Accuracy and precision of 238Pu determination at fg level by alpha spectrometry using 239Pu and 240Pu amount analyzed by ICP-MS

The accuracy and precision of the determination of 238Pu amount at the fg (mBq) level by a combination of alpha spectrometry and inductively coupled plasma mass spectrometry (ICP-MS) were studied using a standard reference material (CRM 136-Plutonium Isotopic Standard, National Bureau of Standards, Washington, D.C., USA). The activity of 238Pu was calculated from the amount of 239Pu and 240Pu obtained by isotope dilution method and ICP-MS combined with the intensity ratio of 238Pu/239,240Pu obtained by alpha spectrometry without adding spike. The results show that approximately 130 fg of 238Pu can be analyzed within an accuracy of 7% bias and with 5-12% of total uncertainty. The examined analytical procedure was applied to the quantitative analysis of 238Pu in water samples representing Safeguards inspection samples. The combination of alpha spectrometry and ICP-MS is useful for quantitative and isotopic analysis in low-level Pu samples. Also the method leads to improving the accuracy of 238Pu determination because efficiency calibration is not necessary for alpha spectrometry.