# A Case Study of the Radon Hazard at the Boundary of a Coal Minefield
## 2 Materials and Methods
### VAR and RFD Measurement  
A total of 46 radon measurement points located within 20 points and outside 26 points in the coal minefield were selected to study soil radon concentrations and RFDs. Soil air probes were collected from 0.6 m-deep wells (80 mm diameter). All wells were drilled at previously selected locations, and the distance between neighboring sample points was 130-190 m. Before measurements, the wells were kept undisturbed for 24 h, covered with a polymer film from above to ensure uniform filling with soil air, and equilibrated between radon and its progeny elements. For soil air sampling, a polyethylene tube (10 mm external diameter) was installed in order to reach the bottom of the well. Inside the tube, there were two silicone tubes (3 mm internal diameter). The air entered the air sampler through one tube and descended into the well through the other, thereby avoiding contamination with outdoor air. Thus, the air was circulated in a closed cycle for 5 min at a rate of 11/m using an AV-07 air sampler (NTM-Zaschita, Moscow, Russia). The ascending flow of air passed the cartridge drier and entered the sampling flask. The total sample volume was 50 mL. After each sampling cycle, the system was ventilated with outside air for 2 min. The VAR was measured in a 50 mL air sample tube with an Alfarad+R (NTM-Zaschita, Moscow, Russia) radiometer using an \"Air Soil\" protocol. This methodology focuses on analyzing the levels of \({}^{222}\)Rn, thereby enabling us to determine the concentration of \({}^{222}\)Rn in the subterranean atmosphere. The radiometer chamber quantifies the decay of \({}^{218}\)Po, which undergoes radioactive decay from \({}^{222}\)Rn. Its half-life is 3.05 min. The uncertainty of this monitor is \(\pm\)30%. This protocol includes corrections for both the standard 50 mL sample tube and the sample collection time. All measurements were performed within 1-8 h of sampling. RFD was assessed using a \"Camera-01\" device developed by NTC-NITON in Moscow, Russia. The camera-01 tool assesses the activity of \({}^{222}\)Rn by gauging gamma and beta radiation from the short-lived descendants of radon--\({}^{214}\)Pb and \({}^{214}\)Bi--in equilibrium with radon. The accuracy of the device is \(\pm\)30%. To reach a radon state and its DPR, the chosen sample was left for 4 h after exposure and then examined using a beta sensor. Radon was accumulated through coal sorption columns (SK-13) and a storage chamber (NK-32). Coal in NK-32 chambers adsorbs \({}^{222}\)Rn on its surface, providing an estimate of its departure rate from the soil, considering the chamber's known area and sensor installation time. A total of 690 RFD measurements were taken at 46 observation sites. The collection chambers were firmly pressed into the loose soil and then securely sealed after measurements to prevent the release of radon into the atmosphere. At every observation point, 15 cameras were installed. The data from these 15 measurements were averaged, and the resulting values were used for subsequent analysis. An internal measurement control was carried out for all measurements. The repeated measurements accounted for 15% of the previously conducted measurements. The acquired values complied with the method measurement errors declared by the manufacturer, which was limited to 30%. Radon is typically a mobile phase that varies depending on various factors, such as geological and meteorological conditions, as well as registration methods. A concentration assessment of soil air radon levels with a declared maximum of 30% is adequate to differentiate between hazardous and safe areas, which may differ significantly. More precise measurement techniques are available, such as the use of CR-39 sensors that have a 10% measurement uncertainty. However, they also possess utility constraints. These sensors can assist in enhancing the radon map at a local level in the future. Our study provides primary data on the land near the mines, enabling us to employ existing equipment.