include the balance of the vegetation samples.
With the inclusion of these
samples, a complete examination of 235°240py correlation with 225U was possible.
RESULTS AND DISCUSSION
Accurate interpretation of the measurement results demands that all reported
specific activities be representative of the extent of sample (vegetation)
contamination, i.e., necessary corrections for local background have been
applied.
Since Area 11 is also known as "Plutonium Valley," it was not at all
surprising to find that the plutonium atom ratios from the four selected
vegetations showed no requirement for background corrections.
Such was not
the case for the uranium.
The observed 738y/235y atom ratios were influenced
by the natural uranium background in degrees varying from slight to significant.
In order to eliminate these natural uranium influences, mathematical expressions
were derived which provided for resolution of the local background (b) and
contaminant (c) fractions via known atom ratios.
Detailed discussion of this
mathematical technique may be found in MCL-TM-76-26 dated April 20, 1976.
Briefly, the atom ratio of 2354 in the contaminant to 73°5U in the background
was calculated from the following general expression: ©
235
235
U,
b
U
- U
o
-wU
c
where
UL = atom ratio of isotope X to *3°U in the local background.
Uy = atom ratio of isotope X to *35U in the observed sample, and
U. = atom ratio of isotope X to 235y in the contaminant or extraneous
source.
Although values of the 235y /235y,
atom ratio may be calculated from Eq. (1)
for each of the uranium isofopic atom ratios (*38y/235y, 236y/235y, and
234y/235y), only the *38y/235y ratios were used. The reason for this decision
was based solely on an inability to accurately define the °36y/235y and
2341/2359 atom ratios of the contaminant.
contaminant *38y/235y atom ratio.
Such was not the case for the
For the type of uranium assumed to have
contaminated the sampling area, an uncertainty in the possible range of
238y/235y atom ratios of no greater than 2% can be accurately predicted.
the 235y /235y,
Once
ratios were determined, the atoms of 235y per gram associated
with the contaminant, *3°U were determined from
2355
—
233)
o
235,
235,
(2)
235.
+1
235,
81