High-Precision Measurement of the ²³⁸Pu/²³⁹Pu Isotope Ratio by TIMS

Area of Science:

• Nuclear Chemistry and Mass Spectrometry
• Isotopic Analysis of Special Nuclear Materials

Background:

• Recertification of uranium and plutonium isotopic standards (SRM 946, 947, 948) is crucial for improving measurement precision.
• Existing methods have limitations in accurately determining the 238Pu content, with uncertainties around 2%.

Purpose of the Study:

• To lower uncertainties in plutonium isotopic compositions, specifically targeting the 238Pu content measurement.
• To achieve a measurement uncertainty for 238Pu content of approximately 0.3%.

Main Methods:

• Proposal of a novel indicator: minima in the (R_MO^(254)/)/(R_M^(238)/) curve for uranium, comparing metal oxide and metal ion isotope ratios.
• Application of thermal ionization mass spectrometry (TIMS) for accurate 238Pu content determination.
• Utilized various enriched uranium spikes and monitored the proposed indicator curve minima.

Main Results:

• Demonstrated the efficacy of the minima indicator in uranium analysis.
• Achieved an external precision of 0.2% for the 238Pu/239Pu isotope ratio determination in SRM 947.
• Successfully combined the indicator monitoring with the use of a 233U spike (with low 238U abundance).

Conclusions:

• The novel indicator and 233U spike method significantly enhance the precision of 238Pu isotopic measurements.
• This approach contributes to the recertification of critical plutonium isotopic standards with improved accuracy.