Interim Baseline Radiological Investigation Report in
Support of the Upton Rare Earth Processing Plant
November 15, 2012
23
magnitude, given the heterogeneity of the site (at least in some areas), differing detector-source
geometry at various sample/survey locations, prominent gamma-ray emitters from the thorium
decay series contributing to the instrument response, and uncertainty in the laboratory
analytical results, especially at low concentrations.
The relationship between the entire data set (n=16) of natural thorium concentrations and gamma
count rates yields a strong correlation coefficient (R
2
= 0.83) with no unusual observations; that
is, observations imparting large leverage on the analysis. The linear regression model for this
iteration, which was used to estimate natural thorium concentrations from the gamma survey data
for the Upton Project is:
Concentration
Th-nat
= 3*10
-4
x cpm – 2.17
Using these equations, radium-226 and natural thorium concentrations in soil were estimated for
each discrete gamma survey measurement. Table 4.6 presents the resulting summary statistics for
the radium-226 and natural thorium concentrations predicted by the linear regression model from
the scanning gamma count rates observed in the Upton Project.
Table 4.6. Statistical Summary of Radium-226 and Natural Thorium Concentrations
Predicted from Gamma Count Rates
Estimator/Endpoint
Radium-226, predicted
(pCi g
-1
)
a
Th-nat, predicted
(pCi g
-1
)
a
Mean (arithmetic)
1.6
2.6
Standard Deviation
0.2
0.6
Median
1.6
2.7
1
st
quartile
1.5
2.2
3
rd
quartile
1.7
3.0
IQR
0.2
0.8
Minimum
0.9
0.4
Maximum
2.7
6.0
Count (n)
38,023
38,023
Notes:
a
Measurements only within the Upton Project.
IQR = interquartile range
pCi g
-1
= picocuries per gram
The median of the predicted concentrations of radium-226 is 1.6 pCi g
-1
and its IQR is
0.2 pCi g
-1
(1.7 less 1.5 pCi g
-1
). This is equal to the median of the laboratory results.
The median of the predicted concentrations of natural thorium is 2.7 pCi g
-1
and IQR is
0.8 pCi g
-1
(3.0 less 2.2 pCi g
-1
). The median of the predicted concentration of natural thorium is
also in good agreement with the median of the laboratory results (2.4 pCi g
-1
).
These observations indicate that gamma surveys can be an effective tool to predict radium-226
and natural thorium concentrations in soil at the Upton Project.
