DISTRIBUTION OF AIRBORNE RADIOACTIVITY
127
spheric testing (January through April 1963). It is apparent, too, that
the fission debris,
even from a Stratospheric source, is associated
with particles in the 0.3- to 1.1- range on arrival at ground level.
This strongly suggests that the assumed small fission-product conglomerates existing in the stratosphere coalesce with or become attached to the larger nonradioactive tropospheric aerosols during
Slow migration of the aerosols to ground level. Thus meteorological
phenomena that affect the size distribution of normal atmospheric
dusts should have a corresponding effect on the fission-product conglomerate seen at ground level. Although as yet no attempt has been
made to correlate the particle-size distribution with weather, the data
for March and April 1964 suggest a progressive decrease in the size
of the radioactive aerosol particles during a prolonged rainy period.
A comparison of the size distribution of radon daughter products
(RaB + C) with the extremes in the fission-product distributions observed through use of this method is shownin Table 3. It is obvious that
Table 3—-COMPARISON OF THE SIZE DISTRIBUTION OF RaB + C ACTIVITY WITH
THE OBSERVED EXTREMES FOR FISSION PRODUCTS
Collection dates
Total
Activity
Activity contribution of
activity,
on initial
arious s1ze groups,
counts/min
filter, %
llp
Weather
ne
Cloud
Rainfall,
cover, %
in.
O6p
O.8p
O13 pn
RaB+C
Mar. 15, 1963
(0810-0840)
136.4
9.0 + 1.2*
4.8
0.5
30.0
64.7
G
0.0
270.5
9.9 + 0.5
3.6
6.8
33.2
56,4
100
0.0
151.8
13.1 + 0.6
aed
16.4
20.1
98.2
0
0.0
Apr. 19 to 22, 1963
3098.8
27.8 + 0.1
21.1
9.0
30
Trace
July 23 to 27, 1964
41.6
o9.6 + 1.4
46.6
2.3
70
Trace
Mar. 27, 1963
(0815—~ 0845)
Mar. 18, 1964
(0830-0900)
.
Fission products
Oct, 4 to 7, 1963
467.8
26.3 + 0.2
13.1
2.7
27.0
51.0
67.2
56.8
0.1
3.1
10
0.0
*Standard deviation, co, based on counting statistics.
RaB+C activity is associated primarily with particles smaller than
0.3 # in diameter. The smallest observed fission-product distribution
peaked near 0.3 4, whereas the largest distribution peaked between 0.6
and 1.1 4. In most cases the contribution of fission products to the
<0.3-u grouping was about 2 to 3% of the total, although on one occasion
(Apr. 19 to 22, 1963) 9% of the fission-product radioactivity fell into
this grouping.
Radiochemical Results
Radiochemical analyses for some long-lived fission products and
for *!°Pb (RaD) have been carried out on seven three-filter collections®
made during 1963. Similar analyses are being performed on one combined month-long collection during each quarter of 1964.