222 Rn calibration procedure for water analyses by liquid scintillation counting

Purpose Among the several methods to measure 222 Rn in water, the liquid scintillation counting (LSC) technique is one of the most sensitive and widely used when analysing non-saline waters by liquid extraction, providing excellent accuracy, precision and low-level detection limits. When using the liquid scintillation counter Tri-Carb 3170 TR/SL (Perkin-Elmer) it is not possible to make an automatic determination of the alpha/beta discrimination parameter, being necessary to evaluate the pulse decay discriminator (PDD) value manually to the 222 Rn determination. In this work, we describe the steps to perform the calibration of that type of equipment. Methods The method consists in the preparation of standard samples from a 226 Ra certified standard solution and the adjustment of the PDD value by hand until the percentage of alpha events reaches a plateau of a near-constant fraction of total events. The process consists in taking several measures between PDD 80 and 200 with 15 min counting time each. Within the plateau, the alpha/beta ratio was used to determine the plateau final range as well as the optimum PDD value. After that, the calibration standards and blanks were measured and the region of interest (ROI) was established by finding the maximum value for the figure of merit (FOM) in the upper and lower limits. Results The optimal PDD value of 148 was established by the alpha/beta ratio, and after that, it was possible to measure 226 Ra standards to perform the calibration. An efficiency of (9.758±0.035) × 10 –1 was obtained. In routine measurements, a detection limit of about 0.50 Bq L –1 is achieved for a 120 min counting time. To evaluate the uncertainty budget, the uncertainties of the net count rate, the efficiency, the sample amount and the decay correction factor were considered. Conclusions A method was developed for the calibration of LSC Tri-Carb 3170 TR/SL with the optimization of the PDD using a 226 Ra standard solution. The acquisition to obtain the plot of the variation of the events fraction in each window (alpha and beta) in function of PDD allowed to evaluate a plateau where the fraction of events was at a minimum. Afterwards, it was possible to measure the set of 226 Ra standards after the equilibrium with 222 Rn and determine the efficiency for 222 Rn with a detection limit of about 0.50 Bq L –1 in routine analyses.