SIZE DISTRIBUTIONS OF RADIOACTIVE AEROSOLS
175
collections. The errors in these ratios are based ona 1o counting
error and the estimated error in the decay-curve resolution as men-
tioned earlier. Data for ‘Ce, Sr, and *Zr are the most complete and
will bé considered in detail. Figure 6 depicts ‘4Ce/*Sr activity ratios
as a function of particle size and altitude as observed for the 1962
collections, and Figs. 7 and 8 show “4Cce/"Sr, ®Zr/"Sr, and Zr/'4Ce
activity ratios from the 1963 collections. It should be noted that the
respective production ratios estimated from the data of Katcoff'! are
approximately 40, 220, and 4.8.
14400/ 9s, Activity Ratios
For the March 1962 collection (see Fig. 6), approximately 130
days after the most recent atmospheric nuclear test with a fission yield
greater than about 1 Mt, there appears to be a wide variation in 44Ce@
and “Sr distributions with particle size. Within the altitude bands at
9 to 15 and 15 to 21 km, however, the “4Ce/*Sr activity ratio is constant among particles with 0.02 u< r< 0.15 uy and r< 0.02 yu. With the
exception of a relatively high “Sr concentration in the 27- to 30-km
altitude band, these two particle size ranges contained about 90% of
the “Ce and Sr activities observed in the March 1962 collections.
We find a more systematic pattern in the M4Ce and “Sr distributions with particle size for the September 1962 collection (see Fig. 6),
which was made during a period of nuclear testing. Although the errors
in the '“4ce/"Sr activity ratios are admittedly large because of the low
“Sr concentrations encountered, the observed variation in size distribu-
tion of ‘Ce and "Sr at each altitude band is never greater than about
a factor of 2. One is tempted to conclude that, within the estimated
accuracy of the observed ratios, there is no chemical fractionation
between ‘44Ce and “Sr in this collection. There is a slight tendency,
however, toward enrichment of Mace, relative to Sr, in the largest
particles in the 9- to 27-km altitude region. Also, the *Ce/*Sr ratios
in September are about a factor of 10 higher than those observed in
March 1962. Both of these obServations support the fact that relatively
fresh debris was collected inSeptember 1962. The decrease in “4Ce/*Sr
activity ratios with increasing altitude is probably a result of the con-
tinuing influence of older debris at 21 to 30 km.
In Fig. 7 a most striking result is observed in the plot of “4ce/*"Sr
activity ratios as a function of particle size and altitude for the May
1963 collection when At is equal to about 150 days. With the exception
of only the largest size fraction at the highest altitude sampled, which
showed a very low “Sr concentration, the “4Ce and "Sr are distributed
in an identical manner among the three particle size ranges. Less
than 5% of the total “4Ce and Sr collected within each of the four
altitude bands for this flight is found on particles with r > 0.15 up. Also,