Advances in low level uranium and plutonium isotope mass spectrometry using multiple ion counting and filament carburization

After upgrading IRMM's mass spectrometric capabilities for certification measurements for uranium and plutonium using large sample sizes during the previous years, now in 2006-2007 we focused on necessary improvements in the area of low-level isotopic analyses for uranium and plutonium. This project was driven firstly by the need for reliable verification measurements for the Nuclear Signatures Measurement Evaluation Programme (NUSIMEP) samples at IRMM, secondly by the need for verification measurements on single uranium oxide reference particles and thirdly by the request from the IAEA's Safeguards Analytical Laboratory (SAL) to provide assistance for this type of analyses through the EC support programme. Improving low-level isotope mass spectrometry for uranium and plutonium at IRMM consisted of three steps. First a new thermal ionization mass spectrometer was acquired in order to have an instrument which can be used for peak-jumping measurements in ion counting mode, and which can be subsequently upgraded with a “Multiple Ion Counting” (MIC) system. This detector system allows the simultaneous detection of up to seven small ion beams with currents of 10-19 – 10-14 Ampere in ion counting mode, corresponding to count rates of 1–60.000 counts per second. As a result of test measurements with the MIC system it turned out that static measurements using the MIC system with a sample-versus-standard type external calibration can be associated with uncertainties even higher than in peak-jumping mode. The second step of improvement to tackle this situation was to implement the principle of \"multi-dynamic\" measurements for both uranium and plutonium measurements. This \"multi-dynamic\" measurement procedure provides an internal calibration of the MIC system and therefore circumvents the need for complicated inter-calibration routines. As a third step, a filament carburization procedure was implemented by which the ionization efficiencies for uranium and plutonium were improved by a factor of 3 and 10, respectively. Results for measurements performed on samples of previous NUSIMEP campaigns will be shown in comparison to results from various techniques employed by participating laboratories.