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High Useful Yield and Rapid Isotopic Analysis of Uranium by Resonance Ionization Mass Spectrometry Michael R. Savina, Brett H. Isselhardt, Andrew Kucher, Reto Trappitsch, Bruce V. King, David Ruddle, Raja Gopal, and Ian D. Hutcheon Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 09 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017
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Analytical Chemistry
High Useful Yield and Rapid Isotopic Analysis of Uranium by Resonance Ionization Mass Spectrometry Michael R. Savina1*, Brett H. Isselhardt1, Andrew Kucher1†, Reto Trappitsch1, Bruce V. King2, David Ruddle3, Raja Gopal4, Ian Hutcheon1 1
Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory The University of Newcastle, Newcastle, Australia 3 National Security Engineering Division, Lawrence Livermore National Laboratory 4 Global Security Computing Division, Lawrence Livermore National Laboratory † Present address: SCIEX, 1201 Radio Rd., Redwood City CA *
[email protected] 2
ABSTRACT: Useful yields from Resonance Ionization Mass Spectrometry can be extremely high compared to other mass spectrometry techniques, but uranium analysis shows strong matrix effects arising from the tendency of uranium to form strongly bound oxide molecules that do not dissociate appreciably on energetic ion bombardment. We demonstrate a useful yield of 24% for metallic uranium. Modeling the laser ionization and ion transmission processes shows that the high useful yield is attributable to a high ion fraction achieved by resonance ionization. We quantify the reduction of uranium oxide surface layers by Ar+ and Ga+ sputtering. The useful yield for uranium atoms from a uranium dioxide matrix is 0.4%, and rises to 2% when the surface is in sputter equilibrium with the ion beam. The lower useful yield from the oxide is almost entirely due to uranium oxide molecules reducing the neutral atom content of the sputtered flux. We demonstrate rapid isotopic analysis of solid uranium oxide at a precision of 4 µm in depth, with >90% of the crater deeper than 0.5 µm, which is deep compared to the 10 nm gold coating. Observing the U and UOx signals in the RIMS spectra during crater sputtering showed a thin oxide layer beneath the gold coating which disappeared after a few seconds (total sputtering times for the craters were 15 minutes). Sputtering yields from the gold and likely substoichiometric oxide layer beneath are unknown, however their contributions to the measured uranium sputtering yield are negligible. For purposes of RIMS analysis, which is extremely surfacesensitive, the sputter-exposed uranium surface picks up a significant amount oxygen in less than a minute even at background pressures below 10-8 torr, likely from residual oxygen and water in the vacuum environment8. Our measurements with Ga+ sputtering on U show rapidly declining U+ signals as a function of time after sputter-cleaning the surface (~25% per minute), with a concomitant increase in the UOx+ signal. To avoid oxide formation during the measurement we sputtered the metal surface with 200 nA of 3 keV Ar+ for 300 µs during the 1000 µs between analysis pulses to remove any surface oxide acquired in vacuum. This kept the surface free of oxygen as evidenced by the near-total lack of UOx+ in the mass spectra, and by the constant U+ signal level as a function of time. The measurements we describe are therefore the maximum achievable useful yield under controlled conditions, and not actual practical useful yields, since uranium is wasted during the cleaning pulse. The intent is to identify and quantify the various effects, and suggest avenues for achieving high useful yield in practice. Figure 2 shows the measured useful yield as a function of the Ga+ primary ion pulse length, and demonstrates a maximum of 24.4 ± 1% for a primary ion pulse length of 95 ns. Figure 2 also shows the prediction of a SIMION model for the LION instrument, which is useful in understanding the instrument and technique, and also in understanding comparisons to SIMS data. The model takes into account several factors that affect useful yield in RIMS: 1. The efficiency with which the sputtering source converts solid material into gas-phase ground state neutral atoms rather than ions, molecules, or excited-state atoms.
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Analytical Chemistry
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Figure 2: Uranium useful yield as a function of primary ion pulse width. The black dashed line is the prediction of a SIMION simulation of the instrument performance. The solid black line shows the ion fraction calculated by the convolution of Equations 1 and 2, and scaled by the detection efficiency (60%) and ground state occupancy (79%).
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The fraction of sputtered material that is ionized by the lasers. 3. The efficiency of photoion transport to the detector. 4. The efficiency of the detector and data collection system. The first factor, the composition of the sputtered flux, is dominantly affected by the oxidation state of the sample surface. The depth of origin of sputtered species is only 1-2 monolayers,22 and it is well known that U sputters overwhelming as uranium oxides when oxygen is present on the surface. Measurements on cuprosklodowskite, a uranium-bearing silicate mineral, showed a RIMS useful yield of 5×10-5, which was attributed to the dominance of oxides in the sputtered flux.23 Willingham et al.10 showed that sputtering U3O8 with Ga+, Au+, Au2+, and Au3+ gave sputtered fluxes with U atom fractions of from 0.6% to 4%, with the remainder being UO and UO2.1 Uranium oxide surfaces are partially reduced by ion bombardment due to preferential sputtering of oxygen, as shown by XPS spectra of uranium dioxide24. This is corroborated by RIMS experiments described below in which the U+/UOx+ ratio in the mass spectrum increases with ion dose, as well as by the study by Willingham et al.10, who showed that the U atom content of the sputtered flux from U3O8 increased as a function of primary ion dose. As described above we ionmilled the surface of the U metal with 3 keV Ar+ to remove the oxide layer and achieve a sputtered flux whose neutral component was essentially all U atoms as indicated by the near-total lack of UOx+ in the RIMS spectra. In the SIMION
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Note that there is an arithmetic error in Table 1 of reference 10. The correct values for the % composition of uranium atoms are 10× higher than reported.
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model we assumed that the sputtered flux consisted entirely of atoms. The population of a low-lying electronic state excited by the sputtering process is also important in uranium RIMS. Laserinduced fluorescence of U atoms sputtered with 3 kV Kr+ (normal incidence) showed that 21±6% of neutral U atoms sputtered from uranium metal are in a low-lying 5K5 excited state at 620 cm-1 above the 5L6 ground state.12 Occupancies of other states were
