The material was prepared by spiking technique, using radioactive reference solutions with low uncertainty from metrological institutes. All dilution steps and material balance of the entire preparation process were tracked by weight measurement.
Spike solution was prepared using calibrated high precision analytical balances. The on-the-job dry mass determination of the raw material was carried out by thermo balance.
The spiking process was carried out in two steps: at first, 1 kg of the material was spiked, then dried in the open air and homogenized using a special Teflon-lined container. To force the blending process and to destroy potential conglomerates 15 ceramic balls were added. In a second step this concentrate was diluted to 20 kg and homogenized in the TURBULA 51 power blender for 60 hours.
The homogeneity of the material was tested in 1 g sample size using gross beta measurement by LSC technique.
The dry content of the material was measured in two steps. At first the optimum temperature was determined by thermo balance in the range of 50-100oC. 90oC was identified as optimum temperature. Further, the average moisture content of the material was determined by drying several test portions of 2 g in an oven at 90 (± 2)oC for 8 hours, and was found to be 3.8 (± 0.1)% with a coverage factor k=1.
After checking for the homogeneity of sample material, altogether 197 bottles were prepared, each containing 100 g of milk powder.
The bottles were labeled IAEA-473, arranged into plastic boxes and sterilized according to ISO standards using gamma-ray irradiation with a total dose of 25 kGy using a 60Co source.
Certified values for massic activities (based on dry mass)
Reference date for decay correction: 1 November 2014
|Radionuclide||Certified value [Bq kg-1]||Uncertainty* [Bq kg-1]|
* The uncertainty is expressed as a combined standard uncertainty with a coverage factor k = 1 estimated in accordance with the JCGM 100:2008 Evaluation of measurement data – Guide to the expression of uncertainty in measurement.
The assigned property values were derived from the certified values specified in the certificates of the radioactive solutions by formulation considering the dilution factors controlled by unbroken chain of weight measurements. Thus they are traceable to the derived SI unit Bq-1 through the certificates issued by the providers.