Vertical distribution of outdoor radon and thoron in Japan using a new discriminative dosimeter.

Passive measurements of outdoor radon and thoron concentrations were conducted from June 1992 to June 1993 at a monitoring station over a soil area (10 m x 6 m) in Chiba city, Japan. The measurement period was divided into 4 parts to investigate seasonal variations of radon and thoron concentrations. Ten passive radon-thoron discriminative dosimeters (R-T dosimeters) were placed in duplicate at 5 different altitudes to show the vertical distributions of outdoor radon and thoron concentrations. Outdoor radon concentrations showed no significant difference within 1.0 m above the ground, and the annual average of outdoor radon concentration was 3.85 +/- 0.19 (SE) Bq m-3. Annual averages of outdoor thoron concentrations at 0.04, 0.15, 0.25, 0.70, and 1.0 m above the ground were 40.5 +/- 4.4, 22.5 +/- 3.7, 13.9 +/- 3.1, 9.5 +/- 2.9 (SE) Bq m-3, and < 9.0 Bq m-3; the lower detection limit of the dosimeter, respectively, and their vertical profiles, n(z) (Bq m-3), were expressed well by the formula n(z) = alpha z beta. Vertical profiles of the atmospheric turbulent diffusion coefficient were also estimated from the observed thoron profiles, as expressed by the power function K(z) = AzB, of which B values were estimated to vary from 1.034 to 1.609 if averaged thoron exhalation rates during the measurement periods were within 0.3 to 2.8 (Bq m-2 s-1).