Detecting Pu in U-bearing Particles by SEM/EDS for Nuclear Safeguards Applications

For nuclear safeguards, several DOE supported programs rely on environmental sampling programs, where particulate materials on filters and swipes are analyzed for compliance. A desired capability for environmental material characterization is detecting Pu when it is intermixed in a predominantly uranium matrix, at the particle level. Several methods exist to measure low abundances of Pu within uranium particles, mainly involving mass spectrometry. For such techniques, however, several challenges exist, including Pu heterogeneity at the particle level, isobaric signal interferences that overlap with U and Pu, complex sample preparation, and overall cost and timeliness of characterization. Here, we use scanning electron microscopy combined with energy dispersive spectroscopy (SEM-EDS) to rapidly and non-destructively analyze single, micron-sized particles, and to detect and quantify percent level abundances of Pu within particles consisting of uranium oxide. Each element analyzed via SEM/EDS has a unique spectrum (e.g. peak locations at a specific energy in keV) that is generated from characteristic X-rays emitted from their electron shells due to excitation from the SEM electron beam. However, some spectral peaks overlap, which hinders the ability to verify the presence of such elements and/or to quantify their abundance within a material. An example of overlapping spectral peaks is shown in Figure 1, for uranium and plutonium. Here, it is seen that the highest intensity peak of Pu (Mα) overlaps with the second-most intense peak of uranium (Mβ). Although this overlap makes it impossible to conventionally quantify low abundances of Pu, we hypothesize the abundance of Pu within a U matrix is directly related by the following relationship: In this work, the relationship between eqn. 1 and the Pu abundance within predominantly U-bearing particles was demonstrated with mixed U-Pu test materials with known Pu abundances produced by Savanna River National Laboratory (SRNL) [1]. These samples consist of spherical particles, with a tightly constrained diameter of approximately one micron, distributed on a doped silicon or graphite wafer. Multiple samples provided by SRNL with a range of Pu abundances were characterized by SEM-EDS. Pure U reference materials and other U materials were also analyzed to establish a robust baseline U Mβ/ U Mα. The samples were measured via large geometry secondary ion mass spectrometry and isotope dilution mass spectrometry at LANL to confirm their actual Pu content. The Pu concentrations for the samples from SRNL were determined to be 0% (pure depleted U), 1.6(±0.33)%, 2.5(±0.42)%, 3(±0.40)% and 7.7(±1.10)% Pu. All SEM imaging and EDS were done using a FEI Quanta 200F field emission SEM coupled with an AMETEK EDAX Octane Elite silicon drift detector. All EDS data were processed using EDAX APEX advanced software. For each sample, three consecutive measurements were taken per particle, with a total of 3 particles per sample, using an acceleration voltage of 20 keV, FEI spot size setting of 4, 30 µm aperture and the following EDS settings: amp time of 3.84 µS, quant method eZAF, and a 5 min dwell. Averaged (Pu Mα + U Mβ)/ U Mα values are as follows: 0.6033 ± 0.0049 for pure U samples, 0.6204 ± 0.0037 for 1.6% Pu, 0.6259 ± 0.0041 for 2.5% Pu, 0.6333 ± 0.0058 for 3% Pu, and 0.6986 ± 0.0153 for 7.7% Pu samples. Uncertainties are the standard deviation of all individual particle measurements per sample, and accurately capture the measurement-to-measurement variabilities per sample. In terms of the averaged (Pu Mα + U Mβ)/ U Mα values and their uncertainties: (1) the 0% Pu sample is resolvable from the 1.6% Pu sample; (2) the 1.6% Pu sample is resolvable from the 3.0% Pu sample; (3) the 2.5% sample is not resolvable from the 1.6% Pu and 3.0% Pu samples; and (4) the 3.0% Pu sample is resolvable from the 7.7% Pu sample. Collectively, these values show a linear relationship between (Pu Mα + U Mβ)/ U Mα and Pu concentration (Figure 2). In addition to counting statistics, two other sources of uncertainty include: (1) true particle-to-particle variability in Pu abundance per sample, which was documented by LG-SIMS analyses (12% to 20% relative variability about averages found within particle datasets); and (2) observed particle drift during EDS analyses that caused variability of triplicate measurements per particle. Although these uncertainties limit how finely Pu abundances can be discriminated with our technique, the calibration curve can determine the presence of percent-level Pu in a U matrix and can also estimate an abundance range. Additionally, other aspects of measurements (e.g. analysis dwell time, beam conditions, etc.) could reduce uncertainties further. However, it is also recognized that factors hindering this characterization technique include: the influence of other elements that may overlap with U (such as K), sample topography, particle size and sample charging. We have demonstrated a method to rapidly and non-destructively detect the presence of Pu within micron sized U-bearing environmental particles via SEM-EDS, when concentrations are too low to be detected by the Auto ID function. Using several test materials with differing Pu abundances, a calibration curve can be generated to estimate percent-level Pu level abundances within unknown U-bearing particle samples. SEM-EDS systems are ubiquitous in organizations that participate in nuclear safeguards/security/verification and monitoring, and are relatively inexpensive compared to other analytical capabilities. In addition, the proposed method is relatively easy and rapid with respect to sample preparation and analysis. As such, this method may likely serve a broad community. Example EDS spectrum of uranium (red), and the position of Pu EDS spectra (yellow lines). Note the overlap at 3.34 keV of the U Mβ and the Pu Mα spectral peaks. SEM-EDS data from samples with Pu from 0% (pure uranium) to 7.7% Pu, showing the relationship between equation 1 (e.g. Fig. 1) and the abundance of Pu within a particle.