_ prSCRIPTION OF THE EXPERIMENTAL EQUIPMENT Sampling of atmospheric aerosols is carried out by drawing air through a ribre filter loaded with asbestos. The air is moved with a turbine having 4 550 watt motor fed by the aircraft generator. There is a diaphragm flowmeter on the pump which allows constant regulation of the volume of air filtered. The air intake is a bent tube passing through one of the windows and facirg forward. The alpha activity of the atmospheric aerosols is measured by a scintilla- tion counter made up of a are ZnS scintillator 20 cm in diameter coupled to a 54 AVP photomultiplier by a plexiglass light guide. are fed to a portable scaler (Figure 1). have 550 watt motors. The shaped output pulses In the captive balloons the turbines The current is carried by the cables whose weight is 21 kg/km and whose core is a nylon thread having a 50 kg breaking strength. The voltage loss in the cables are compensated for at the surface by an autotransfcrmer so that the supply to each motor is maintained at 220 volts. The electrical re- sistance of the cable is 500 ohms per km. IV. ° FILTER ACTIVITY A. Stepwise Measurements The radon is assumed to be in equilibrium with its daughters. The radon con- centration corresponding to a given level is calculated directly from the filter activity. Figure 2 shows the calculated variation of the total number cf alpha particles emitted by the filter as a function of time during and after collection. The sensitivity of the apparatus is 5 x 10713 curies/kg for a 20 minute sample of radon daughters.