiency of STD059 was 71–73%, less than that for IAEA standards prepared in 2006–2007 (74.3–78.5%). Spectral quenching parameters (SQP) and Channel ratios (R) were similar between the STD059 and IAEA standards (C-1 to C-5), suggesting insignificant quenching of STD059. The temporal reduction in counting efficiency was possibly caused by density quenching, although the scintillator concentration of the solvent was lower than 15.9 g/L during the study period. According to the counting efficiency of STD059, we determined that the 14C activities of the IAEA standards were within 0.10-1.81 pMC of the values recommended by Rozanski et al. (1992), while the C-1 and C-4 samples showed~2% of modern carbon contamination, which resulted in a +1.0 pMC excess.
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