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 General Engineering Laboratories, LLC Radiochemistry Division, Charleston, SC 29407

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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

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

DLC MDA

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

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 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

Sample Preparation: A Key to Low Level Detection

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

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

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

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

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%)

Result<DLCResult<TPUResult<MDA

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