A New Resonance Ionization Mass Spectrometry Scheme for Im

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A New Resonance Ionization Mass Spectrometry Scheme for Improved Uranium Analysis Michael R. Savina, Reto Trappitsch, Andrew Kucher, and Brett H. Isselhardt Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02656 • Publication Date (Web): 21 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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Analytical Chemistry

A New Resonance Ionization Mass Spectrometry Scheme for Improved Uranium Analysis Michael R. Savina1*, Reto Trappitsch1, Andrew Kucher1†, Brett H. Isselhardt1 1 †

Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory Present address: SCIEX, 1201 Radio Rd., Redwood City CA

ABSTRACT: Resonance Ionization Mass Spectrometry (RIMS) combines tunable laser spectroscopy with mass spectrometry to provide a high-efficiency means of analyzing solid materials. We previously showed a very high useful yield of 24% for analysis of uranium using three lasers to excite and ionize atoms sputtered from metallic uranium and uranium dioxide. A new resonance ionization scheme using only two lasers achieves a higher useful yield of 38% by accessing both the ground electronic state and a lowlying electronic state of atomic uranium that is significantly populated by sputtering. The major loss channel in analyzing uranium dioxide is the formation of UOx molecules during sputtering. Pre-bombardment of the surface with 3 keV noble gas ions prior to analysis reduces the surface and results in a sputtered flux with a greatly enhanced proportion of atomic U. This method of surface reduction results in uranium useful yields as high as 6.6% for uranium dioxide analysis, compared to 2% from previous work.

Resonance Ionization Mass Spectrometry (RIMS) is increasingly used in actinide detection and characterization.1-6 RIMS uses lasers tuned to electronic transitions to ionize elements selectively, and is useful for analyzing materials rapidly primarily because it avoids the need to remove or resolve isobars. Thus solid samples can be analyzed directly, i.e. without prior chemical treatment. Because of the generally high efficiency of resonance ionization, RIMS can achieve very high useful yield (defined as the number of atoms detected normalized by the number removed from the sample). The major limitation in determining the useful yield of uranium measurements by ion sputtering followed by resonance ionization is the atom/oxide ratio in the sputtered flux.7,8 Because of this effect, we obtained a very high useful yield of 24% for U atoms sputtered from reduced uranium metal, but only 2% for U atoms from uranium dioxide.9 In fact, the useful yield from virgin uranium dioxide is only 0.4%; extensive physical reduction of the surface by preferential sputtering of oxygen using 15 keV Ga+ was necessary to achieve a 2% useful yield. Preferential sputtering of oxygen from metal oxides is a well-known phenomenon,10 and high-resolution X-ray photoelectron spectroscopy has shown that bombarding uranium dioxide with 3 keV Ar+ produces uranium valence states down to U0, with an estimated surface composition of UO~1.4.11 An additional significant effect on uranium useful yield in RIMS is the fraction of U atoms that sputter into a low-lying electronic state rather than the ground state. Wright et al. found that 17±4% of U atoms sputtered from uranium metal are in the 5K5 state at 620 cm-1, with essentially all of the rest in the ground state.12 Resonance ionization spectroscopy (RIS) schemes that do not access both states are necessarily limited in their achievable useful

yield. Our previous work used a three-color RIS scheme that accessed only the U ground state. In this work we demonstrate two new one-color RIS schemes for the analysis of U, one of which accesses atoms in the ground while the other accesses atoms in the 5K5 state. When applied together the result is simultaneous excitation from the ground and excited states and a much improved useful yield. We use the combined scheme to determine the effect of bombarding uranium dioxide with 3 keV noble gas ions from He to Xe. We use Ga+ sputtering followed by resonance ionization to examine the bombarded surfaces, and quantify changes in sputtering yield, surface reduction, and U useful yield. Experimental Samples The uranium dioxide sample was CRM 125A, a standard reference material from the US Department of Energy New Brunswick Laboratory (>99.9% UO2, 4.23% enriched in 235U). This material comes in the form of a sintered powder fabricated into fuel pellet suitable for a nuclear reactor, ~1 cm in diameter and ~1 cm long. We cut a disc ~2 mm thick from the pellet and glued it to a sample stub using a conductive acrylic adhesive. The metal sample consisted of a piece of isotopically natural uranium (0.72% 235 U) ~1 cm long, 3 mm wide and 3 mm thick potted in epoxy. Both samples were polished to provide a smooth surface at the level of ~1 µm RMS roughness. The metal was coated with a gold layer ~50-100 nm thick to protect it from oxidation and provide conductivity across the epoxy surface to prevent sample charging. Instruments and Methods All RIMS measurements were made on the LION (Laser Ionization Of Neutrals) instrument at Lawrence Liver-

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more National Laboratory. In brief, material is sputtered from a solid by means of a pulsed ion beam and ionized above the surface by one or more pulsed lasers tuned to electronic resonances in the element of interest. The ions are then detected by time-of-flight mass spectrometry. Details are given elsewhere.9,13 In this work, a MagnetTOF discrete dynode detector with an ion pulse width of 550 ps (FWHM) was used instead of the microchannel plate used previously. A Hiden IG20 ion gun was used to bombard the solid uranium dioxide prior to analysis. The ions were He+, Ne+, Ar+, Kr+, and Xe+ at an incident angle of 60º from the surface normal. The ion energy was 3 keV, with a spot size at the sample of ~100×200 µm and a beam current from 18 to 23 nA depending on the gas. Ion currents were determined using a Faraday cup. The noble gas ion beams were rastered over ~350×350 µm areas on the surface. Dosing times ranged up to 16 minutes. A liquid metal ion gun was used to sputter material from the bombarded surfaces for RIMS analysis. We used an IonOptika IOG 25AU, which delivered 69Ga+ ions at 15 keV with a spot size of ~1 µm at an incident angle of 60º from the surface normal. The Ga+ gun was run at a repetition rate of 1 kHz with an instantaneous current of 42 pA and a pulse width of 160 ns. It was rastered over a 25×25 µm area in order to mitigate any effects due to surface roughness and to ensure the sampling of many grains within the CRM 125a pellet (grain size up to ~10 µm). For experiments comparing the efficiency of surface reduction by bombardment with Ga+ vs. noble gas ions, uranium dioxide was bombarded with the Ga+ beam in continuous mode for specified durations, followed by analysis in pulsed mode. For U metal analysis the Ga+ ion gun was run at a duty cycle of 80%, i.e. the gun was on for 800 of the 1000 µs between analysis pulses. Using the previously measured value of 7.93 for the U sputtering yield,9 the removal rate was 0.05 monolayers/second ( ML/s). The vacuum chamber pressure was 8×10-9 Torr. Even assuming that all of the gas in the chamber is oxygen, this pressure corresponds to an arrival rate at the sample surface of 0.003 ML/s. Thus the U removal rate was at least 16× higher than the O arrival rate, keeping the surface clean and reduced. The Ti:Sapphire lasers were essentially identical to those described previously.13 The pulse repetition rate was 1 kHz. Pulse energies ranged up to 0.72 mJ per pulse for second harmonic beams (350-500 nm). The bandwidths

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were 0.2 to 0.3 cm-1. For the combined two-color RIS scheme the beams were aligned in a nearly co-linear fashion such that they intersected in the ionization region of the mass spectrometer ~1 mm above the surface. The angle between the beams was 99% of sputtered atoms exist in either this 5K5 or the 5L6 ground state.12 (All state energies reported in this study refer to the 238U isotope unless otherwise noted.) The angular momentum selection rule (∆J=0,±1) forbids the transition from the 5K5 state to the 5L7 intermediate state in the three-color scheme, and therefore this scheme cannot access those atoms. To address this issue we developed a new RIS scheme,

Table 1: State assignments and level energies relative to the isotopic ground states for the schemes in Figure 1. Bound and Rydberg state energies and assignments are from refs. 21 and 36; the AI state energy is determined from this work.

State

Assignment

Ground Excited Bound Rydberg AI

L6 K5 J=6 J=5 J=5,6,7

5

5

235

U (cm-1)

0 620.318 25235.792 ---

238

U (cm-1)

0 620.323 25235.750 49851.3 50471.5

Figure 1. The resonance ionization schemes used in this work. The proposed autoionizing state is shown as a dotted line. This drawing is not to scale; exact state energies are given in Table 1.

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Figure 2. Saturation curve for the one-color RIS scheme originating on the ground state in Scheme B. Filled circles: measured total U ion signal as a function of laser irradiance. Dashed line: Fit to a first-order ionization rate model (see text for details).

which combines two one-color two-photon schemes. Figure 1 shows the transitions involved; exact state energies are given in Table 1. Ground state atoms are excited to a J=6 state at 25,235.750 cm-1.17 A second photon of the same color has sufficient energy to ionize uranium. A second laser excites U atoms from the 5K5 state to the same J=6 state. This transition was previously observed in RIMS as part of a different RIS scheme.18 Results Saturation curves Figure 2 shows the U atom signal as a function of laser irradiance for the one-color two-photon scheme originating on the ground state, i.e. only one laser tuned to 396.263 nm. Hereafter we refer to this laser as the ground state laser for simplicity. The dashed lines are fits to a first-order twolevel rate model:19 1 /

(1)

where N is the observed number of ion counts, Ni is the number of ion counts at zero irradiance (i.e. ions created by alternate ionization processes), Nmax is the number of ion counts that would be observed at 100% ionization, I is the laser irradiance, and Isat is the saturation irradiance. This function does not describe a real multilevel system, particularly for the first transition, however it provides a semiquantitative measure of the effectiveness of the ionization process. Figure 2 shows that the U atom signal is nearly saturated using only the ground state laser. The fit to the data implies that the observed signal at the maximum irradiance of our laser is ~90% of the maximum achievable. The twophoton energy is 50,471.467 cm-1, while the ionization potential (IP) of uranium is 49958.4 cm-1.20 For Equation 1 to have physical meaning in this case, we assume that the

Figure 3: Top: Saturation curves for the excited state RIS scheme alone (filled circles), excited state RIS scheme with ground state laser added at 5.9 MW/cm2 (open circles), and the expected signal level if the two lasers were to ionize uranium independently (filled triangles).

transition from the ground state to the bound excited state is fully saturated at low irradiance and that the second transition is to an autoionizing (AI) state whose spontaneous radiative decay rate is much slower than decay via electron emission. Under these assumptions, this approximates a simple two-level system, which is what is modeled by the dashed line in Figure 2. Given the excellent fit to the two-level model, we hypothesize that the two-photon energy of 50,471.467 cm-1 lies close to an autoionizing state. The AI state must have odd parity (since the intermediate state at 25236 cm-1 is even) and a J-value of 5, 6, or 7. The closest known oddparity AI state we have found in the literature is at 50476.8 cm-1 with a J–value of 4, 5, or 6.21 This state lies some 5 cm-1 above our two-photon energy, and has a reported FWHM of only 1-2 cm-1, making it an unlikely candidate for the strong ionization that we observed. At present we consider it likely that the scheme terminates on or near a new odd AI state with J = 5, 6, or 7. Figure 3 shows the U atom signal as a function of laser irradiance for the one-color two-photon process originating on the 5K5 state, i.e. only one laser tuned to 406.249 nm (filled circles). Hereafter we refer to this as the excited state laser. The ion signal was much weaker in this case as expected from the lower occupancy of the excited state, however it scales linearly with the laser irradiance and is therefore not saturated. The total energy of an atom starting from the 5K5 state and absorbing two 406.249 nm photons is 49851.213 cm-1, which is 107.2 cm-1 below the uranium IP. An electric field gradient lowers the effective IP according to ∆ 2√ in atomic units,22 which corresponds to ∆ 6.1√, when ∆E is the change in IP in cm-1 and F is the field gradient in V/cm. The field gradient in the ionization region of the LION mass spectrometer is 550 V/cm, which corresponds to a lowering of the IP of 143 cm-1. We



therefore expect two-photon ionization from the excited state laser, particularly if there is a Rydberg state at or near 49851.2 cm-1. In fact, Ray et al.21 report an odd-parity Rydberg state with J=5 at 49851.3 cm-1, which is easily overlapped at our laser bandwidth. The fact that the signal is not saturated indicates that either the transition cross section is relatively low or the rate of decay by electron emission is slow compared to the decay rate by photon emission. The open circles in Figure 3 plot the signal obtained as the irradiance of the excited state laser was varied when the ground state laser was present at a constant irradiance of 5.9 MW/cm2, which is in the saturated region of Figure 2. This is not a simple saturation curve, since now all five states in Figure 1 are connected. Given that the one-color scheme originating on the ground state is saturated at 5.9 MW/cm2 and ionization from the Rydberg state is negligible at low intensities of the excited state laser, we interpret the initial sharp rise in ion counts (as the ES laser irradiance goes from zero to 0.07 MW/cm2) as saturation of the 5 K5→J=6 transition, followed by a transition to and ionization from the 50471 cm-1 AI state by the ground state laser. When both lasers were present at full power simultaneously, the ion signal exceeded the sum of the two acting independently. (For comparison, the triangles in Figure 3 show the signal expected if the two lasers were to ionize U independently.) We interpret this as evidence that the ionization rate from the Rydberg state is slow compared to that from the AI state. This is also indicated by the linear behavior observed in Figure 3. Ion Bombardment Experiments Uranium Metal Figure 4 shows the 235U count rate from natural uranium metal (filled circles) as a function of Ga+ ion dose using the combined two-color RIS scheme. We consider only

Figure 4: Filled circles, left axis: 235U ion counts from U metal as a function of 15 keV Ga+ dose. The initial slow rise is due to removal of a gold coating. Open circles, right axis: fraction of UO+ + UO2+ ion counts in the RIMS spectrum, showing the reduction of the oxide layer as the surface is bombarded.

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235

U in this analysis since the 238U signal was off scale at the primary ion current that we used. The initial slow rise in 235U signal (the relatively flat region between 0 and ~2.5×1015 Ga+/cm2) is due to the removal of the gold coating. That the signal is not zero in this region indicates some mixing of the gold and uranium, either from the coating process or due to mixing induced by the Ga+ beam itself. Once the coating is gone the uranium atomic ion signal rises sharply, indicating the removal of the oxide layer beneath, and then stabilizes once a reduced metallic surface is exposed. Figure 4 also plots the fraction of UOx+ in the RIMS spectrum, [UOx], calculated as the sum of UO+ and UO2+ ion counts divided by the sum of sum of U+, UO+, and UO2+ ion counts. Because the ionization efficiencies differ for these species, this is not a quantitative measure of the relative amount of UOx+ in the sputtered flux, but it is useful as an indicator of the oxidation state of the surface (see discussion of Equation 2 below). While the overall signal level was very low until the gold layer was sputtered off, it was dominated by UO+ and UO2+. The high value of [UOx] observed below ~2.5×1015 Ga+/cm2 indicates that the uranium present in the gold layer is highly oxidized. The UOx signal drops off sharply after the coating is removed and reaches an equilibrium in which the UOx+ peaks in the RIMS spectrum represent less than ~0.6% of the total signal. The solid line in Figure 4 is a fit to Equation 1, where the laser irradiance I is replaced by Ga+ dose (ions/cm2) and the saturation irradiance Isat is replaced by the dose required to “saturate” the surface (i.e. the dose that produces 1-e-1 ≈ 63% of the maximum effect). For doses ≫ Isat (~4.5×1015 cm-2) we assume that the sputtering yield is equal to the previously measured value of 7.93±0.31,9 and calculate a useful yield of 38.3±1.6%. This is 1.6× higher than the useful yield of 24% we previously obtained using a three-color RIS scheme that does not access the 5K5 excited state. This is not a direct measurement of the occupancy of the 5K5 state since a new detector with a higher efficiency was used for this measurement. The unequal U ionization efficiencies for the two RIS schemes demonstrated in Figures 2 &

Figure 5: RIMS spectra taken after dosing uranium dioxide to equilibrium with 15 keV Ga+ and 3 keV He+, Ne+, Ar+, Kr+, and Xe+.


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Figure 6: Uranium atomic ion count rate from uranium dioxide as a function of ion dose. In this context, dose refers to bombardment with 3 keV He+, Ne+, Ar+, Kr+, Xe+ and 15 keV Ga+ prior to analysis with 15 keV Ga+.

3 prevent us from estimating the excited state occupancy. Uranium Dioxide Figure 5 shows a series of RIMS spectra taken after bombarding uranium dioxide to equilibrium (i.e. the point at which a stable sputter yield is achieved) with either Ga+ or the noble gas ions. The count rate is lowest for Ga+ bombardment. Across the noble gases, the count rate rises from He+ to Ar+, then declines again, with Ar+ dosing producing by far the highest count rate. Peaks corresponding to U+, UO+, and UO2+ are present in all spectra, but in varying proportions. The U+/UO+ ratio is lowest in the Ga+ treated sample at 0.47, while the Ar+ treated sample has U+/UO+ ~ 1. Bombarding with the other four noble gas ions gives U+/UO+ ~ 0.6. Figure 6 shows the change in the U+ count rate as uranium dioxide is bombarded with Ga+ and noble gas ions. At the smallest doses the effect of the ion beam is to clean the surface of adsorbed materials without significantly affecting the bulk material. Once the cleaning is complete, the U atom count rate rises with ion dose as the surface is reduced, and levels off as the surface comes to equilibrium with the ion beam. The striking feature in Figure 6 is the high U count rate when the surface is bombarded with Ar+ compared to the other noble gases. This implies that Ar+ causes much higher surface reduction than the other ions. As a proxy for the surface reduction, we use the fraction of atomic uranium ion counts in the sputtered flux:

!

"# " # $"% # $"%&#

(2)

where U+, etc. are the integrated ion counts of each species in the RIMS spectrum. We note that [U] as defined in Equation 2 is not the actual fraction of U atom either on the surface or in the sputtered flux, but rather a proxy. The relationship between [U] and the U atom fraction in the sputtered flux should be close to linear, since the only missing terms are the secondary U+ yield, which we previously

Figure 7: Uranium atomic ion fraction observed in the RIMS spectrum obtained from uranium dioxide as a function of ion dose. In this context, dose refers to bombardment with 3 keV He+, Ne+, Ar+, Kr+, Xe+, and 15 keV Ga+ prior to analysis with 15 keV Ga+.

showed to be negligible compared to neutral U,9 and the photofragmentation of UOx to U+, which we observed to be less than 3% of the U RIMS signal by detuning the laser from the J=6 resonance and measuring the background. We therefore expect [U] to scale reasonably well with the extent of reduction of the surface. In Figure 7 we plot [U] vs. the ion dose. The trend observed in the spectra of Figure 5 and the count rate data of Figure 6 is clear: the apparent extent of reduction is much higher for Ar+ than the other ions. However, Figures 6 and 7 have a discrepancy with regard to bombarding with Ga+ vs. the noble gas ions other than Ar+. Excluding Ar+, the observed reductions for noble gas bombardment are all equal and significantly higher than for Ga+, but the U+ count rates are essentially the same for both Ga+ and noble gas bombardment. If oxide sputtering is the dominant loss channel, we expect that the ordering in Figures 5 and 6 to be the same, i.e. more reduced samples giving higher U+ count rates. Figure 8(a) shows the calculated useful yields as a function of [U]. Here we used the previously determined value of 2.68 for the sputtering yield9 to determine the useful yields. The data for Ga+ and the noble gas ion treatments do not lie along a single curve as would be expected based on the extent of surface reduction. Further, each shows an anomaly at high reductions, with the apparent useful yield continuing to rise while the extent of reduction is essentially constant. This strongly suggests that the sputtering yields for the various ion-dosed surfaces are not the same, and that they change during bombardment. We had previously determined a U atom sputtering yield of 2.68±0.06 for 15 keV Ga+ at 60º incidence, however this is accurate only for uranium dioxide surface equilibrium with the Ga+ beam. The Ga+ dose for analysis in this work was typically a few times 1012 ions/cm2, which is far short of equilibrium and much too low to have a noticeable effect on the sputtering yield of a previously-dosed surface.



The questions here are: what is the sputtering yield for 15 keV Ga+ on a uranium dioxide surface that has been previously modified by a 3 keV noble gas ion beam, and does it vary as the noble gas is varied from He to Xe? Further, what is the sputtering yield as a function of the noble gas ion dose? We can estimate the sputtering yield as a function of ion dose by examining the amounts of U+, UO+, and UO2+ in the RIMS spectra. The ionization probabilities of species in the sputtered flux are constant for a fixed laser irradiance, and therefore the total ion signal serves as a proxy for the amount of neutral material in the sputtered flux. Knowing the ionization probabilities would allow us to calculate the sputtering yields. Neglecting secondary ions, which we previously showed account for only ~1% of the sputtered flux, the sputtering yield for uranium-bearing species (atoms + molecules) is )

'( *+ ,

- ,.

∑

*0 ,0

(3)

where N’ is the number of primary ions incident on the surface during the analysis, Ni is the number of uraniumbearing ions of type i observed in the RIMS spectrum, εi is the ionization efficiency of the ith species, and εT, εD are the transmission and detection efficiencies of the instrument. This treatment assumes that the laser/plume overlap and TOF-MS transmission and detection efficiencies are the same for each species under consideration. We fix the species in the sputtered flux as U, UO, and UO2. We have not observed ions larger than UO2+ in RIMS spectra, even though UO3+ and much larger UxOy+ cluster ions have been observed in laser ablation mass spectrometry experiments.23-25 Indeed, UO3 dominates the vapor in some experiments.26 Clusters larger than UO2 are attributed to collisions and condensation in the gas phase. In our case, the sputtered flux under the analysis conditions is largely collisionless; we therefore do not expect clusters larger than UO2 to form in any significant quantity. This is cor-

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roborated by the fact that we never observe uraniumbearing ions larger than UO2+ in our sputtered flux, either as laser-ionized neutrals or secondary ions. We therefore neglect clusters larger than UO2 and write Eq. 3 in terms of U, UO, and UO2:

'( *+

) * 8 9 ,- ,. ,9

*9: ,9:

*9:& ,9:&

;

(4)

The ionization efficiency as defined here consists of both the ionization probability for an atom or molecule interacting with the laser and the fraction of the sputtered flux overlapped by the laser beam. As described previously, we estimated the ionization probability and overlap for U atoms from the saturation curves (Figs. 2 and 3) combined with modeling of a sputtered plume assuming a Sigmund-Thompson energy and angular distribution of ejected species.9 Using this approach for the current data yields an estimate of 0.57 for εU. In this case, the ionization step of the excited state scheme is not completely saturated and therefore our estimate is a lower bound. We estimated εT as 0.8 from SIMION modeling as described previously and we used εD = 0.86 from the detector manufacturer’s information. This leaves the ionization efficiencies for UO and UO2 as the only unknowns in Eq. 4. Since both of these species are far from saturation, we cannot estimate their ionization

Table 2: Uranium sputtering yields and corresponding useful yields calculated from Eq. 3 for UO2 samples dosed to equilibrium with various ions. aPrevious work accessing only the ground state. bThis work. Dosing Uranium Uranium Ion energy Sputtering Useful Yield (keV) Yield Gaa 15 2.68 2.0% Gab

15

2.68

3.3%

He

3

2.52

4.3%

Ne

3

2.68

4.4%

Ar

3

3.16

6.6%

Kr

3

2.47

4.5%

Xe

3

2.15

4.5%

Figure 8: Useful yield as a function of the observed fraction of uranium atomic ion in the RIMS spectrum obtained from uranium dioxide. a) Useful yield calculated assuming a constant sputtering yield of 2.68. b) Useful yield calculated using a variable sputtering yield as described in the text.


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probabilities from our laser spectroscopy. To estimate the values of εUO and εUO2 we used previously determined values for the sputtering yields for two different materials, uranium dioxide (2.68±0.06) and uranium metal (7.93±031). This gives two independent equations and allows us to solve for the two unknowns. The solutions for εUO and εUO2 depend very strongly on the values of εU and the sputtering yields of U metal and uranium dioxide. Using count rate data from U metal (Figure 4) together with the count rate data for Ga+ dosing of uranium oxide (Figure 6) gives a negative value for εUO when the sputtering yields are constrained to their mean measured values, making this method unsuitable for determining either absolute ionization probabilities or the absolute composition of the sputtered flux. However, allowing the sputtering yields for uranium dioxide and U metal to vary within their measured uncertainties makes it possible to find physically reasonable solutions for εUO and εUO2. The sputtering yields for UO2 at equilibrium calculated from Eq. 4 vary by less than 2% across a range of reasonable values for εU when we constrain the solutions such that the Ga+ equilibrium sputtering yields for U metal and uranium dioxide are within one standard deviation of their measured values. The fit parameters we found are εU = 0.57, SY(UO2) = 2.69, SY(U metal) = 7.65. This gives εUO = 0.12 and εUO2 = 0.07. Table 2 gives sputtering and useful yields calculated from Eq’n 4. The sputtering yields for noble gas ion dosing vary significantly from that of the Ga+ equilibrated surface, and the calculated useful yields vary accordingly. The highest sputtering yield is for the Ar+ equilibrated surface, which also has the highest useful yield at 6.6%. The other noble gases all fall in the range between 4.3 and 4.5%, while Ga+ bombardment gives a useful yield of 3.3%. Figure 8(b) shows the useful yields determined using sputter yields calculated from Eq. 4 vs. the extent of reduction [U],

and does not have the anomalies seen when the nominal sputtering yield value of 2.68 is used for all cases as in Figure 8(a). As expected, the useful yield obtained in this work for the Ga+ equilibrated surface is higher than the one we obtained previously using a RIS scheme that accesses only the uranium ground state. Table 2 lists the previous result along with the new ones. Similar to what we observed on U metal, the useful yield increases by 1.65× for the Ga+ dosing case. Noble gas ion dosing always gives higher useful yields compared to Ga+ dosing, with the Ar+ case a factor of two higher. This is due to the stronger reduction effect as observed in Figures 5 and 7. We attribute surface reduction to strong preferential sputtering of oxygen. Mixing induced by the ion beam could conceivably play a role by dredging up uranium atoms from the sub-surface while driving oxygen atoms deeper into the bulk; however there is at present no evidence for this effect in uranium dioxide. Our TRIM27 calculations indicate oxygen sputtering rates 3.4× to 4.4× higher than U sputtering rates for the experimental conditions used in this work. In the remainder of this discussion we consider only preferential sputtering as a surface reduction mechanism. Much of the enhanced reduction noted for noble gas ion bombardment is likely due to the lower energy used (3 keV vs. 15 keV). Preferential sputtering of light atoms from compounds is generally enhanced at lower ion impact energies as the energy transferred to the light atoms exceeds that of the heavy atom and thus produces a lower sputtering threshold for collisional sputtering.28 Because collisional sputtering is not the only process operating, the maximum preferential sputtering may occur at energies significantly above threshold.29 This may be the case here, though we have not attempted to determine the optimal ion impact energy. While there is a great deal of literature on the preferential sputtering of light atoms from compound targets (see reviews by Gnaser,30 and Smentkowski31), there is little on the relative reducing powers of the noble gases as bombarding ions. In general, preferential sputtering is favored when the mass difference between the metal and oxygen is large and the binding energy of oxygen is less than that of the metal. The effect is most prevalent for near-threshold sputtering.28 At low ion energies, the maximum energy transfer per atom scales as the reduced mass of the projectile plus target atom: ?@ ?& & @ $?&

> ?

Figure 9: Bars, left axis: Uranium useful yield after bombardment of uranium dioxide with 3 keV noble gas ions and 15 keV Ga+. White circles, right axis: Ordering of the relative energy transfer to O and U calculated from Eq. 5, scaled to He+. Black circles, right axis: Relative energy transfer calculated using TRIM, scaled to He+.

(5)

where M1 and M2 are the masses of the projectile and target atoms, respectively. Thomas III et al.29 showed that 1 keV Ne+ was most effective at reducing SiO2, likely due to the optimal mass match between Ne and O, although Ar+, whose mass is a close match for SiO, was equally effective within error. Varga and Taglauer32 observed that the reduced mass effect was dominant for 1 keV bombardment of Ta2O5 where the difference in atomic masses of target at-



oms is large. In that case reduction was most effective for H+ bombardment, followed by He+, Ne+, Ar+, and Xe+. They attributed the effect to relative maximum energy transfer from the projectile to Ta and O (e.g., g(H,O) / g(H,Ta), etc.), which is maximized at low mass. The same trend was also observed for 1 keV He+ vs. Ar+ bombardment of several compounds.33 He+ was a more effective reductant than Ar+, and the effect was strongest where the mass difference between the light and heavy constituents of the target was greatest. Thus, for low energy bombardment, the projectile mass at which preferential sputtering is maximized is not necessarily the one closest to the light atom mass. Figure 9 shows the useful yields measured on uranium dioxide after dosing to equilibrium with noble gas ions, along with the relative energy transfers calculated from Equation 5 (e.g., g(O,He) / g(U,He), etc.) and relative energy transfers modeled by TRIM.27 For both the reduced mass and TRIM calculations the energy transfers are scaled to the value calculated for He+. For the TRIM model we calculated the relative energy transfer to U and O in the top nm of material. To account for reduction we repeated the calculation for UO2, UO1.5, and UO1.0; however the ordering observed in Figure 9 did not change. Neither reduced mass nor TRIM energy transfer considerations agree with the data. For purely collisional sputtering, the yield depends on the amount of energy deposited in the first two atomic layers, which depends on the penetration depth of the incident ion. This varies with the ion’s mass (lighter ions penetrating deeper). Our noble gas ion energy was 3 keV, which is significantly higher than the 1 keV used in the literature cited above and may explain some of the shift away from a pure reduced mass effect as a lower fraction of the ion’s energy is deposited in the first two atomic layers as the mass of the ion decreases. At higher energies first-order collisional processes such as primary and secondary knock-

Figure 10: The useful yield for U analysis as a function of the amount of uranium removed by bombardment with various ions prior to RIMS analysis. The uranium consumption was calculated using TRIM.

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ons become less important and other processes such as defect formation and oxygen diffusion take on increased importance, especially as the dose increases.30 For example, Leinen et al.34 found that 10 keV Ar+ was always more effective than 3.5 keV Ar+ in reducing TiO2, BaTiO3, Al2TiO5, and NiTiO3. At present we do not have an explanation for why Ar+ is significantly more effective in reducing uranium dioxide than the other noble gases, nor for why the other noble gases are all equally effective despite their wide differences in mass. The data of Figure 7 show that preferential sputtering of O reduces uranium dioxide and thereby allows resonance ionization to achieve high useful yields for U analysis; however the sputtering itself wastes U atoms and thereby reduces sample utilization efficiency. To estimate the amount of U lost in the pre-sputtering process we used TRIM to calculate U sputtering yields for the various projectile ions. Predicted sputtering yields are very sensitive to the surface binding energy used in the calculation, which changes during ion bombardment as the surface chemical composition and roughness changes. These changes are not calculable in TRIM. We used TRIM to calculate the relative sputtering yields as the mass of the noble gas ion changes, and to provide an estimate of the U loss. Figure 10 plots useful yield vs. the amount of U sputtered for each projectile ion, and shows increasing U loss as the projectile mass increases, with the effect saturating at the Kr mass. Sputtering with 15 keV Ga+ is also shown, for comparison. While not strictly quantitative, the calculation shows that by far the most efficient sample utilization is achieved with He+ dosing, even though Ar+ dosing provides the highest useful yield at equilibrium. In terms of the amount of U sputtered, the surface reaches equilibrium with the He+ beam much faster than it does with the other noble gases. Assuming a UO2 unit cell dimension of 0.5 nm and four U atoms per unit cell,35 Figure 10 implies that equilibrium is reached after the loss of ~4 layers of U atoms. Since all surface analysis techniques generally include a cleaning step to remove surface soils, this is a negligible loss. However, the surface will slowly re-oxidize even in UHV, and will require periodic He+ bombardment to re-establish the surface reduction. Conclusions A new two-color uranium resonance ionization scheme improves the RIMS useful yield by accessing both the ground and low-lying 5K5 state. A useful yield of 38% was achieved for U metal analysis. Bombarding uranium dioxide with noble gas ions reduces the surface via preferential sputtering of O and achieves a useful yield of 6.6% for 3 keV Ar+ bombardment at equilibrium. In terms of sample utilization efficiency, TRIM modeling indicates that bombarding with 3 keV He+ prior to analysis with 15 keV Ga+ provides the best result even though the measured useful yield at equilibrium is only 4.3%, because far less U is lost during the bombardment phase prior to analysis.


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Acknowledgements This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work was supported by the Laboratory Directed Research and Development Program at LLNL under project 14-ERD-082, by the U.S. Department of Homeland Security's National Technical Nuclear Forensics Center, by the National Nuclear Security Agency Office of Defense Nuclear Nonproliferation Research and Development, and by the Defense Threat Reduction Agency through award no. HDTRA 135636-M. LLNL-JRNL-752818. References (1) Erdmann, N.; Betti, M.; Kollmer, F.; Benninghoven, A.; Grüning, C.; Philipsen, V.; Lievens, P.; Silverans, R. E.; Vandeweert, E. Anal. Chem. 2003, 75, 3175-3181. (2) Erdmann, N.; Kratz, J.-V.; Trautmann, N.; Passler, G. Anal. Bioanal. Chem. 2009. (3) Gruning, C.; Huber, G.; Klopp, P.; Kratz, J. V.; Kunz, P.; Passler, G.; Trautmann, N.; Waldek, A.; Wendt, K. Int. J. Mass Spectrom. 2004, 235, 171. (4) Raeder, S.; Hakimi, A.; Stöbener, N.; Trautmann, N.; Wendt, K. Anal. Bioanal. Chem. 2012, 404, 2163-2172. (5) Trautmann, N.; Passler, G.; Wendt, K. D. A. Anal. Bioanal. Chem. 2004, 378, 348-355. (6) Franzmann, M.; Bosco, H.; Hamann, L.; Walther, C.; Wendt, K. J. Anal. At. Spectrom. 2018. (7) Goeringer, D. E.; Christie, W. H.; Valiga, R. E. Anal. Chem. 1988, 60, 345-349. (8) Willingham, D.; Savina, M. R.; Knight, K. B.; Pellin, M. J.; Hutcheon, I. D. J. Radioanal. Nucl. Chem. 2013, 296, 407-412. (9) Savina, M. R.; Isselhardt, B. H.; Kucher, A.; Trappitsch, R.; King, B. V.; Ruddle, D.; Gopal, R.; Hutcheon, I. Anal. Chem. 2017, 89, 6224-6231. (10) Seah, M. P.; Nunney, T. S. Journal of Physics DApplied Physics 2010, 43. (11) Senanayake, S. D.; Waterhouse, G. I. N.; Chan, A. S. Y.; Madey, T. E.; Mullins, D. R.; Idriss, H. J. Phys. Chem. C 2007, 111, 7963-7970. (12) Wright, R. B.; Pellin, M. J.; Gruen, D. M.; Young, C. E. Nucl. Instrum. Methods 1980, 170, 295-302. (13) Stephan, T.; Trappitsch, R.; Davis, A. M.; Pellin, M. J.; Rost, D.; Savina, M. R.; Yokochi, R.; Liu, N. Int. J. Mass Spectrom. 2016, 407, 1-15. (14) Schumann, P. G.; Wendt, K. D. A.; Bushaw, B. A. Spectrochim. Acta, Part B 2005, 60, 1402. (15) Isselhardt, B. H.; Prussin, S. G.; Savina, M. R.; Willingham, D. G.; Knight, K. B.; Hutcheon, I. J. Anal. At. Spectrom. 2016, 31, 666-678. (16) Isselhardt, B. H.; Savina, M. R.; Knight, K. B.; Pellin, M. J.; Hutcheon, I. D.; Prussin, S. G. Anal. Chem. 2011, 83, 2469-2475. (17) Kiess, C. C.; Humphreys, C. J.; Laun, D. D. J. Res. Natl. Bur. Stand. 1946, 37, 57-72. (18) Raeder, S.; Fies, S.; Gottwald, T.; Mattolat, C.; Rothe, S.; Wendt, K. Hyperfine Interact. 2010, 196, 71-79.

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A New Resonance Ionization Mass Spectrometry Scheme for Im

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