Improvement of the 241Pu/239Pu isotope ratio measurement by thermal ionisation mass spectrometry - An approach to account for the 241Am in-growth after plutonium purification

Abstract

Reliable determinations of plutonium isotope ratios are required in the nuclear field. Thermal ionisation mass spectrometry (TIMS) is used for this purpose. Commercially available TIMS instruments cannot distinguish between 241Am and 241Pu. A purification step before measurement to remove americium from plutonium solutions is required. Nevertheless, the amount of 241Am in-growth due to 241Pu decay (half-life 14.3 years) in the time between plutonium purification and TIMS measurements can be significant for an accurate determination of the 241Pu/239Pu isotope ratio. This paper presents a method, with a focus on nuclear safeguards, to correct for the 241Am in-growth after plutonium purification. In addition, the method provides an estimate for the americium ionisation efficiency to allow an even more accurate determination of the 241Pu/239Pu isotope ratio.