This Standard Reference Material (SRM) has been developed in cooperation with member laboratories of the International Committee for Radionuclide Metrology and other experienced metrology laboratories.
The SRM consists of approximately 150 grams shellfish powder that was freeze-dried, pulverized, bottled, and sterilized. The SRM is intended for use in tests of measurements of radioactivity contained in matrices similar to the sample, for evaluating analytical methods, and as a generally available calibrated "real" sample matrix for laboratory intercomparison.
The certified massic activity or activity ratio value for 40K, 90Sr, 137Cs, 210Pb, 228Ra, 228Th, 230Th, 232Th, 234U, 235U, 238U, 238Pu, 239Pu + 240Pu, 241Am, 228Th/232Th, 230Th/232Th, 234U/238U, 235U/238U, 238Pu/(239Pu + 240Pu)was determined from the evaluated average of the individual laboratory means. This approach was selected because of the well-behaved normal distribution of the laboratories' data. The combined standard uncertainties (uc) for each of the certified values were expanded with the Students't-statistics. The expanded uncertainty (U) is taken to be the 95 percent confidence interval. The 95/95 tolerance limits means that NIST is 95 % confident that 95 % of the population of SRM measurements fall within the specified limits. The tolerance limits are used when the number of replicates is small (n<5), e.g. when the material is used as a periodic QC sample. The values for radionuclides activities and isotopic ratios (208Tl, 212Pb, 214Pb, 214Bi, 226Ra, 241Pu, 242Cm, 244Cm) for which insufficient numbers of data sets or for which unresolved discrepant data sets were obtained are reported as a mean value of all reported data and the range of reported results. No uncertainties for those values are provided because no meaningful estimates could be made. The massic activities and mass ratios are not certified at this time, but may be certified at some future time if additional data become available. Users are invited to submit measurement data to contribute to the certification process.