Calculation of the Combined Standard Uncertainty in the Radiochemical Determination of Pu isotopes, ²⁴¹Am and ⁹⁰Sr in Environmental Samples

J. Moreno, P.R. Danesi, K. Burns, P. De Regge,

International Atomic Energy Agency, Seibersdorf Laboratory,

A-2444, Seibersdorf, AUSTRIA.

The combined standard uncertainty in the activity concentration of ⁹⁰Sr, ²⁴¹Pu, Pu alpha-emitting radioisotopes and ²⁴¹Am was estimated both algebraically and using a spreadsheet method. The sensitivity parameters and major factors contributing to the combined uncertainties were identified. The radiochemical procedure involved the sequential determination of these radionuclides in a single environmental sub-sample. The analysis of a-emitters was performed by isotope dilution a-spectrometry using ²⁴²Pu or ²³⁶Pu and ²⁴³Am tracers. Strontium-90 was measured by liquid-scintillation counting after radiochemical separation using the double energy windows method. The activity concentration of ²⁴¹Pu was measured by liquid-scintillation counting using the same plutonium source prepared for a-spectrometry analysis.

Calculation of the combined uncertainties in the recovery factors, efficiency and activity concentrations using the algebraic method based on differentiation and the spreadsheet method yielded essentially the same results in all cases. The present paper will show the calculation of combined uncertainties using illustrative examples.

One example concerns the calculation of the Sr recovery factor in the IAEA reference materials Soil-6, IAEA-135 and IAEA-367. While the major contributors to the combined uncertainty in the recovery factor for Soil-6 and IAEA-135 were the uncertainty in the concentration of the Sr standardised carrier solution and uncertainties associated with weighing the final Sr source; the major contributor in reference material IAEA-367 was associated with the concentration of natural Sr in this sample. In the case of the activity concentration of ⁹⁰Sr, the major contributors were those associated with the counting statistics, especially the count rate and background rates in the ⁹⁰Sr and ⁹⁰Y windows and; with the efficiency of the liquid-scintillation counter for the detection of ⁹⁰Sr. For IAEA-367, the contribution of the Sr recovery factor in the combined uncertainty in the activity increased to 22 %. In Soil-6 and IAEA-135, the most important sensitivity parameters were the gross count rate and background rates in the ⁹⁰Sr window. In sample IAEA-367, which contains high activity levels of ⁹⁰Sr, the most important sensitivity parameter was the gross count rate in the ⁹⁰Sr window.

For samples such as IAEA-135 containing elevated levels of ²⁴¹Pu, the calculation of the combined uncertainties in the activity showed that the major contributors were: i) the recovery factor (76 %), ii) the efficiency of the by liquid-scintillation counter for the detection of ²⁴¹Pu (21 %) and iii) the ash to dry ratio f_ad. (2 %). The most important sensitivity parameters were: i) the gross count rate of ²⁴¹Pu, ii) the Pu recovery factor, iii) the efficiency and iv) the mass of sub-sample taken for the radiochemical analysis.

Another example concerns the determination of the activity concentration of ²⁴¹Am in IAEA-135. To correct the activity concentration of ²⁴¹Am back to the reference date involves not only correction for decay of ²⁴¹Am but also correction for ingrowth from ²⁴¹Pu. The correction of Am originating from ²⁴¹Pu was one important factor contributing to the combined uncertainty in the activity (6 %). The major contributors were those associated with counting statistics: the count rate of ²⁴¹Am (30 %), the count rate of ²⁴³Am (30 %); and also with the ash to dry ratio (17 %) and the activity of the tracer (15 %).