H. Obayashi

H." Yamanishi

K. "Okamoto

T. Iida

National Institute for Fusion Science (NIFS-Repository)

§5. Measurement Method of Thoron Exhalation Rate from Wall Okamoto, K., lida, T. (Dept. Nucl. Sch. Eng., Nagoya Univ.) Obayashi, H., Yamanishi, H. We have studied a new type of passive monitors wi th solid state nuclear track detector (SSNTD). Figure 1 shows the passive integrating 222 Rn and 220 Rn cup monitors. To control 222 Rn or 220 Rn exchange rate, we have made a 5 mm filtered opening for 222 Rn monitor and four openings of diameter 20 mm for 220 Rn monitor. stainless steel opening ( ¢5 ) ( a ) ( r50 ) opening ( ¢ 20 ) ( b) Fig. 1 Passive integrating (a) 222 Rn and (b) 220 Rn cup monitors. Calibrations of the 222 Rn monitors were performed in high 222 Rn concentration air. The calibration factor was derived to be (4. 17Jt0. 24) Calibration experiment of the 220 Rn monitors is quite difficult because of the short half-life of 220 Rn (55.6 s). Therefore, the calibration factors of both monitors have been calculated by using molecular diffusion model . The calculated factors were 4. 71X10- 8 and 0. 62X10- 8 tracks·cm- 2 (Bq·m- 8 ·h)- 1 for 222 Rn and 220 Rn monitors, respectively. The 220 Rn concentrations have been measured with six pairs of cup monitors in the various dwellings. The results in a dwelling with soil wall are shown in Fig. 2. The 220 Rn concentration at a distance of X m from wall could be expressed by following equation 1 > 294 (1) where Q(X) is the 220 Rn concentration at a distance of X m from wall in Bq·m- 8 , ETn is the 220 Rn exhalation rate from wall in Bq·m- 2 ·s- 1 , 1 .. Tn is the 220 Rn decay rate in h- 1 , D is the effective diffusion coefficient of 220 Rn in air in m2 ·s- 1 , and X is the distance from wall in m. "? 1 0 .j c--------.-----,----,---.----,-----, E [J3!. Oct. l 991 - 28. Jan. l 992 g l. Mar.- 14. May l 992 '-.../ 1 O 3 ;c . . . 18. Ju n. -3 1. Aug. 1992 § ·· f ~ · f ·... 8.Scp. - IO.Ilec.IQ92 i !O ' · ~ -.i } ; J r l ~r ; 0 u c 101 ~-~-~-~--~-~-~ r o zo 40 60 so 1 oo 120 Dist ance from wall (em) Fig. 2 The distributions of 220 Rn concentrations indoors. The values of ETn and D have been determined by using the least squares method. The calculated values of ETn ranged from 0.8 to 6.4 m2 ·s- 1 in the dwellings with soil wall. The exhalation rates from soil walls show more than ten times as much as those from brick and heavy concrete ranging from 0.01 to 0.11 Bq m- 2 ·s- 1 • The annual mean effective dose equivalent due to 220 Rn progeny was expected to be 0. 67 mSv·y- 1 in the dwellings with soil wall 2 >. Since the half-life of 222 Rn is 3.8235 d. we could not use the same procedure for measuring the 222 Rn exhalation rate from wall. Then, we thought a new equipment that consists of three boxes between which filters are positioned to make the difference of 222 Rn concentrations. When we set the equipment on the wall, we could obtain the 222 Rn exhalation rate from the differences. REFERENCES 1) Katase, A. et al. : Health Phys . .§_!(1988) 283. 2) Guo, Q. et al: J. Nucl. Sci. Techno I. (in printing). 

Measurement Method of Thoron Exhalation Rate from Wall

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