The radioactivity of the atmosphere resulting from natural sources has been monitored continuously for a period of two years. The method consists of collecting on moving strips of filter paper the daughter products of radon and thoron decay—principally Pb212 and Pb214. The highest activities found were 5 × 10−13 curies per liter of atmosphere and the lowest values in the order of 10−14 curies per liter. Changes in radioactivity may take place very rapidly so that a very high value may fall to one of the low values within an hour or less. Recent history of the air mass being sampled determines its content of these radioactive isotopes of natural origin. Maximal values are always characterized by still air held over land. Minimal values have been associated with air masses having been held over the sea where replenishment of radon is much reduced. The turbulence of winds causes mixing of radon with greater masses of air with correspondingly diluted levels of radioactivity.

In spite of the high degree of variability in atmospheric radioactivity from day to day or even from hour to hour, averages of the mean hourly activities when taken over a year in Berkeley, California show a well defined diurnal fluctuation. In this respect, data for 1955 are very close to those for the year 1956. On the yearly average, maximum activity occurs at 6 a.m. and minimum activity at 6 p.m. The averaged radioactivity of the atmosphere in the morning is about 40 per cent greater than the activity in the evening. This difference is probably a reflection of the tendency for offshore winds to migrate inland during the afternoon hours.

Samplings of stagnant air in wells and caves showed very high activity as compared to the open atmosphere. As much as 10−10 curies per liter of air have been observed in enclosed atmospheres.

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