Interim Baseline Radiological Investigation Report
November 14, 2012
Bear Lodge Project, for Rare Element Resources, Inc.
Rev.1
11
Table 3.2. Statistical Summary of Gamma Count Rates Outside Project
Estimator/Endpoint
Gamma Count Rate
(cpm)
Mean (arithmetic)
12,841
Standard Deviation
7,163
Median
9,818
1
st
quartile
7,129
3
rd
quartile
16,540
IQR
9,411
Minimum
4,254
Maximum
40,469
Count
24,522
Notes:
cpm = counts per minute
IQR = interquartile range
3.3
Derivation of Exposure Rates Based on HPIC Correlation
One-minute integrated counts from 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 3.3 presents the results of the paired
measurements. Exposure rates are reported in microRoentgens per hour (µR/hr). 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) = (5x10
-4
) x cpm + 9.42
Plotting of the data from the HPIC and sodium iodide detector indicates that, with a correlation
coefficient, R
2
, of 0.98, the sodium iodide detector response explains 98 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 3.4 presents summary statistics for the exposure rates that are
predicted from the population of scanning gamma count rates observed in the Project.
Figures 3.3 and 3.4 present the exposure rates predicted from all of the scanning gamma count
rates observed within the Project and on Miller Creek Road (portions of the road outside the
Project), respectively. 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 five colors --from dark blue to
red-- based on its value: exposure rates below 15 µR/hr and above 100 µR/hr, are depicted as
dark blue and red, respectively.
