*2.1. 220Rn Monitor and Measurement Periods*

The solid-state nuclear track detector (SSNTD) was developed at the National Institute of Radiological Sciences (Chiba, Japan) as a 222Rn and 220Rn discriminative monitor [2]. The monitor consists of two electroconductive hemispheres and there are two polycarbonate films installed in the center of the two hemispheres. To isolate and separate the progeny species of 222Rn and 220Rn, a glass fiber filter is located in the first hemisphere. Therefore, only gaseous 222Rn and 220Rn can pass through the filter and enter the first hemisphere. This monitor has two different diffusion chambers which have relatively large and small ventilation rates. This system has been developed based on the large difference in half-lives of 222Rn and 220Rn. After being exposed, the film was first subjected to chemical etching with a mixed solution of 8 mol L−<sup>1</sup> KOH and 20% C2H5OH at 30 ◦C for 30 min [23]. Then the films were electrochemically etched at 800 V and 2000 Hz for 2 h. A control film, which was exposed to particles from an 241Am source and which had been etched simultaneously with the sample films, was also prepared to assure the stability of the etching condition. The track density was converted to the average 220Rn concentration by the calibration factor after subtraction of the background track density, i.e., 3.5 ± 1.8 tracks cm−2. In the case of the three month long exposure period, the detection limit (DL) for the concentration of 220Rn was estimated to be 7.4 Bq m−<sup>3</sup> (k = 1.65), the definition of DL being based on the definition of Currie [24]. Four monitors were used in the survey to determine the mean annual 220Rn concentration. Consequently, the DL for the mean annual 220Rn concentration was estimated to be about 1/2 of DL value specified above. The measurements were carried out for four successive three-month periods to cover a whole year (i.e., January–March, April–June, July–September and October–December) for estimation of the mean annual indoor 220Rn concentration. The survey was carried out for four years (January 1993–June 1996) and conducted in the same manner as reported previously [23].
