Traceability of performance evaluation materials

One of the most critical elements of a performance evaluation (PE) program for radioactivity measurements is the traceability of the PE materials to the national standards. The requirements and criteria for the production of traceable environmental and radiobioassay PE materials have been defined by ANSI N42.22 and ANSI N13.30 standards. It is important to note that use of traceable source materials does not necessarily ensure the traceability of subsequently derived PE materials unless verification measurements exist in conjunction with the preparation processes. This paper describes the protocol currently used by NIST for the preparation and verification of air filter, acidified water, spiked soil, synthetic urine, and synthetic fecal PE materials for low-level radioactivity measurements. The process involves gravimetric dilutions and mixing of primary radionuclide NIST Standard Reference Materials (SRMs), addition of the derived master solution to sample matrices, and subsequent verification measurements. Several gamma-emitters were used to trace the gravimetric dilutions and spike addition through an unbroken chain of gamma comparison measurements. The massic activities of alpha- and beta-emitters in the diluted solutions and PE samples were also measured by radiochemical methods and compared with their gravimetric values. A correlation analysis demonstrated that the gamma emitters quantitatively followed ⁹⁰Sr, ²³⁸U, ²³⁸Pu, and ²⁴¹Am throughout the dilution and spiking and can be used as effective process monitors. The statistical results from t-tests, box plots, and normal probability tests suggested that traceability of radionuclides in the PE materials to their primary standards can be verified to within 1%, with an overall precision better than 2% (1s).