Plutonium assay and isotopic composition measurements in nuclear safeguards samples by inductively coupled plasma mass spectrometry

Abstract

We describe analytical methods used to accurately and precisely determine the Pu amount and isotopic composition of nuclear material obtained from chemically digested and purified environmental swipe samples using a multicollector inductively coupled plasma mass spectrometer. Analyses of certified reference material solution standards at a concentration of ∼0.2 pg mL⁻¹ are presented to demonstrate that the isotopic composition of Pu materials with a ²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu, and/or ²⁴²Pu/²³⁹Pu ratio ≥6.3 × 10⁻² can be accurately measured with a precision <1% relative expanded uncertainty (coverage factor k = 2), and the isotopic composition of Pu materials with a ²⁴⁰Pu/²³⁹Pu, ²⁴¹Pu/²³⁹Pu, and/or ²⁴²Pu/²³⁹Pu ratio ≥3.8 × 10⁻⁴ can be accurately measured with a precision <7% relative expanded uncertainty (k = 2). Blank environmental swipe sample measurements and measurements of Pu solutions containing ≤5 fg total ²³⁹Pu are used to constrain the ²³⁹Pu detection limit between 0.10 and 0.58 fg, assuming that entire swipe samples are used for Pu assay, and that Pu loss during chemical separation is negligible. Results from materials prepared to simulate IAEA environmental swipe samples are presented to demonstrate that, following digestion, chemical purification, and analysis by mass spectrometry, the Pu amount and Pu isotope ratios are accurate, with a precision comparable to that of isotopic composition measurements of Pu solution certified reference materials analyzed at a concentration of ∼0.2 pg mL⁻¹. The relative expanded uncertainty of the Pu amount in environmental swipe samples containing ∼0.32 and ∼10.5 pg total Pu is 1.7% (k = 2) on average.