Interim Baseline Radiological Investigation Report in
Support of the Upton Rare Earth Processing Plant
November 15, 2012
11
3.3
Derivation of Exposure Rates Based on HPIC Correlation
One-minute integrated counts made using the sodium iodide detector were correlated to exposure
rate measurements, made with a High Pressure Ion Chamber (HPIC) at eleven locations. Figure
3.1 shows the locations of the measurements. Table 3.2 presents the results of the paired
measurements. Exposure rates are reported in microRoentgens per hour (µR hr
-1
). Gamma
measurements are reported as cpm.
The HPIC was in current calibration and function checked daily, before and after use. Calibration
and function forms for the HPIC are provided in Appendix A.
The linear regression model for the eleven pairs of measurements is:
Exposure rate (µR hr
-1
) = 5x10
-4
x cpm + 5.76
Plotting of the data from the HPIC and sodium iodide detector indicates that, with a correlation
coefficient, R
2
, of 0.99, the sodium iodide detector response explains 99 percent of the variance
in the exposure rate; thus, the model strongly fits the data.
Figure 3.2 is a plot of the linear regression. This model was used to convert the gamma count
rate data to exposure rates. Table 3.3 presents summary statistics for the exposure rates that are
predicted from the population of scanning gamma count rates observed in the Upton Project.
Figure 3.3 presents the exposure rates predicted from all of the scanning gamma count rates
observed within the Upton Project. This figure also shows the two areas of differing gamma
count rates. As stated in Section 2.1, the gamma survey results are expressed as exposure rates to
allow a more meaningful context for interpretation than the relative magnitude of individual
gamma readings. Each datum is assigned to one of three colors —from dark blue to green—
based on its value.
Table 3.2. Integrated Gamma Count Rates and
High Pressure Ion Chamber Measurements
Measurement
Location
Easting
a
Northing
a
Sodium Iodide
Detector Response
(cpm)
HPIC Response
(µR hr
-1
)
PIC-01
526635.3
4883935.2
15958
14.4
PIC-02
525096.5
4883855.1
13915
13.2
PIC-03
525987.9
4882773.8
13950
13.2
PIC-04
527194.8
4883317.2
18367
15.6
PIC-05
527179.8
4884002.4
22100
17.6
Notes:
a
Coordinate system is NAD 1983, UTM Zone 13N, meters.
cpm = counts per minute
µR hr
-1
= microRoentgens per hour
