larger particles, to particles as small as 0.02, is certainly unreliable, as
is the use of the airplane comparison in indirectly determining the particle
size.
Also adding to the uncertainty in the filter efficiency problem, is the
question of flow-rate.
It has been assumed that a linear velocity of 100 feet
per minute prevails at all times and altitudes.
It may perhaps be necessary
to determine the flow rate for individual collections, again based on the fan
law, once the particle size and efficiency curves are established.
Such re-
finement is probably not warranted at present.
D.
Radiochemical Analysis.
The stratospheric filters are routinely analyzed
for the following fission products:
Table 3
Isotope
Isotopes Analyzed
Half-life
Ba-140
12.8 days
Zr-95
Sr-90 is,
-
65 days
Ce-14.4
275 days
Cs-137
28.8 years
Sr-89
50.5 days
Sr-90
27.7 years
of course, the isotope of principal concern in evaluating the
long-term hazard of delayed fallout.
Cs-137, with a similar half-life, fission
yield and mode of formation serves as a check on the data and is also of in-
terest as a long-term fallout hazard.
,
NAS
Information on the observed Cs-137/Sr-90 ratio in fallout is available
from other sources and can be compared with the balloon filter results.
Baus et ai'13) of the Naval Research Laboratory have determined the ratio in
air filtered at the ground at a series of stations along the 80°W meridian
extending from Thule, Greenland, to Punta Arenas, Chile, using monthly
collections.
Stewart et a(t) in England have made measurements using a
world-wide precipitation network making monthly collections.
storedg\+?
)
in Norway, used Stewart's data, together with some additional measurements in
Norway, to conclude that there is a difference in behavior of the two isotopes,
and suggests that the Sr-90 is more easily captured by rain.
If this is a
result of some physical parameter, like particle size, a systematic distribution of the Cs-137/Sr-90 ratio with altitude should be seen in the
balloon data.
10
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