Development of Marinelli beaker standards containing thorium oxide and application for measurements of radioactive environmental samples

Marinelli (reentrant) beakers are recommended for measurement of low-activity radioactive environmental samples, in both liquids and solid phases. The preparation of Marinelli beaker standards containing ²³²ThO₂ in a secular equilibrium with its daughter nuclides was studied. Standards were prepared by mixing of known amounts of solid ThO₂ and various solids (e.g. CaCO₃) to reach desired density (e.g. 1.1 g/cm³ for CaCO₃). Measurements of calibrated Marinelli standards with HPGe detector showed that the energy dependence of the efficiency is similar to that of a point source, i.e. an almost linear dependence of log efficiency vs. log energy in the 200-2000 keV range, however the parabolic correlation fits better. The validity of these standards was checked by comparison to three certified reference Marinelli standards containing radio-cesium, radio-potassium and ²²⁶Ra in environmental samples. Quite good agreement was obtained between our standards and the published certified data. The limit of detection for the determination of radio-cesium by γ-ray spectrometry under the prevailing experimental conditions is 0.07 Bq (1.9 pCi), for 1.0 kg of environmental samples.