Transcript Powerpoint

A Comparison of Techniques
Used to Evaluate Low Level
Radiochemical Data
Theresa L. Parrotte, Scott C. Moreland,
J. Stan Morton Ph.D., James B. Westmoreland
email: [email protected]
General Engineering Laboratories, LLC
Radiochemistry Division, Charleston, SC 29407
• General Engineering Laboratories (GEL).
Offers a complete range of environmental testing
•Organics
•Inorganics
•Radiological
•Bioassay
•Consulting
Introduction
• Radiochemistry laboratories provide data in the form:
result + uncertainty & detection limit
• Data users may need to make a “detection decision”
based on this data
• The method used to make this decision must be carefully
selected or the results can be misleading
Detection Limit Principle
Signal to Noise
Image courtesy of the AccuNet/AP Photo Archives ©2000
How do we calculate the minimum
signal distinguishable from the noise?
• “Math is the Language of Science” -unknown
• Statistical Models are useful to predict method
sensitivity (signal to noise threshold)
• Why use Statistics?
– Radiological measurements are random in nature
– We must make estimates based on a single measurement
Poisson Distribution
1.000
95% confidence level
0.800
0.600
0.400
0.200
0.000
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Decision Level Concentration (Critical Level)
ts
1.645* B(ts)(1  )
tb
DLC 
K * ts
Where: B = background count rate (cpm)
ts = sample count time (minutes)
tb = background count time (minutes)
K = constant used to convert to activity/unit
Poisson Distributions
1.000
0.800
DLC
MDA
0.600
0.400
0.200
0.000
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32
Minimum Detectable Activity
ts
3  3.29* B(ts)(1  )
tb
MDA 
K * ts
Where: B = background count rate (cpm)
ts = sample count time (minutes)
tb = background count time (minutes)
K = constant used to convert to activity/unit
3 Approaches Evaluated
• Comparison of the Result with the Decision Level
Threshold (DLC) (a.k.a critical level)
• Comparison of the result with the 2 sigma total
propagated uncertainty (TPU)
• Comparison of the Result with the Minimum
Detectable Activity (MDA)
Total Propagated Uncertainty
TPU 
 2  N 2 * (REeff 2  REali2  RErec2  ...)
K
Where:  = variance of the net sample count rate (cpm)
N = net sample count rate
REeff = relative error of the detector efficiency
REali = relative error of the aliquot
RErec = relative error of the recovery
K = constant to convert to activity/unit
Sample Preparation:
A Key to Low Level Detection
Sample Counting:
Backgrounds, efficiencies and sample count
times are critical
Example Data
Result 2*TPU DLC MDA
Am-241 0.023 0.061 0.035 0.161
Is the result less than the DLC?
Yes
Is the result less than 2*TPU?
Yes
Is the result less than the MDA?
Yes
Example Data
Result 2*TPU DLC MDA
Pu-239 0.008 0.011
0.006 0.023
Is the result less than the DLC?
No
Is the result less than 2*TPU?
Yes
Is the result less than the MDA?
Yes
Example Data
Result 2*TPU DLC MDA
Th-230 0.132 0.044 0.012 0.200
Is the result less than the DLC?
No
Is the result less than 2*TPU?
No
Is the result less than the MDA?
Yes
Summary Data
1639 records were evaluated with result < MDA
• 1366 were also less than the TPU (83%)
• 927 were also less than the DLC
(57%)
• 273 were less than the MDA but greater than
the DLC and the 2*TPU level
(17%)
Activity Summary
0.5
Result<DLC
Result<TPU
Result<MDA
0.29
0.3
DLC
MDA
Result
TPU
0.2
0.14
0.1
Replicates (increasing activity)
40
37
34
31
28
25
22
19
16
13
10
7
4
0
1
Activity
0.4
Conclusions
• Comparing results to MDA alone is not
recommended for making a detection decision
• Comparing results to DLC is recommended
but can be problematic at low background
count rates (alpha spectrometry)
• Comparing the result to the 2*TPU can be
helpful in detection decision making
Questions?
Contact Information:
James Westmoreland
[email protected]
843.556.8171