Description
There is always a need for a supply of natural matrix standards for the metrology community and the network of radioanalytical laboratories to validate their radioanalytical methods, control the quality of their measurement process, compare measurements results within and between laboratories/programs, monitor lab performance over an extended period of time. They are also used to support the traceability and credibility of measurement results, especially for testing quick turnaround times on measurements of a similar matrix. The goal of international comparison CCRI(II)-S15 is to certify the mushroom material for its 40K and 137Cs content. Thirteen international labs, including NIST, received ~ 400 g of material from Korea Research Institute of Standards and Science to perform the measurements.
Using multiple measurement methods improves the robustness of the overall final results, and we considered several methods to assay the mushroom material: non-destructive and destructive (ashed and dissolved) gamma-ray measurements based on comparison measurements of the received mushroom material and so-called "calibration" or "standard" sources that consisted of a similar material spiked with a calibrated solution of radionuclides of interest. An additional approach complimented both methods (and consisted of using computed ("virtual") instead of measured efficiencies. Potassium hydrogen phthalate was used to prepare a 40K solid calibrated source and potassium nitrate to prepare liquid calibrated sources. Efficiency calibrations for 137Cs were made using 1 mol/L nitric acid and KNO3 solutions spiked with known activities of 137Cs in the case of the destructive approach. Direct spiking of a low-density (0.4 g/cm3) blank material or over-spiking of the same mushroom material with a 137Cs calibrated solution raised concerns over potential losses/contamination from easily dispersed powder and achieving an acceptable level of radionuclide homogeneity. Therefore, the 137Cs efficiency for solid samples was calculated based on Geant4-based Monte Carlo modelling. The modeling approach was validated by comparison of efficiencies for liquid sources. Monte Carlo calculations of efficiency ratios "mushroom powder/liquid" were used together with the measured liquid efficiencies to obtain efficiencies for the non-destructive measurements.
The results showed a good agreement between non-destructive and destructive approaches for both radionuclides, with a difference from -0.1 % to 1.6 % that is at the level of combined measurement uncertainty. And the difference between experimental and modeled efficiencies ranged from -1.9 % to 0.3 %.
This poster will present the details of the project and NIST results of the measurements of the mushroom material as part of the mushroom CRM certification process.
