Preparation and homogeneity of a monazite sand matrix: A candidate reference material for activity concentration measurements 40K, 226Ra, 228Ra and 214Pb

The production of Certified Reference Materials (CRM) is an essential activity to improve and maintain a coherent measurement system worldwide. CRMs are analytical standards of high metrological reliability used for calibration, quality control, method validation and determination of the accuracy of results. This work focuses on the development of a monazite-rich sand standard according to ISO/IEC 17034. "Praia da Areia Preta", located in Guarapari, southeastern Brazil, is known for its above-average concentration of ²³²Th. Although widely studied, there is no metrological reference (standard) for this type of sand (monazite). Homogeneity is one of the most important characteristics of CRMs, verified by the statistical method of Analysis of Variance (ANOVA) for the radionuclides in this study. Homogeneity is defined as the degree to which a property is randomly distributed throughout the material. In practice, a CRM candidate is only considered sufficiently homogeneous when the contents of its various vials do not differ sufficiently from each other to affect their intended uses, as specified in Guide 33. The mean activity concentrations and their uncertainty for each radionuclide in this study were, respectively: (33.8 ± 1.4) Bq kg^-1 for ⁴⁰K; (44.56 ± 0.36) Bq kg^-1 for ²²⁶Ra; (416.12 ± 0.68) Bq kg^-1 for ²²⁸Ra; and (43.96 ± 0.33) Bq kg^-1 for ²¹⁴Pb. The CRM candidate samples are homogeneous for the entire prepared batch. The results for Pb and Bi (²²⁶Ra) showed close values, indicating good measurement and equilibrium of the decay chain.