摘要：A new analytical method for the determination of 222Rn in air was studied. The method was based on the separation and determination of daughter 214Bi which was in radioactive equilibium with 222Rn and the procedure was as follows. The air was sampled in a 5 liter round-bottomed flask, then 250 ml of Bi and Pb carrier solution adjusted to pH 1.0 with nitricacid was added, and the air-borne 214Pb and 214Bi, the daughter products of 222Rn, were absorbed in this solution by standing and shaking 5-6 times. 214Bi in the solution together with Bi carrier was extracted by shaking with 1% cupferon solution and chloroform, re-extracted with 25% sulfuric acid, and precipitated by the addition of 10% sodium sulfide solution. The precipitate was dried and radioactivity of 214Bi was measured with a low background gas-flow counter. The radioactivity of 222Rn can be calculated by the equations 1 and 2 shown below. [numerical formula] Where NBi is radioactivity (dpm) of 214Bi at the time of separation of 214Bi from 222Rn, t1 is the time (min) from the separation to the beginning of measurement, t2 is the time (min) from the separation to the end of measurement, f is the counting efficiency of 214Bi, and A is the total count of 214Bi in t2-t1. [numerical formula] Where NRn is the radioactivity (dpm) of 222Rn at the time of air sampling, t is the time (days) from the sampling to the separation of 214Bi. The sensibility of this method was 5×10-12 Ci 222Rn/liter of air, so that this method would be available for the determination of 222Rn in air in places such as the bath room of radioactive spa where extraordinary high content of 222Rn is expected in air. Using the above method, determination of 222Rn content in air was carried out at Misasa and Masutomi spa areas where many natural radioactive fountains are distributed. The 222Rn concentration in outdoor air was very low in both spa areas, while the air in bath room showed high concentration of 222Rn in both spa areas. The mean value of 222Rn content in air of three bath room were 42×10-12 Ci/liter at Misasa and 13×10-11 Ci/liter at Masutomi. These 222Rn contents are 13 and 4 times higher, respectively, than 1/30 of the "maximum permissible concentration in air"for occupational exposure recommended by ICRP.
