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

Abstract

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 232ThO 2 in a secular equilibrium with its daughter nuclides was studied. Standards were prepared by mixing of known amounts of solid ThO 2 and various solids (e.g. CaCO 3) to reach desired density (e.g. 1.1 g / cm 3 for CaCO 3). 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.