*2.2. 220Rn Calibration Experiments*

The 222Rn and 220Rn calibration experiments were performed in a standard radon chamber at the National Radiological Protection Board in the UK and at the 222Rn–220Rn mixed chamber of Waseda University, Tokyo, respectively [25]. 220Rn conversion factor was evaluated to be 0.0098 ± 0.0016 (tracks cm−<sup>2</sup> per Bq m−<sup>3</sup> d).

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