Application of electrolytic enrichment to minimize the detection limit in the case of low-level tritium activity measurement on LSC

Abstract

Tritium is a radionuclide, difficult to analyze because of its low beta energy (maximum 18.6 keV). Tritium beta particle has a range in the air of 6 mm, and this leads to difficulty of detection. Liquid Scintillation Counting (LSC) is the accurate technique to measure tritium in liquid samples. Considering the low-level activity of tritium in environmental water (drinking, surface, groundwater, precipitation), it is necessary to perform method which can increase tritium activity to be measurable at liquid scintillation counter and to reduce the minimum detectable activity (MDA). MDA is significant factor in radioactivity measurements providing higher sensitivity of counter. For that purpose, electrolytic enrichment of the water samples must be included when determining the tritium concentration. Each electrolytic run contains 15 samples and one spike water. When electrolytic enrichment technique is applied, initial volume of each distillate sample is 250 ml. Reduction of the detection limit can be achieved by increasing the enrichment factor during electrolysis by reducing the initial volume of the samples 10-15 times. If system works at 5 A, it takes about 5 days, or 600-700 Ah. Enrichment factor for the given electrolytic cell represents the ratio between net count rate of the spike water enriched during spike run and the net count rate of the spike water before enrichment, after background subtraction. The calculation of the tritium enrichment factor includes enrichment parameter value, number of ampere-hours for the enrichment run, initial and the final volume of the sample, and the Faraday constant. For measurement on Liquid Scintillation Counter, the samples are mixed with the scintillation cocktail in ratio 8:12. MDA, calculated for 13 electrolysis, conducted during one year (implying the same detection efficiency of 25.7 %, and the same measurement time of samples and background of 18000 s), decreases from 0.41 Bq/l to 0.26 Bq/l, as the enrichment factor increases from 7.99 and 11.98. Volume reduction was in the range 13-20 times (the final volumes of the samples in each electrolytic run are normalized at one value).