406 KALKSTEIN, THOMASIAN, AND NIKULA PROCEDURES Stratospheric filter samples* have been obtained by WU-2 aircraft up to an altitude of 21.3 km, Cadmium is chemically separated by cadmium sulfide precipitations, purified by basic ferric acetate and palladium sulfide scavengings, and finally electroplated from a cyanide solution. Yields generally range between 40 and 80%. Counting of the samples is accomplished by measuring the silver K X rays (about 22 kev energy) with a small, thin scintillation detector and a pulseheight analyzer. Conversion to absolute decay rates is based on an intercalibration performed by several laboratories on a '®Cd source circulated by the AEC Health and Safety Laboratory. RHODIUM REVIEW The '°Cd experiment is the second tracer experiment, Its predecessor was the '*Rh tracer associated with the Orange rocket shot of Aug. 12, 1958, detonated during Operation Hardtack I at about 43 km above Johnston Island.’ In part, the !9°Cd tracer experiment was meant to check findings from the !Rh experiment as well as to provide new information, Briefly, the major findings of the '**Rh experiment were that there were (1) a delay of over a year before the appearance of a sizable fraction of the tracer at about 20 km, (2) an apparent major influx from high altitudes into the lower stratosphere taking place at high latitudes in the winter season, and (3) a predominance of the debris at 20 km at high latitudes (above 30°) in both hemispheres with conSiderably lower concentrations at low latitudes. CADIUM RESULTS The first observations of !°Cd in the stratosphere were reported by Salter.‘ Beginning in December 1962, balloon samples collected at above 30 km in the southern hemisphere showed the presence of the tracer. Our studies to date have been confined to altitudes of about 20 km and below. The first of our samples that obviously indicated the pres- ence of '°8Cd were collected in mid-1963; however, some earlier sam- ples indicated the possible presence of this tracer. The results obtained to date are shown in Fig. 1. *Samples have been provided through the assistance of Capt, Adrian Polk of the Defense Atomic Support Agency, by H. W. Feely of Isotopes, Inc., and by the 4080th Strategic Wing, U. S, Air Force, Davis-Monthan Air Force Base,