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Abstract No.: MoP-119
Session: Inductively Coupled Plasma MS/Inorganic MS
Presentation date: Mon, Aug 28, 2006
Presentation time: 09:50 – 11:20

Determination of Isobarically Interfered Plutonium Isotopes Using Thermal Ionization Mass Spectrometry

Suresh Kumar Aggarwal1, Alamelu Devanathan1, Pravin M. Shah1

1 Bhabha Atomic Research Centre, Mumbai, India

Correspondence address: Suresh Kumar Aggarwal, Bhabha Atomic Research Centre, Fuel Chemistry Division, Trombay, Mumbai 400 085, 400 085 India.

Keywords: Ionization, Thermal; Isotope Dilution; Isotope Ratio MS; Mass Spectrometry, Isotope Ratio.

Novel aspect: Atomic and molecular ion formation during simultaneous thermal ionisation mass spectrometry of uranium and plutonium.


Thermal Ionization Mass Spectrometry (TIMS) is an inter-continentally recognized analytical technique for the determination of amounts of different isotopes of plutonium (Pu) in any Pu sample, required for different applications in nuclear science and technology. This is because of the high precision and accuracy inherent in this analytical technique when using a magnetic sector based, fully automated mass spectrometer equipped with multiple Faraday cup detectors for simultaneous collection of all the Pu isotopes. The data obtained on 238Pu and 241Pu isotopes would have uncertainties due to the isobaric interferences from 238U and 241Am, respectively. Plutonium is generally associated with traces of U due to high U/Pu amount ratios in irradiated fuel samples whereas Am is formed in the Pu sample due to radioactive decay of the short-lived isotope 241Pu (14.4 yr). These interferences are conventionally minimized by purifying the Pu sample from U and Am, using a suitable separation and purification procedure. However, there is lack of confidence in the data on 238Pu and 241Pu obtained by TIMS due to the ubiquitous presence of traces of U, in spite of separation and purification and the in-growth of 241Am in purified Pu sample due to the beta decay of 241Pu (14.4 yr). The U contamination can occur from the laboratory environment, apparatus and reagents whereas the 241Am in-growth problem will be more severe in samples containing appreciable amounts of 241Pu and elapsed time after purification.

Experiments were, therefore, undertaken by using synthetic mixtures of U + Pu and Pu + Am to investigate the formation of atomic (M+) and molecular ions (MO+, MO2+) under different heating temperatures of ionization and vaporization filaments of a multiple filament assembly in TIMS. These studies have provided an insight into the accurate determination of 238Pu using Pu+ ion and on-line accounting of the isobaric interference from 238U, in each scan, using interfering element correction methodology. This is done by using 235U as a monitor isotope and pre-determining 235U/238U amount ratio, from the same filament loading, using UO+ ion. During the studies on Pu + Am mixtures, it was observed that the evaporation and ionization efficiency of Am versus that of Pu lies in the range of 0.3 and 1.2 under constant filament temperatures. It was observed that 241Pu amount can be determined in presence of 241Am by using lower heating temperatures of ionization and vaporization filaments, using M+ ion. This paper would present details of the studies carried out by TIMS for determination of 238Pu and 241Pu in real-life samples.