Measurements of radon 220 (thoron) in the atmosphere below 50 centimeters

Direct measurements of Rn220 (thoron) have been made from the ground surface to a height of 50 cm, using apparatus adapted from that of G. W. Israël. Activity profiles are similar to those calculated by Jacobi and André and by Wormell; this supports their assumption that in the 50-cm height range a linear increase of diffusivity with height property describes the turbulent diffusion of radioactive emanations from the soil surface, at least for turbulence of moderate intensity. The maximum activity at ground level, as expected, occurs at night during intervals of very low wind speed; it reaches values 75 to 150 times as high as the typical activities at 50 cm and 1 m. The Rn220 flux through the soil surface at our site is approximately 2.1×10−15 curie cm−2 sec−1, or 0.006 atom cm−2 sec−1. The activities of Rn220 plus Po216 observed under conditions of very low turbulent diffusion are capable of yielding an ion production rate of about 250 ion pairs cm−3 sec−1 in the lowest 10 cm of the atmosphere over appreciable time intervals; this seems adequate to explain the presence of the shallow stratum of relatively high space charge density previously reported to exist under these conditions.