RESUMEN

We present a description of the capabilities and performance of the NAval Ultra-Trace Isotope Laboratory's Universal Spectrometer (NAUTILUS) at the U.S. Naval Research Laboratory. The NAUTILUS combines secondary ion mass spectrometry (SIMS) and single-stage accelerator mass spectrometry (SSAMS) into a single unified instrument for spatially resolved trace element and isotope analysis. The NAUTILUS instrument is essentially a fully functional SIMS instrument with an additional molecule-filtering detector, the SSAMS. The combination of these two techniques mitigates the drawbacks of each and enables new measurement paradigms for SIMS-like microanalysis. Highlighted capabilities include molecule-free raster ion imaging for direct spatially resolved analysis of heterogeneous materials with or without perturbed isotopic compositions. The NAUTILUS' sensitivity to trace elements is at least 10× better than commercial SIMS instruments due to near-zero background conditions. We describe the design and construction of the NAUTILUS, and its performance applied to topics in nuclear materials analysis, cosmochemistry, and geochemistry.

RESUMEN

We present the ability to conduct single micrometer-sized uranium particle age-dating measurements on particles that are younger, smaller, and less enriched in 235U content than previously reported. Specifically, we use large geometry secondary ion mass spectrometry (LG-SIMS) to precisely measure the 230Th/234U radiochronometer, combined with a systematic treatment of relevant parameters such as particle size, enrichment, and age, to achieve this development. We describe the necessary requirements for instrument background, interference rejection, abundance sensitivity, and other instrumental conditions that allow for this advance in single-particle uranium age-dating. We introduce the use of statistics developed by Feldman and Cousins to generate 95% confidence intervals in particle age, even when 230Th daughter ions are not detected. For particles where counts are limited and are of identical isotopic signatures, we provide an option for aggregating individual measurements of single particles to reduce measurement uncertainty, as if the measurement had been performed on one larger particle. The methodology is validated on a range of certified reference materials and 'real-world' samples, ranging in age from 15 to 60 years, and on individual particles ranging in equivalent size from 0.6 to 6.8 micrometers. Additionally, we provide model age calculations for particles ranging in size from 1.0 to 3.0 micrometers across enrichments ranging from natural uranium to highly-enriched uranium and on ages ranging from 0 to 60 years. Experimental results compare well with the predicted model ages, providing realistic guidance for expectations of single micrometer-sized uranium particle age-dating measurements. The age-dating capabilities described herein are directly relevant to the International Atomic Energy Agency (IAEA) and its mission to safeguard nuclear materials and monitor member state nuclear programs.

RESUMEN

A commercial secondary ion mass spectrometer (SIMS) was coupled to a ± 300 kV single-stage accelerator mass spectrometer (SSAMS). Positive secondary ions generated with the SIMS were injected into the SSAMS for analysis. This combined instrument was used to measure the uranium isotopic ratios in particles of three certified reference materials (CRM) of uranium, CRM U030a, CRM U500, and CRM U850. The ability to inject positive ions into the SSAMS is unique for AMS systems and allows for simple analysis of nearly the entire periodic table because most elements will readily produce positive ions. Isotopic ratios were measured on samples of a few picograms to nanograms of total U. Destruction of UH(+) ions in the stripper tube of the SSAMS reduced hydride levels by a factor of ∼3 × 10(4) giving the UH(+)/U(+) ratio at the SSAMS detector of ∼1.4 × 10(-8). These hydride ion levels would allow the measurement of (239)Pu at the 10 ppb level in the presence of U and the equivalent of ∼10(-10 236)U concentration in natural uranium. SIMS-SSAMS analysis of solid nuclear materials, such as these, with signals nearly free of molecular interferences, could have a significant future impact on the way some measurements are made for nuclear nonproliferation.

RESUMEN

Time-of-flight-secondary ion mass spectrometry (TOF-SIMS) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) were used for characterization and identification of unique signatures from a series of 18 Composition C-4 plastic explosives. The samples were obtained from various commercial and military sources around the country. Positive and negative ion TOF-SIMS data were acquired directly from the C-4 residue on Si surfaces, where the positive ion mass spectra obtained were consistent with the major composition of organic additives, and the negative ion mass spectra were more consistent with explosive content in the C-4 samples. Each series of mass spectra was subjected to partial least squares-discriminant analysis (PLS-DA), a multivariate statistical analysis approach which serves to first find the areas of maximum variance within different classes of C-4 and subsequently to classify unknown samples based on correlations between the unknown data set and the original data set (often referred to as a training data set). This method was able to successfully classify test samples of C-4, though with a limited degree of certainty. The classification accuracy of the method was further improved by integrating the positive and negative ion data using a Bayesian approach. The TOF-SIMS data was combined with a second analytical method, LA-ICPMS, which was used to analyze elemental signatures in the C-4. The integrated data were able to classify test samples with a high degree of certainty. Results indicate that this Bayesian integrated approach constitutes a robust classification method that should be employable even in dirty samples collected in the field.

Asunto(s)

RESUMEN

We report results of measurements of isotopic ratios obtained with atom probe tomography on U3O8 reference materials certified for their isotopic abundances of uranium. The results show good agreement with the certified values. High backgrounds due to tails from adjacent peaks complicate the measurement of the integrated peak areas as well as the fact that only oxides of uranium appear in the spectrum, the most intense of which is doubly charged. In addition, lack of knowledge of other instrumental parameters, such as the dead time, may bias the results. Isotopic ratio measurements can be performed at the nanometer-scale with the expectation of sensible results. The abundance sensitivity and mass resolving power of the mass spectrometer are not sufficient to compete with magnetic-sector instruments but are not far from measurements made by ToF-SIMS of other isotopic systems. The agreement of the major isotope ratios is more than sufficient to distinguish most anthropogenic compositions from natural.

Asunto(s)

RESUMEN

The application of surface analytical techniques such as time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS) is explored as a means of differentiating between composition C4 plastic explosives (C-4). Three different C-4 samples including U.S. military grade C-4, commercial C-4 (also from the United States), and C-4 from England (PE-4) were obtained and analyzed using both ToF-SIMS and XPS. ToF-SIMS was able to successfully discriminate between different C-4 samples with the aid of principal component analysis, a multivariate statistical analysis approach often used to reduce the dimensionality of complex data. ToF-SIMS imaging was also used to obtain information about the spatial distribution of the various additives contained within the samples. The results indicated that the samples could potentially be characterized by their 2-D chemical and morphological structure, which varied from sample to sample. XPS analysis also showed significant variation between samples, with changes in the atomic concentrations, as well as changes in the shapes of the high-resolution C 1s and O 1s spectra. These results clearly demonstrate the feasibility of utilizing both ToF-SIMS and XPS as tools for the direct characterization and differentiation of C-4 samples for forensic applications.

RESUMEN

The surface chemistry and in-depth distribution of the composition of a poly(ethylene oxide) (PEO)-containing biodegradable poly(L-lactic acid) (PLLA) blend matrix system have been investigated using X-ray photoelectron spectroscopy (XPS). This study reports detailed quantitative compositional information using a novel numerical method for determining depth profiles. The PEO system studied is an amphiphilic Pluronic P104 surfactant, PEO-b-poly(propylene oxide) (PPO)-b-PEO. The extent of phase separation is analyzed by determining the surface enrichment of the PEO component via measurement of chemical composition at the polymer-air interface. For this blend system, the combination of the PPO component in the Pluronic surfactants drives the formation of a surface excess of Pluronic in the blends with PLLA. The surface excess profile shows a rapid increase in Pluronic surface composition versus bulk Pluronic mass fractions of 1-5%, but the profile levels off above bulk Pluronic mass fractions of 5%.

Asunto(s)

RESUMEN

Cluster secondary ion mass spectrometry (cluster SIMS) employing an SF5+ polyatomic primary ion sputter source in conjunction with a Bi3+ analysis source was used to obtain three-dimensional molecular information in polymeric-based drug-eluting stent coatings. The formulations of the coatings varied from 0% to 50% (w/w) sirolimus drug in poly(lactic-co-glycolic acid) and were prepared on both MP35N metal alloy coupons and bare metal stents. All cluster SIMS depth profiles obtained indicated a drug-enriched surface region, followed by a drug-depletion region, and finally a constant bulk composition region, similar to previous data obtained in polymeric blend systems. The drug overlayer thickness was determined to increase with increasing sirolimus content. Sample temperature was determined to play an important role in the resulting depth profiles, where it was shown that the best profiles were obtained at low temperatures (-100 degrees C). At these temperatures, molecular signals typically remained constant through the entire depth of the film (approximately 6.5 microm) in some cases, as opposed to the typical 1 microm-2 microm depth limit, which is achievable at room temperature. The 3-D imaging capabilities of cluster SIMS were successfully demonstrated and indicated a significant amount of subsurface domain formation in the 25% and 50% sirolimus samples, but not in the 5% sample, which was homogeneous. These results clearly illustrate the utility of cluster SIMS for probing the 3-D structure in polymeric-based drug delivery devices.

Asunto(s)

RESUMEN

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) was utilized to obtain characteristic mass spectra from three different smokeless powders and six different black powder samples. In these mass spectra, peaks indicative of both the organic and inorganic additive constituents in the gunpowders were observed. TOF-SIMS was able to successfully differentiate between the different black and smokeless gunpowder samples analyzed with the aid of principal components analysis (PCA), a multivariate statistical analysis approach often used to reduce the dimensionality of complex data. TOF-SIMS was also used to obtain information about the spatial distribution of the various additives contained within the gunpowder samples. SIMS imaging demonstrated that that the samples could potentially be characterized by their 2-D structure, which varied from sample to sample. These results clearly demonstrate the feasibility of utilizing TOF-SIMS as a tool for the characterization and differentiation of gunpowder samples for general forensic applications.