204 PERKINS, THOMAS, AND NIELSEN Special case where the two gamma rays have equal energy, it also serves to illustrate the change in efficiency with energy. It is evident that with this relatively high geometry system, coincidence counting rates can be measured with a reasonably high efficiency. This, together with the system’s extremely low backgrounds and elimination of most of the Compton interference, makes the device an extremely sensitive and selective instrument. EXPERIMENTAL RESULTS AND DISCUSSION OF MEASUREMENTS OF AIRBORNE RADIONUCLIDES NEAR GROUND LEVEL Air sampling? near ground level was performed with a continuously operating vacuum pumpthat pulled air at 100 cu ft/min through a membrane filter of 5-u pore size (AM-1, Gelman Instrument Company). These membrane filters have been shown to be essentially absolute for the collection of fallout radionuclides.’ The filters are composited on a monthly or semimonthly basis, pressed into a stan- dard geometry of '/ in. thick by 1 in. in diameter, and counted on the multidimensional gamma-ray spectrometer. The air samples were obtained at a point 15 ft above the ground 6 miles north of Richland, Wash. The annual precipitation in this area is only about 8 in. and dry deposition accounts for a significant fraction of the fallout. In Fig. 6 the concentration in air of 13 radionuclides is recorded for the period January 1962 through September 1964. A tabulation of data is given in Table 1. All these radionuclides except *°*Pu were measuredby the direct spectrometric analysis. With the exception ofPu, which is a residue of the nuclear devices exploded, these radionuclides result from fission and neutron activation during atomic tests and from cosmic-ray interactions with nuclei in the atmosphere. The radio- nuclides ®Zr—*Nb, '°®Ru, '°sb, "Cs, and “Ce are fission products; presumably nuclear testing is their main source in the atmosphere. The radionuclides “Mn, “Co, *y, ‘Sb, and Cs are formed as neutron-activation products during nuclear testing; and "Be and Na are produced continuously in the atmosphere by interactions of cosmic rays. The fallout rates of many of the fission products are measured at sampling locations the world over® and perhaps their main value here is to serve as a base line for comparing the concentrations of less abundant radionuclides in the atmosphere. Sodium-22 was first reported to be present in the atmosphere by Marquez‘ in 1957, who found it in rainwater at Rio de Janeiro, Brazil, in a concentration of 0.017 dis/min per liter. It is produced naturally by cosmic-ray spallation of argon in the atmosphere and also results from the reaction *°Na(n,2n)**Na during tests of nuclear weapons, Its potential value as a tracer of atmospheric circulation has been recog-