1. 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: jbw@mail.gel.com General Engineering Laboratories, LLC Radiochemistry Division, Charleston, SC 29407

2. General Engineering Laboratories (GEL). Offers a complete range of environmental testing Organics Inorganics Radiological Bioassay Consulting

3. 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

4. Detection Limit Principle Signal to Noise Image courtesy of the AccuNet/AP Photo Archives ©2000

5. 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

7. Decision Level Concentration (Critical Level) Where: B = background count rate (cpm) ts = sample count time (minutes) tb = background count time (minutes) K = constant used to convert to activity/unit

8. DLC MDA

9. Minimum Detectable Activity Where: B = background count rate (cpm) ts = sample count time (minutes) tb = background count time (minutes) K = constant used to convert to activity/unit

10. 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)

11. Total Propagated Uncertainty Where:  = variance of the net sample count rate (cpm) N = net sample count rate RE eff = relative error of the detector efficiency RE ali = relative error of the aliquot RE rec = relative error of the recovery K = constant to convert to activity/unit

12. Sample Preparation: A Key to Low Level Detection

13. Sample Counting: Backgrounds, efficiencies and sample count times are critical

14. Example Data Is the result less than the DLC?Yes Is the result less than the MDA?Yes Is the result less than 2*TPU?Yes

15. Example Data Is the result less than the DLC?No Is the result less than the MDA?Yes Is the result less than 2*TPU?Yes

16. Example Data Is the result less than the DLC?No Is the result less than the MDA?Yes Is the result less than 2*TPU?No

17. 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%)

18. 0.29 0.14 Result<DLCResult<TPUResult<MDA

19. 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

20. Questions? Contact Information: James Westmoreland jbw@mail.gel.com 843.556.8171