(B) Measurement of Air-Borne Radioactivity by Gamma-Ray Scintillation Spectrometry P. F. Gustafson Argonne National Laboratory Introduction Soil is unquestionably the medium in which total accumulation and a running inventory of fallout radioactivity at a given location may be measured most directly. The major difficulty encountered in the direct measurement of fission product activity in soil, particularly by y-ray spectrometry, is due to the relatively high concentrations of natural radioactivity which lead to errors of 10 to 25 percent in the determination of fission products in soil. Such errors preclude accurate determination of the monthly differences in soil fission activity (caused by the radioactive decay of old debris and the deposition of fresh fallout) which in general amount to only a few percent of the total accumulation. In order to observe quantitatively the variations in fall- out rate it is necessary therefore to refer to air or rain samples. It was for this reason, namely to supplement the soil fallout measurements, that we have turned to the investigation of fission product radioactivity in surface air. The usefulness of fission product radioactivity in surface air as an index of fallout rate is predicated on the following conditions: 1. The mixing time in the troposphere is sufficiently rapid so that sampling at one or a limited number of sites over a wide area will be representative of the average air concentration over the entire area. 2. The removal of radioactivity by local precipitation does not reduce permanently the concentration of activity at subsequent times at that location below the level pertaining at other sites in the area which did not experience similar precipitation. 3. The average concentration of fission products in the troposphere should bear a reasonably constant relationship to that found in the adjacent stratosphere. The rapidity of mixing in the troposphere is illustrated by the work of Lockhart et al.! which indicated that debris from Hardtack I in surface air spread from 10 to 50 degrees N latitude in a matter of a few weeks. given time, the concentration in surface air differed by less than a factor of 10 over the entire region. Ata Simi- larly, in the spring of 1959 comparable concentrations of fission activity were observed simultaneously from the Central United States to Northern Norway.” Except for dry deposition and impingement on surface materials (vegetation and minor variations in terrain) the removal of air borne debris occurs via rain and snow. During prolonged rains, a decrease in concentration of activity with time has been noted, implying a depletion of activity per unit volume of air; however, replacement by masses of air not similarly depleted is rapid, and a lower value of activity in air does not persist for more than a few hours under moderate rainfall conditions. That this does not materially change the average air concentrations over periods of weeks or months is shown by the findings of Hyvinden* in which air levels were almost identical during the same month between localities of widely varied rainfall, Condition 3 is fulfilled in that approximately 10° higher concentrations are observed on the