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RADIATION STANDARDS, INCLUDING FALLOUT
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All samplng referred to herein was condugted at the Argonne National Laboratory site; hence the dose values derived from these data pertain rigorously only
to this location. Due to the similarity in deposition observed at other sites having
comparable precipitation and located nearby in lattude, the dose values may
be presumed to have consderably wider applicability.
As illustrations of the three test situations, Hardtack I (April—July 1958)
was chosen for the low equatorial stratospheric injection case, Soviet October
1958 for low polar stratospheric injection, and the Orange and Teak shots
(100,000 feet over Johnston Island in August 1958) for the high-altitude case.
The choice of these series and shots is not arbitrary, since the production of
Ww", in Hardtack I and Rh’ in the Orange shot provided tracers for these
events. Activity ratios between appropriate nuclides have been used to determine the Soviet portion of the remaining debris.
Modifications of Dunning’s method for obtaining the infinite plane dose from
surface depostion were used to caleulate the air gamma ray dose rate. The
modifications take into account the actual distribution in depth, determined
experimentally, of each of the various fission products in soil. The attenuation
and dose buildup factors based upon the vertical distribution in soil are included
in the dose equations which are given in appendix I for each isotope. In essence,
the dose rate so calculated is no longer the usual infinite plane dose, but is
analogous to that obtained by applying corrections for weathering and terrain
roughness to the infinite plane values. Further reduction in the actual dose to
an individual may arise due to shielding by dwellings and other structures.
Shielding factors currently in use range from 0.2 to 0.7 times the open field
dose.
During late 1958 and throughout 1959, soil measurements at Argonne indicated
the deposition of 40 millicuries per square mile’ of Cs” arising from the Soviet
October 1958 tests. Furthermore it was shown from coneurrent air and precipitation studies that almost all of the Soviet debris came down within 1 year after
the series ended. Assuming that this quantity of Cs*” came from 12.5 megaton
of fission, 3.2 millicuries per square mile* of Cs'” was produced per megaton of
fission and was deposited within 1 year. The open-field dose from this deposition during the first year amounted to approximately 0.1 milliroentgen. The dose
from short-lived nuclides during this interval was 3.83 milliroentgen as shown
in table I. The corresponding integral doses for 30-year (genetic dose) and
70-year (lifetime dose) intervals are also shown in table I. The mean stratospheric residence time of this debris was 8 to 10 months as derived from Cs’
concentration in surface air according to the methodillustrated in figure 1. The
ratio of short-lived fission product dose to that from Cs'” for 30- and 70-year
intervals has been calculated as a function of mean stratospheric residence time
and is shown in graphical formin figure 2. The ratio of dose from short-lived
emitters to that from Cs” for the Soviet October 1958 tests is shown in tabular
form in table II for the 30- and 70-year intervals. The mean residence time
corresponding to these values as obtained from figure 2 is indicated in
parentheses.
For Hardtack I, the assumption has been miade that 1.6 millicuries per square
mile’ of Cs” per megaton offission will be deposited at Argonne instead of 3.2
millicuries per square mile® per megaton as the debris is presumed to divide
equally between Northern and Southern Hemispheres. In addition the longer
mean stratospheric residence time deduced from the W*™ data (15 to 18 months)
means that 1.6 millicuries per square mile” per megaton will be an upper limit
for Cs™, because a portion of the nuclide will decay before reaching the ground.
Approximately half of the total Hardtack Cs’ reached the ground during the
first. year after the series, hence the dose from this nuclide pertaining to the
deposition of 0.8 millicuries per square mile’ in 1 year has been computed (table
T). The corresponding dose from short-lived fission products wag obtained
using the dose ratio found upon computing the various dose contributions from
the observed deposition of Hardtack debris. The 30- and 70-year integral doses
for Cs™ and short-lived components are alse indicated in table I.
The dose ratios
are indicated in table II with corresponding values of mean residence time as
derived from figure 2 shown in parentheses as before.
The reasonably good
uereement between observed mean stratospheric residence time and those found
from figure 2 is somewhat surprising when one realizes that 2 constant rate of
deposition is implicit in the data plotted in figure 2.
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