1° 433 Zz 31 ~Y wa = Z a Oo W _ ce > 2 1 W < 10° 0 | 0.2 ! 0.4 | 0.6 ENERGY, MEV | 0.8 | 1.0 1.2 ALTITUDE, 1000 FT 957,—Nib ACTIVITY, PC/45-MIN SAMPLING TIME Fig. 4—Gross fission-product gamma-ray spectrum. Fig. 5—Variation of %Zr—%Nb activity of stratospheric and tropo- Spheric air samples. No samples were collected between August and November 1963. Data were obtained with sampler shown in Fig. 1. Since the sampling altitude was generally maintained near 30,000 ft on the outward flight and near 40,000 ft on the return flight and since the height of the tropopause varied from flight to flight, the data show that the distance below the tropopause generally determines the ac- tivity levels of the tropospheric samples. Thus there is a concentra-