*2.3. Soil Parameters and Its Activity Concentrations of 226Ra and 228Ra*

Five soil core samples from the ground surface to 5 cm depth were collected using a stainless-steel soil sampler, which had a volume of 100 mL [26]. Dry bulk density, soil particle density, porosity, and soil textures were evaluated after drying samples for 24 h at 110 ◦C. The dry bulk density *ρ*<sup>b</sup> was calculated as the mass of the dried soil divided by the soil volume. The soil particle density *ρ*<sup>s</sup> was evaluated using a specific gravity bottle according to the test procedure of Japan Industrial Standards (JIS) A1202 [27]. The porosity ε was calculated by *ε* = 1−*ρ*b/*ρ*s. Soil particle size distribution was evaluated using a standard stainless-steel sieve for 0.075–2.0 mm particle size range and the sedimentation analysis for particles below 0.075 mm was made according to the test procedure of JIS A1204 [28] to determine the soil textures of the samples. The percentages of sand, silt, and clay for each soil sample were evaluated using the sample particle size distribution curves.

The activity concentrations of 226Ra and 228Ra were evaluated using a high-purity germanium semiconductor detector (GEM-100210, ORTEC, USA) with a relative efficiency of 30%, 1.85 keV energy resolution (FWHM) at 1.33 MeV of 60Co. The efficiency calibration of the detector was made using the standard volumetric sources which are contained 109Cd, 57Co, 139Ce, 51Cr, 85Sr, 137Cs, 54Mn, 88Y, and 60Co supplied by Japan Radioisotope Association. The detector was enclosed in a shielding made out of compacted lead of 10 cm of thickness. Each soil sample was enclosed in a cylindrical polypropylene container (U8 type container, 100 cm3) after drying for 24 h at 110 ◦C. The prepared soil sample was then enclosed in an air-tight container for 40 days to allow radioactive equilibrium between 226Ra and 222Rn to be reached. The measurement time was set as 80,000 s. In this study, the weighted average concentration of 214Pb and 214Bi were used as the 226Ra concentration in the soil samples by counting photons in the photoelectric peak channels of 352 keV for 214Pb and 609 keV for 214Bi. 228Ra was measured by counting photons in the photoelectric peak channel of 911 keV for 228Ac. The uncertainty for the activity concentration was evaluated taking into account the uncertainties of the counts for the sample and background. Coincidence summing, self-attenuation and decay corrections were applied using software (Gamma Studio, SEIKO EG&G, Tokyo, Japan).

#### *2.4. Comparison of the Exhalation Rates with the Other Methods*

#### 2.4.1. Accumulation Chamber with Scintillation Cell

The stainless-steel accumulation chamber was set on the ground surface. Radon gas exhaled from the ground was accumulated for 1.5 to 3 h. Then, the radon gas inside the accumulation chamber was collected into a scintillation cell (Pylon 300A, Pylon Electrics, Inc., Toronto, Ontario Canada) at a sampling flow rate of 0.5 L min−<sup>1</sup> and a sampling time of 5 min. After 3.5 h, the alpha counts from radon gas in the scintillation cell were

measured using a portable radiation monitor (AB-5, Pylon Electrics, Inc., Toronto, Ontario Canada) [29]. The radon exhalation rate by grab sampling can be calculated by applying Equation (5).

$$E\_{\rm Rn} = \frac{(N - N\_{\rm b}) \cdot CF \cdot V \cdot \lambda\_{\rm Rn}}{S \cdot \left[1 - \exp\left(-\lambda\_{\rm Rn} \cdot t\right)\right]} \tag{5}$$

Here, *N* and *N*<sup>b</sup> are the count rates of the sample and background (cpm), *CF* is the conversion factor from count rate to radon concentration (27.0 Bq m−<sup>3</sup> cpm−1) [30], *<sup>V</sup>* is the volume of the accumulation chamber (1.4 × <sup>10</sup>−<sup>2</sup> <sup>m</sup>3), *<sup>λ</sup>*Rn is the decay constant of radon (2.1 × <sup>10</sup>−<sup>6</sup> <sup>s</sup>−1), *<sup>S</sup>* is the area under the accumulation chamber (9.9 × <sup>10</sup>−<sup>2</sup> <sup>m</sup>2), and *T* is the accumulation time (s).

#### 2.4.2. In Situ Radon and Thoron Exhalation Rate Monitor

Radon and thoron exhalation rates from the ground was also measured with an *in-situ* radon and thoron exhalation rate monitor (MSZ). The details of the method have been described by Saegusa et al. [31]. The monitor was composed of an accumulation chamber (volume, 13 L), a ZnS(Ag) scintillation detector with an aluminized mylar sheet, a light guide, a photomultiplier tube, a pulse counting part and scaler, and a timer. The area of an acrylic board coated with ZnS(Ag) scintillator was 0.12 m2. Count rates were recorded over consecutive 30-s intervals during a total recording period of 30 min after the monitor was set up on the ground. The conversion factors from count rates of 10 min and 30 min to exhalation rates of radon and thoron were 0.521 ± 0.040 mBq m−<sup>2</sup> <sup>s</sup>−<sup>1</sup> cpm−<sup>1</sup> and 18.1 ± 3.2 mBq m−<sup>2</sup> s−<sup>1</sup> cpm<sup>−</sup>1, respectively. The measurement uncertainties for radon and thoron exhalation rates using the MSZ have been reported as ~20% and ~6%, respectively [11].

#### **3. Results and Discussion**
