172
DREVINSKY AND PECCI
It is of interest to compare these results with total beta-activity
data from collections of June and October 1960 over Minneapolis with
At of about 660 days. The vertical profiles’ of total beta activity for
total sample observed in 1960 showed a nearly constant concentration
{about 60 net counts per minute time geometry factor per cubic meter
(S.T.P.)| over the 16- to 30-km altitude region. This would be con-
sistent with the concept of a more uniformly mixed lower stratosphere
with respect to debris of this age. The concentration was higher by a
factor of about 15 than that obtained for the 9- to 15-km altitude band.
In general, most of the total beta activity in 1960 was on particles
with 0.02 u< r< 0.15 » except for the 27- to 30-km altitude band within
which most
r< 0.02 yu.
of
the
beta
activity
was associated with particles of
individual Radionuclides
For each flight vertical profiles of both total concentration and
concentration within each size range and the activity —size distribution
for “4Ce, “Sr and ®Zr are very similar to each other and to the total
beta activity. There is little variation over all flights. There is a
notable persistence of higher levels of radioactivity on particles with
r < 0.02 » at 21 to 30 km in the 1963 collections; this could be because
of the relatively greater influence at these levels of debris from higher
altitude sources. On particles with r < 0.02 u at 21 to 30 km, there is a
higher percentage of ‘4Ce and “Sr in 1963 than in 1962. It should be
especially noted that a large fraction of 57r is foundon the smallest
of the particles in the May 1963 collection. The total concentrations of
“4Ce, Sr, and *Zr which we observed at each altitude interval are
in reasonably good agreement with those reported by Salter’® for filter
collections over San Angelo, Tex., during the same time periods in
1962 and 1963.
The limited data on “Mn show that most of this radionuclide is on
particles with 0.02 u< r< 0.15 u with a peak concentration at 15 to 21
km in May 1963, but on particles with r < 0.02 yu at the concentration
peak of 21 to 27 km in August 1963. Data for ‘4°Ba concentrations are
sparse, although most of the “°Ba seems to be on particles with
. 0.02 u<r<0.15 pat 15 to 21 km as Shown by the March 1962 results.
It is clear that most of the 4Ce and “Sr radioactivity has been
found to be associated with the particle sizes 0.02 u< r< 0.15 » and
r<0.02 4H. In 13 out of 16 samplings within the altitude bands, the
4Ce/"Sr activity ratios in these two size ranges were identical within
the estimated error limits, and there is little indication of chemical
fractionation between these two nuclides in stratospheric nuclear
debris whose age is predominantly several days to 240 days. Clearly,
there is no evidence for such fractionation in the May 1963 collections.