On the Quality Control for the Determination of Ultratrace-Level ²³⁶U and ²³³U in Environmental Samples by Accelerator Mass Spectrometry

Abstract

Our recent attempt to determine ultratrace-level ²³⁶U and ²³³U in small-volume seawater samples was challenged by high and unstable procedure blanks in our environmental radioactivity laboratory, which used to be a spent fuel research facility. Through intercomparison experiments with different laboratories and background checks on the chemical reagents and laboratory dust, the resuspended U-bearing dust was identified as the dominating source of the ²³⁶U and ²³³U contamination. With the implementation of background control (especially dust control) measures, the procedure blanks and detection limits of ²³⁶U and ²³³U for the radiochemical separation procedure have been significantly improved by three orders of magnitudes. With well-controlled blanks, the analytical precision for ²³⁶U and ²³³U predominantly relies on the AMS counting statistics. Background check and dust control are strongly recommended before the analyses of environmental-level long-lived radionuclides (such as ²³⁶U and ²³³U) that are conducted in the former or active nuclear facilities, even if clearance of radioactivity relevant for radioprotection was achieved.