8
NAVAL RESEARCH LABORATORY
Direct impaction by smallfibers is a significant factor in the deposition of radioactive
atomic bombparticulates. Wire and cloth meshes oriented to face into the wind collected,
in some instances, as much as 10 to 100 times the activity deposited on the same horizontal
area by fallout. The screen and fallout collections were approximately equal during periods
of heavy fallout with rain. It seems reasonable to postulate similar impaction phenomena
for vegetation or any other loosely compacted filamentary substance. Except for instances
when activity is rained out of upper-level strata of fairlv limited extent, the available data
indicate that impactionaccounts for a large part of the total deposition. Variations in
screen mesh sizes from 40 to 200 and in the electrical charge had relatively small effects
on the amountof activity collected.
Additional work carried out over a longer period of time will be required in order to
define clearly the relationship between the varicus deposition processes. It is possible,
however, that the cloth screen may provide a simple meansof collection, since it com-
bines someof the advantages of both the air-filter and gummed-paper methods.
Pack-filter measurements for the differentiation of old and new fission debris have
- not, by themselves, given entirely conclusive results. Apparent differences were found
in the particle size distributions similar to those observed during Operation GREENHOUSE.
There is some evidence for increased activity in the large size fraction at short times
after a test. It is also possible that the observed effects may be due to fundamental problems associated with the method of measurement.
ae
REFERENCES
1.
Baus, R. A., Blifford, I. H., Friedman, H., and Lockhart, L. B, Jr., “Radio-
activity of the Air,” NRL Report 4509, March 1955 (Confidential)
2. ° Blifford, I.-H., Lockhart, L. B., Jr., and Rosenstock, H. B., “On the Natural
‘Radioactivity in the Air,” J. Geophys. Research, 57:499-509, 1952
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