Publications

The recently updated technical standard for the Department of Energy Laboratory Accreditation Program (DOELAP) may soon require accredited laboratories to empirically verify the estimated minimum detectable activity (MDA) for the nuclides of interest measured by in-vivo detection systems. The Radiation Protection Sample Diagnostics (RPSD) program is the SNL on-site DOELAP accredited laboratory that provides in-vivo measurements of ingested gamma-emitting nuclides (or to prove the lack of significant ingested gamma-emitting nuclides) for the internal dosimetry program administered by Radiation Protection Dosimetry Program (RPDP). Currently, the main nuclides of concern for RPDP include cesium-137 and cobalt-60 as specified in the Statement of Work between the two programs. Historically, MDAs for the RPSD whole-body counting system (WBC) were calculated annually as a-priori values by averaging the critical levels (LC) of any twelve subjects with undetected Co-60 and Cs-137 and assuming MDA is twice the decision level. The purpose of this technical basis document is to evaluate the method and process that validates the a-priori MDA of the RPSD WBC.

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Numerous events have or could have resulted in the inadvertent uptake of radionuclides by fairly large populations. Should a population receive an uptake, valuable information could be obtained by using liquid scintillation counting (LSC) techniques to quickly screen urine from a sample of the affected population. This study investigates such LSC parameters as discrimination, quench, volume, and count time to yield guidelines for analyzing urine in an emergency situation. Through analyzing variations of the volume and their relationships to the minimum detectable activity (MDA), the optimum ratio of sample size to scintillating chemical cocktail was found to be 1:3. Using this optimum volume size, the alpha MDA varied from 2100 pCi/L for a 30-second count time to 35 pCi/L for a 1000-minute count time. The typical count time used by the Sandia National Laboratories Radiation Protection Sample Diagnostics program is 30 minutes, which yields an alpha MDA of 200 pCi/L. Because MDA is inversely proportional to the square root of the count time, count time can be reduced in an emergency situation to achieve the desired MDA or response time. Note that approximately 25% of the response time is used to prepare the samples and complete the associated paperwork. It was also found that if the nuclide of interest is an unknown, pregenerated discriminator settings and efficiency calibrations can be used to produce an activity value within a factor of two, which is acceptable for a screening method.