4
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NAVAL RESEARCH LABORATORY
In previous experiments, it was found that wires and screens placed at right angles
to the wind would collect radioactive atomic bomb debris. In view of the many possibilities
which exist for similar collections, both in nature and man-made structures, it was felt
that a repetition of someof the earlier work might be valuable. Table 3 gives comparative
disintegration rates for weekly collections made by the NRL filter equipment, gummed
paper, andan 80-meshstainless-steel screen.- The screen was mounted on a vane so as
to face into the wind. The activity was removed from the screens by repeated washing with
acetone and nitric acid, after which the washings were evaporated and the residue ignited
and counted. Table 3 shows that the total activity collected by the screen was roughly
comparable to that of a filter collection of approximately 3 x 10° cubic feet of air and, in
some cases, was as much as 10 to 100 times that deposited on an equal horizontal area by
fallout. The screen has been found to be of the order of 1% efficient in the absence of rain
(on the basis of estimated air flow through it). For periods when there was a marked
increase in fallout without a corresponding increase in the air concentration, the screen
collections were more nearly equivalent to those of the gummed papers. - There are indications that precipitation will wash activity from the screen; this may be the cause of the
low screen collection of 4/18-4/25.
TABLE 3
Comparison of One-Week Filter, Gummed-Paper, and
Screen Collections at Washington, D.C.
Date of
Collection
4
2/21
2/28
3/7?
3/14
3/21
3/28
Gummed/
(d/m)
(d/m)
Paper
Stainless-
jRainor
(d/m)
(in. )
Steel Screen}
Snow
2/28
3/7
3/14
3/21
3/28
4/4
8, 400
2,500
4,900
5, 200
7,500
6, 700
340
200
92
5,100
2,000
64
3,100
2,100
3,400
1,700
11,000
71,700
1.22
1.78
0.13
1.13
0. 43
0
4/11 - 4/18|
14,000
130,000
75,000
1.50
4/25 - 5/2
2, 000
700
1,900
4/4
=
-
Filter
Collector
-4/11
8, 000
4/18 - 4/25
5/2 - 5/9
8,900
320
2,300
5/9 - 5/16
5/16 - 5/23}
5/23 ~ 5/31
9, 800
9,300 10,000
110, 000
31,000
Total
207, 200
200,316
5,500
9, 700
13, 000
3,800
1,500
14,000
2,600
4, 700
24, 000*
0.07
0.47
0.40
0.01
1.62
0.50
1.11
156,000 -
*Cloth screen
Ordinary cheesecloth* (about 40 mesh) can be used in place of the metal screens.
The
so..3d ¢heeseclothcan be ignited and counted in the same manner as the gummed paper. Some of
the results are shown in Table 4. The cheesecloth seemsto be less susceptible to loss of
radioactivity through the washing action of rain than does the screen. Flags made of
fone
cheesecloth also collect fission activity from the air, but with only about 1/10 the
efficiency of the vane-mounted cloth.
Table 5 comparesthetotal activity collected by gummedpapers, air filters, and 1-sq ft
cloth screens over a two-month period. The measurements included the periods of maximum
air and fallout concentrations. The cloth screens collected about 50% more activity than
either gummed papersorfilters.
*The cheesecloth used in these experiments was found to have an activit’ of about 100 d/m
per sq ft which could be removed by washing, believed to be caused by otassium-40 present in the sizing.
.
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