Lprtplittirtiuy
2 POINTS REPRESENT
14956 SOIL ANALYSES
|
Sr? JUNE 1957
a
|
Sr9° SURFACE DEPOSITION LEVELS, MC/MI”
I
25 -—-
4
Sr99 JUNE 1956
{Oo
—s
5
—d
°
ne
Le
a
pouty
30°N
°
|
60° N
|
tf
tet
|
30°N
ft
oO?
EQUATOR
tet
|
|
°
30°S
SS
fers
7
_|
60° S
LATITUDE
Fig. 2—Surface deposition levels of Sr*® from soil analyses.
estimated from the lower curve of Fig. 2. He also estimated the stratospheric reservoir at
about 2.4 megacuries (24 megaton equivalents of fission yield). His predictions were based on
1955 soil analyses, his model of tropospheric and stratospheric fallout, and a general knowl-
edge of the megatons of fission devices detonated during the spring and summerof 1956. Using
his deposition values and the world Sr® production up to that time, 24 megaton equivalents
still in the stratosphere would be possible only provided local fallout from surface detonation
of megaton weapons was about 25 per cent.
Values for the world total production (about 5.5 megacuries to mid-1957) and deposition
of Sr® may be used to estimate the present magnitude of the stratospheric reservoir. Fallout
measurements from United States megaton detonations in the Pacific suggest that 50 + 17 per
cent of fission debris falls out locally. The rest is partitioned between tropospheric and
stratospheric fallout. Since the fallout time of tropospheric debris is of the order of 20 to 30
days, the material not accounted for by local fallout plus total world-wide deposition must
still be in the stratospheric reservoir. Such a material balance calculation estimates the
stratospheric Sr®™ content at 1.11 + 0.93 megacurie, or the equivalent of 11.1 + 9.3 megatons
of fission yield. The average is about one-half the value estimated by Libby.”
High altitude air sample measurements suggest that considerable specific fractionation
of fission debris is occurring. If, however, it is assumed that serious fractionation of Sr,
Pu®*? and Cs!" does not occur,” it is possible to estimate the general distribution of these
nuclides in relation to Sr®’. Since their radiological half-lives are long compared to the period
of testing and the stratospheric storage time, their general distribution should be in direct
ratio to their total production relative to that of Sr® (Table 1). On this basis, average maxi-
mum surface deposition levels of Pu’*®® and Cs!*" in the north temperate latitudes (by mid-1957)
would be about 1.2 and 25 mc/sq mile, respectively.
Present world-wide distribution of Cs!’ and Pu’*®, estimated on the above basis, are
compared with Sr® in Table 2.
The estimated levels are general averages only and assumeno fractionation and uniform
distribution within the respective areas. Actually, this general picture is greatly over-
287