SLIDE 10
Analysis of samples.
Soil samples were analyzed for
127
t,
129
25
149r, !45sp,
1376, ,
1355p, , and 60¢9, The methods used for neutron activation analysis were
described by Brauer (Br74) and Keisch (Ke65). JTodine was separated from soils
according to the method of Studier (S$t62).
Once separated, the iodine was
irradiated with neutrons in a nuclear reactor, purified to reduce levels of
interfering nuclides and then measured by gamma spectroscopy (Br80).
In order
to perform quality control, comparison samples containing known amounts of
125,
127, and 1295 were irradiated with each set of fodine samples isolated
from Marshall Islands' soil.
Following irradiation the fodine samples were further purified and then
precipitated onto the detecting media.
The number of atoms of activated
nuclide were determined by gamma spectroscopy measurements of the
radioactivity produced in the soil sample and in a comparison sample.
The
number of initial comparison atoms and resulting comparison activity were used
to determine a production ratio.
The production ratio was applied to the soil
sample activity and the number of atoms of activated nuclide per gram of soil
was estimated.
Corrections to the soil activity were made based on results
for soil sample blanks, comparison sample blanks and method yield.
Individual
counting errors were normally less than 5% although a few samples approached
20%.
A least squares fitting was performed on results for nuclide soil
activity per unit soil mass vs days post detonation using linear, exponential,
logarithmic and power function models.
Sample results varied from their best
fit value by as much as a factor of 9 and by an average factor of 2.5 over the
period 1955 to 1977.
The best fitting function was determined based on a comparison, ef the
coefficient of determination for each nod¢ hy Functions used with
I results
for Rongelap soil are plotted here. For
I soil results, the best
function was exponential. All four fitting functions were generally
predicting soil activity per gram at times after 600 days post BRAVO
nuclides. Significant departure between functions occurs during the
fitting
useful in
for all
period
several hours out to one year post BRAVO.
For example, at 0.5 days the
difference between ghe exponential and power function estimates spans 5 orders
of magnitude for 12 I.
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