Application of TDCR Counting for Primary Standardization of Radon-in-Water Samples

Radon-222 concentration measurements have broad applications in hydrological and geological studies as well as health protection studies concerning drinking water. Elevated levels of ²²²Rn in water pose a significant health risk especially through accumulation in indoor air. Upon inhalation, radon and its short-lived progeny cause radiation exposure, which increases the risk for developing cancer. In this regard, the metrological assurance of the radon-in-water measurements is very important for ensuring compliance with safety regulations. Liquid scintillation counting (LSC) is one of the most widely used methods for ²²²Rn in water measurements. Thus far, the calibration for this method is typically performed using standardized 226Ra solutions. However, this calibration method has certain flaws and limitations due to the accumulation of ²¹⁰Pb in the radium solution, as the solution ages, leading to interference with LS sample measurements and posing challenges for a sound calibration. This study is focused on the primary standardization of radon-in-water samples using Triple to Double Coincidence Ratio (TDCR) counting technique. The method relies on measuring a liquid scintillation sample containing ²²²Rn in equilibrium with its short-lived progeny. We stress on the fact that as the TDCR method is an absolute method, the calibration sources of LSC measurements can be standardized directly, thus largely removing difficult and timely procedures to prepare dedicated LSC standards. We will outline the procedure for preparing radon-in-water samples and their standardization. Practical examples will be given, illustrating the entire approach. Additionally, we will examine subtle effects related to the radon-in-water LSC measurements like the effect of the vial type (clear or diffusive), counting efficiency and temporal stability.