Evaluation of uncertainty and detection limits in 210Pb and 210Po measurement in water by alpha spectrometry using 210Po spontaneous deposition onto a silver disk

Abstract

An easy and accurate method for the determination of 210Pb and 210Po in water using 210Po spontaneous deposition onto a silver disk is proposed and assessed for its detection capabilities according to the ISO Guide for the expression of uncertainty in measurement (GUM) and ISO Standard 11929-7 concerning the evaluation of the characteristic limits for ionizing radiation measurements. The method makes no assumption on the initial values of the activity concentrations of 210Pb, 210Bi and 210Po in the sample to be analyzed, and is based on the alpha spectrometric measurement of 210Po in two different aliquots: the first one measured five weeks after the sampling date to ensure radioactive equilibrium between 210Pb and 210Bi and the second after a sufficient time for the ingrowth of 210Po from 210Pb to be significant. As shown, for a recommended time interval of seven months between 210Po measurements, the applicability of the proposed method is limited to water samples with a 226Ra to 210Pb activity ratio CRa/CPb≤4, as usual in natural waters. Using sample and background counting times of 24h and 240h, respectively, the detection limit of the activity concentration of each radionuclide at the sampling time for a 1L sample typically varies between 0.7 and 16mBqL−1 for 210Pb in water samples with an initial activity of 210Po in the range 0–200mBqL−1, and between 0.6 and 8.5mBqL−1 for 210Po in water samples with an initial activity of 210Pb in the same range.