A cost-effective rock sample unit for quality control and intercomparison of 222Rn measurements

Reliable radon-222 measurements are essential for a wide range of hydrological, geological, and environmental applications, including the study of surface water - groundwater interactions and the quantification of groundwater discharge. Despite the widespread use of ²²²Rn detectors, routine verification of instrument performance and measurement stability remains limited, particularly in laboratories with constrained financial and technical resources. This study presents the development and evaluation of a cost-effective rock sample unit designed to support quality control, calibration checks, and inter-laboratory comparison of ²²²Rn measurements.

The system is based on acidic plutonic igneous rock purchased from commercial suppliers, selected for their naturally elevated and stable ²²²Rn production. The rocks were enclosed in a simple, airtight container assembled using readily available components, including a standard garden filter and plastic tubing. This configuration allows ²²²Rn generated within the rock matrix to accumulate in a closed volume and be circulated through commonly used ²²²Rn detectors without the need for specialized or commercial equipment. Equal amounts of material were placed in each rock sample unit, which were then sealed and stored for 21 days to allow ²²²Rn to reach secular equilibrium with its parent radionuclides. Initial characterization of the rock units was performed at the IAEA Isotope Hydrology laboratory. Each unit was analysed three times using a standardized protocol consisting of six measurement cycles of 30 minutes each. Measurements were conducted using RAD7 and RAD8 ²²²Rn detectors from Durridge, which are widely applied in environmental and hydrological studies. The results demonstrated stable and reproducible ²²²Rn concentrations across repeated measurements, confirming the suitability of the rock units as reference sources for quality control purposes.

Following this initial validation, the previously measured rock sample units were distributed to participating laboratories in Argentina, France, Malaysia, and Morocco. Each laboratory applied the same measurement protocol and used their routinely operated ²²²Rn detectors (RAD7 and RAD8).

To support the interpretation of the observed variability, contextual information was considered, including the age of the instrument, the date of last recalibration, the intensity of use, the type of water typically analysed (saline or non-saline; surface water or groundwater), and the range of ²²²Rn concentrations normally encountered. This approach enabled the assessment of the significance of deviations under different operating conditions and allowed the evaluation of the robustness of measurements obtained with calibrated versus non-calibrated instruments.

This exercise showed that even simple comparison of ²²²Rn responses obtained from the rock units provides valuable insight into the performance of the instrument and detect the potential measurement drift related to the lack of calibration. The results demonstrate that these cost-effective rock sample units represent a practical and accessible tool for strengthening ²²²Rn measurement quality assurance. Their simplicity, low resource requirements, and reproducibility make them particularly suitable for routine checks, contributing to the improved comparability of ²²²Rn data.