# Radon Levels of Water Sources in the Southwest Coastal Region of Peninsular Malaysia
## 2 Materials and Methods
### Radon Activity Measurements  
Direct measurement of radon concentrations in the water samples was conducted at the sampling sites using a RAD7 portable radon detector (DURRIDGE Company Inc., Billerica, MA, USA) complete with a set of RAD7 H\({}_{2}\)O accessories. The RAD7 equipment used for the measurements included: the RAD7 electronic detector, a 250 mL water vial  
Figure 1: Sampling points in an area of the southwest coast region of Peninsula Malaysia (using ArcGIS 10.4.1).  
with an inserted aerator, and the desicant tube supported by a retort stand, as shown in Figure 2.  
For each sample, a completely filled vial was carefully sealed to avoid loss of radon or water, and measurements were performed using a closed-circuit technique, a closed-loop aeration design in which the air and water volumes are constant and independent of flow rate. Accordingly, no decay correction was required [14]. During the aeration phase, air was recirculated through the water, continuously extracting the radon gas until a state of equilibrium was achieved, obtained in approximately 5 min. In the equilibrium state, no more radon could be removed from the water sample. The working principle of the schematic diagram of the RAD7 detector in a closed-circuit setup is presented in Figure 3.  
Figure 3: The schematic diagram of the RAD7 detector in a closed-circuit setup.  
Figure 2: RAD7 equipment for the radon measurements in the water samples: (1) RAD7 detector; (2) 250 mL water vial with aerator; (3) tube of desiccant.  
The RAD7 radon detector was operated followed the protocol designated as Wat 250 and the process was starting with a 5 min aeration phase. During this phase, the extraction efficiency (the percentage of radon removed from the water to the air-loop) was high, at approximately 94% for a 250 mL sample. The exact value of extraction efficiency depends somewhat on the ambient temperature but is almost always well above 90% [15]. During the aeration phase, the presence of thoron gas from the water to the air-loop is neglected due to its short half-life and small diffusion length. The concentration of thoron is expected to decrease very sharply as one moved away from its source [7]; however, if the area has high background radiation it needs special attention to measure radon gas, such as the areas in China, India, and Brazil with thorium rich soils [16]. A previous study, in 2020, successfully measured thoron in surface waters using a novel method for assessing the sensitivity of a mobile thoron-in-water detection setup, thus, achieving quantitative measurements of thoron-in-water concentration in the field using an experimental approach of RAD AQUA accessories coupled with a RAD7 detector [17]. The measurement technique was very useful and could be applied for future study in the measurement of radon and thoron concentrations at the locations with high background radiation, for example, in mines.  
The RAD7 radon detector makes use of alpha spectrometry to detect radon concentrations in the air, collecting positively charged carriers from alpha particle to form the electric signal; moreover, allowing radon and thoron (\({}^{220}\)Rn) to be distinguished [14; 15]. In particular, the high electrical field within the active surface of the RAD7 detector attracts the positively charged particles from the polonium daughters, \({}^{218}\)Po at 6.00 MeV and \({}^{214}\)Po at 7.76 MeV, and converts them to electrical signals that act as a measurement of radon concentration in air. At the end of the analysis, the RAD7 results from the sample analysis are provided in terms of radon concentration, in units of Bq m\({}^{-3}\) or Bq l\({}^{-1}\). The RAD7 minimum detectable activity (MDA) concentration is 0.004 Bq l\({}^{-1}\)[18]. Before each measurement is carried out, the RAD7 detector needs to be set up in purging mode for 10 min to remove high humidity and radon in the detector system.