Protocol for the Collection, Transfer and ANSI/AARST MW-RN 2020 Measurement of Radon in Water 33 11.3 Calculations 11.3.1 Decay correction Radon decays with a half-life (t 1/2 ) of 3.82 days. Therefore, a correction for decay shall be incorporated into the calculation of concentration using the following: A 0 = A t e l t (3) where A 0 = decay corrected activity (activity at time of collection), A t = measured activity, l = decay constant for radon (0.6931 / t 1/2 ), and t = time from sample collection to the midpoint of the measurement period (same units of time as used for t 1/2 . Reported radon activities above 300 pCi/L (11 Bq/L) shall include an uncertainty calculated at the two sigma (95% confidence) level. 11.3.2 Relative percent difference (RPD) RPD is used to evaluate the difference between two measurements when neither one can be assumed to be more accurate than the other. The RPD compares the difference between two measurements divided by their mean, which in this case is the best estimate of the true concentration. RPD is always positive and is used as an estimate of imprecision. The RPD is determined by dividing the difference in the duplicate values (A-B) by the average (mean) of the two duplicate values. RPD values for duplicate samples shall be less than 10%. RPD = [(A – B) / mean] * 100 (4) where A = the larger result, B = the smaller result, and mean = the average of the two results. The user shall calculate and report the relative percent difference for duplicate samples. 11.3.3 Liquid scintillation activity The radon concentration ( A 0, in Bq/L) is determined using the measured count rate for the region of interest (ROI) from the spectrometer. While other formulae are possible, the following formula can be used to calculate the radon activity ( A 0 ) in the water sample at the time and date of collection: A 0 = (cpm - bcpm) / (60 * V * e * DK) (5) where cpm = measured count rate (counts/minute), bcpm = background count rate (counts/minute), 60 = conversion factor between dpm and Bq, V = water volume (L), e = detector efficiency (cpm/dpm), and DK = decay factor from sample collection to the midpoint of the measurement, determined using e - l t . 11.4 Safety warnings and waste management Organic-based scintillation fluids are commonly used for radon measurement and some of these pose a significant harm to humans. Since radon is soluble in solvents such as toluene and xylene, cocktails containing those solvents may be used for radon analysis. Lipophilic xylene-based scintillation fluids

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