Daily and annual courses of natural atmospheric radioactivity

Measurements of the radon-decay products in the atmosphere over a 6-year period have been made with a monitor which precipitates fine airborne particulate matter onto a moving metallic tape. A calibration of the apparatus showed that the mean value for radon content at Socorro, New Mexico, is 2.4×10−13 curie/liter, with an average diurnal fluctuation of a factor of 3.1 between maximum and minimum values. The diurnal variation is attributed to the amount of vertical mixing due to eddy diffusion in the lower atmosphere. The gustiness in air motion near the ground is taken as a measure of the mixing that occurs, and it is measured with a hot-wire anemometer. An annual variation in the atmospheric radioactivity is found which gives values during the fall months that are about twice those during the spring. This variation can also be explained in terms of the mixing that occurs at low levels as judged from mean wind-speed data. Values for the coefficient of vertical diffusion are calculated from measurements of the exhalation rate of radon from the ground and the concentration of radon near ground level as determined from the monitor data. The mean value of the height-independent diffusion coefficient is 6.7×104 cm2/sec. Maximum values of as high as 55×104 cm2/sec are found in the late afternoon of the month of April. Minimum values of the order of 2.0×104 cm2/sec are found in the early morning hours in the fall months.