Geogenic mapping of radon and thoron concentration in Perak State Malaysia

Abstract

Radon and its daughter isotopes exist in the natural environment and can significantly contribute to human exposure to ionizing radiation. Knowing an environment’s radon levels can help the radiological protection agencies in policymaking. This research seeks to quantify the activity of radon (222Rn) and thoron (220Rn), and their mapping, to identify risk areas in Perak state, Malaysia. The RAD7 detector coupled to soil probe, air sampling, and RAD-H2O accessories was used to determine 222Rn and 220Rn activity in soil gas and outdoor air, and also 222Rn activity in water respectively. The RADON-JOK equipment was used to measure the soil gas permeability. A portable NaI (Tl) survey meter was used to measure the terrestrial gamma dose rate. The 222Rn concentration in soil gas ranged from 0.11 to 434.5 kBq m-3 with a mean of 18.96 ± 6.66 kBq m-3. The soil gas permeability ranged from 5.2×10-14 to 5.2×10-12 m2, with a mean of 5.65×10-13 m2. Geogenic radon potential (GRP) values were computed from the 222Rn activity and soil gas permeability data. The range of GRP values was from 0.04 to 154.08. High GRP values were identified at Perak Tengah (36.95), Kampar (43.42), Kuala Kangsar (44.78), Hillir Perak (54.34), and Manjung (154.08) districts. Statistical correlation analysis indicates that the estimated GRP data is strongly correlated with the measured 222Rn in soil gas. The 222Rn activity in water samples was in the range of 0.04 to 3.98 Bq l-1, with a mean of 0.69 ± 0.27 Bq l-1. The annual effective dose for inhalation and ingestion of 222Rn in water ranged from 0.102 to 9.954 µSv y-1 and 0.102 to 10.112 µSv y-1 respectively. The mean annual effective dose of inhalation and ingestion were 1.73 and 1.76 µSv y-1 respectively and were below the world average of 2 µSv y-1. The 222Rn activity in outdoor air ranged from 5.79 to 5110 Bq m-3, with a mean of 320.03 Bq m-3 which is higher than the EPA level of 14.8 Bq m-3. The 220Rn activity in the soil gas range from 0.0 to 562.58 kBq m-3 with a mean of 37.69 kBq m-3. The highest 220Rn activity was recorded in the granite and Triassic-Jurassic geological formations of the study area. Elevated 220Rn activity was found in peat, riverine, and granite source soils. The 220Rn activity in outdoor air ranged from 0 to 4226.70 Bq m-3, with a mean of 228.12 Bq m-3 which is higher than the UNSCEAR value of 10 Bq m-3. The gamma dose rate results ranged from 98.31 to 3769.10 nGy h-1 with a mean of 446.27 nGy h-1. The mean total effective dose from 222Rn, 220Rn, and outdoor gamma dose exposure of 1825 h (5 h per day) was 3.75 mSv y-1 which is above the world average of 2.4 mSv y-1. Pioneer radon, thoron, and GRP maps were developed to highlight radon-prone points in the area under investigation. The maps assist in human health risk assessment and risk reduction since it indicates the source of radon and thoron.