3
to October 1952 for any United States
stations except Salt Lake City, However,
the values reported may be taken as a
satisfactory approximation of the total
gamma dose delivered to date. TL.fallout prior to October 1952 was of a low
order compared with that during the sub-
sequent period.
As expected, the high accumulations
of mixed fission products (Fig. 1) exist
in the vicinity of the proving grounds in
Nevada and in the vicinitv of the Pacific
test area. The highest recorded deposition, Grand Junction, Colo., is 740 milli-
curies per square mile as of | January
1956. The values are lower everywhere
else, the next highest deposition being 290
millicuries per square mile at Iwo Jima
in the Western Pacific Ocean. The lowest
value is Lagos, Nigeria, with 33 milli-
curies per square mile. In general, the
mixed fission products data are characterized by a high degree of regional uniform-
ity in which, with few exceptions, adjacentstations agree within + 50 percent.
The estimates of strontium-90 are
shown in Tables | and 2. Outside of the
United States, the observed range is from
1.8 millicuries per square mile in Peru to
24 millicuries per square mile at Iwo
Jima. In the United States, the accumulation varies from 2.1 at San Francisco
to 23 at Salt Lake City. For most of the
world covered by this network, the range
of 2 to 5 millicuries per square mile may
be taken as a representative estimate of
the distribution of strontium-90.
With respect to the gamma dose, the
average value for the United States is
higher thanit is for the rest of the world.
The range of values in the United States
is relatively narrow, 6 to 49 millirads,
except for Salt Lake City (160), Grand
Junction (120), and Albuquerque, N.M.
(110). The representative dose for east-
ern United States is about 15 to 20 milli-
rads, with slightly higher values in the
Middle West and lower values on the
West Coast.
The cumulative gamma dose at the
foreign stations is in the range of 4 to 23
millirads, except for some of the Pacific
Table 2. Fallout at stations within continental United States, October 1951 to September
1955.
.
Station
Mixed fission
proce)
Sr*
(me/mi’)
Albuquerque, N.M.
Atlanta, Ga.
Billings, Mont.
Binghamton, N.Y.
400
120
160
61
20
3.8
5.7
2.2
110
16
24
7.8
Chicago, Il.
140
5.3
2+
Boise, Idaho
Dailas, Tex.
Des Moines, lowa
160
170
170
4.2
18
3.3
8.4
4.9
3.6
21
120
13
49
18
17
New York, N.Y.
Philadelphia, Pa.
Pittsburgh, Pa.
Rapid City, $.D.
110
110
100
150
4.2
4.6
4.1
6.1
17
16
13
25
St. Louis, Mo.
Salt Lake City, Utah
San Francisco, Calif.
200
680
47
6.0
23
2.1
27
{60
5.8
Scottsbluff, Neb.
Washington, D.C.
200
86
6.3
3.0
26
il
New Orleans, La.
Rochester, N.Y.
Seattle, Wash.
170
for collection of total fallout activity is
taken to be 63 percent, and all of the
data have been corrected accordingly.
A source of error which is more diffhcult to assess is that inherent in the assumption that the decay of the radioactivity is proportional to ¢-1-?, Application
of this decay law requires that the age
of the debris be known with some cer-
tainty, and this has become increasingly
difficult during the past 2 years, Prior
to 1954, it was possible to predict the
decay characteristics of a sample accurately because at any given time the
debris was known to haveoriginated from
the most recent test series. After each
series, the daily fallout would diminish
rapidly and would ordinarily be unde-
1952, the yields from detonations were
relatively low, and the bulk of the debris
where fallout is greatly hastened by precipitation and other factors, In contrast,
the detonation of devices having yields
noted previously, the over-all efficiency
stable region of the earth’s atmosphere,
efficiency
of
the
tance in interpreting reported values. As
13
11
the period October 1951 to September 1955.
gummed film is of fundamental impor- .
collection
3.5
27
* Gammadose for the period October 1952 to Seprember 1955, except for Salt Lake City, which covers
equivalent to megatons of TNT produces
clouds of radioactive debris which pierce
the tropopause and becomedistributed in
The
3.7
89
was distributed below the tropopause,
Discussion
5.7
99
was deliberately conservative.
tion was intended to provide a tentative
upper limit of the estimated dose and
29
28
140
740
88
200
130
110
These gamma values are somewhat
lower than the average estimate of 100
milliroentgens for the United States as
reported by Dunning (5) whose calcula-
millirads.
6.1
6.2
16
Detroit, Mich.
Grand Junction, Colo.
Jacksonville, Fla.
Memphis, Tenn.
Minneapolis, Minn.
New Haven, Conn.
tectable before the next series of tests
started.
The rapid diminution in fallout from
tests conducted prior to the spring of
1954 can be explained by the fact that.
except for Operation Ivy in November
islands, where the rangeis from 13 to 150
9.2
Gammadose*
(mrad)
the stratosphere. From this relatively
the particles descend slowly, and fallout
to the earth’s surface occurs over a period
of time which is measured in years rather
than weeks or months. Thetraces of rela-
tively dld debris from high-yield devices
become mixed with the debris of subse-
quent detonations. This being the case,
neither the decay characteristics of a sample nor the relative abundance of the
long-lived isotopes can be predicted from
theory.
In recent months, the procedure has
been changed and, where necessary,either
individual or pooled samplesare followed
for decay.
There is no practical way in which the
accuracy of the estimates of accumulated
mixed fission products can be tested experimentally. However, it is possible to
test the validity of the strontium-90esti-
mates by direct radiochemical analyses of
the soils from thevicinity of the sampling
stations. In October 1955, immediately
following the sampling period covered by
this report, soil samples were collected
from 17 widely scattered locations in the
United States. The relationships between
the predicted and measured values are
given in Figure 2. It is seen that the data
consist of two groupings, in which [4
points show a reasonably satisfactory re-
gression of measured on predicted values.
The correlation coefficient for these data
is 0.70. The otherthree points forgations
near the Nevadatest site show low ratios