David Willingham

Femtosecond laser ablation multicollector ICPMS analysis of uranium isotopes in NIST glass

We utilized femtosecond laser ablation together with multi-collector inductively coupled plasma mass spectrometry to measure the uranium isotopic content of NIST 61x (x = 0, 2, 4, 6) glasses. The uranium content of these glasses is a linear two-component mixing between isotopically natural uranium and the isotopically depleted spike used in preparing the glasses. Laser ablation results match extremely well, generally within a few ppm, with solution analysis following sample dissolution and chemical separation. In addition to isotopic data, sample utilization efficiency measurements indicate that over 1% of ablated uranium atoms reach a mass spectrometer detector, making this technique extremely efficient. Laser sampling also allows for spatial analysis and our data indicate that rare uranium concentration inhomogeneities exist in NIST 616 glass.

High resolution isotopic analysis of U-bearing particles via fusion of SIMS and EDS images

Image fusion of secondary ion mass spectrometry (SIMS) images and X-ray elemental maps from energy-dispersive spectroscopy (EDS) was performed to facilitate the isolation and re-analysis of isotopically unique U-bearing particles where the highest precision SIMS measurements are required. Image registration, image fusion and particle micromanipulation were performed on a subset of SIMS images obtained from a large area pre-screen of a particle distribution from a sample containing a mixture of several certified reference materials (CRM) U129A, U015, U150, U500 and U850, as well as a standard reference material (SRM) 8704 (Buffalo River Sediment) to simulate particles collected on swipes during routine inspections of declared uranium enrichment facilities by the International Atomic Energy Agency (IAEA). In total, fourteen particles, ranging in size from 5–15 μm, were isolated and re-analyzed by SIMS in multi-collector mode identifying nine particles of CRM U129A, one of U150, one of U500 and three of U850. These identifications were made based on the measured isotopic composition which was accurate to a few percent of the certified value and which proved to be consistent with the respective National Institute of Standards and Technology (NIST) certified atom percent values for 234U, 235U, 236U and 238U for the corresponding CRMs. This work represents the first use of image fusion for nuclear safeguards application, resulting in improved accuracy and precision of isotope ratio measurements for U-bearing particles. Implementation of image fusion is essential for the identification of particles of interest that fall below the spatial resolution of the SIMS images.

Image segmentation for uranium isotopic analysis by SIMS: Combined adaptive thresholding and marker controlled watershed approach

A novel approach to particle identification and particle isotope ratio determination has been developed for nuclear safeguard applications. This particle search approach combines an adaptive thresholding algorithm and marker-controlled watershed segmentation (MCWS) transform, which improves the secondary ion mass spectrometry (SIMS) isotopic analysis of uranium containing particle populations for nuclear safeguards applications. The Niblack assisted MCWS approach (a.k.a. Seeker) developed for this work has improved the identification of isotopically unique uranium particles under conditions that have historically presented significant challenges for SIMS image data processing techniques. Particles obtained from five National Institute of Standards and Technology (NIST) uranium certified reference materials (CRM U129A, U015, U150, U500, and U850) were successfully identified in regions of SIMS image data (1) where a high variability in image intensity existed, (2) where particles were touching or were in c...

Characterization of extreme ultraviolet laser ablation mass spectrometry for actinide trace analysis and nanoscale isotopic imaging

We demonstrate a new technique for trace analysis that has nanometer scale resolution imaging capability: Extreme Ultraviolet Time-of-Flight Laser Ablation Mass Spectrometry (EUV TOF). We describe the characterization of this technique and discuss its advantages. Using the well-standardized NIST 61x glasses, the results show the EUV TOF spectra contain well defined signatures of U, Th, and their oxides, with far fewer spectral interferences than observed in Time-of-Flight Secondary Ion Mass Spectrometry (SIMS TOF). We demonstrate that the ratio of U and Th ions to the oxide ion signatures is adjustable with EUV laser pulse energy. Sample utilization efficiency (SUE) which measures the ratio of detected ions to atoms in the ablated volume was used as a measure of trace analysis sensitivity of EUV TOF. For U and Th, SUE is 0.014% and 0.017%, respectively, which is comparable to SIMS TOF in the same mass range. In imaging mode EUV TOF is capable to map variations in composition with a lateral resolution of 80 nm. Such high lateral resolution enabled mapping of the isotope distribution of 238U and 235U in closely spaced micron-size uranium oxide particles from isotope standard materials. Trace elemental sensitivity and nanometer spatial resolution gives EUV TOF great potential to dramatically improve the state-of-the-art laser ablation/ionization mass spectrometry and elemental spectro-microscopy for applications such as geochemical, forensic and environmental analysis.

Rate equation model of laser induced bias in uranium isotope ratios measured by resonance ionization mass spectrometry

Resonance Ionization Mass Spectrometry (RIMS) has been developed as a method to measure uranium isotope abundances. In this approach, RIMS is used as an element-selective ionization process between uranium atoms and potential isobars without the aid of chemical purification and separation. The use of broad bandwidth lasers with automated feedback control of wavelength was applied to the measurement of the 235U/238U ratio to decrease laser-induced isotopic fractionation. In application, isotope standards are used to identify and correct bias in measured isotope ratios, but understanding laser-induced bias from first-principles can improve the precision and accuracy of experimental measurements. A rate equation model for predicting the relative ionization probability has been developed to study the effect of variations in laser parameters on the measured isotope ratio. The model uses atomic data and empirical descriptions of laser performance to estimate the laser-induced bias expected in experimental measurements of the 235U/238U ratio. Empirical corrections are also included to account for ionization processes that are difficult to calculate from first principles with the available atomic data. Development of this model has highlighted several important considerations for properly interpreting experimental results.

Combining a Convolutional Neural Network and Watershed Segmentation for Identifying U-Bearing Particles in Secondary Ion Mass Spectrometry Images

Secondary ion mass spectrometry (SIMS) has been employed as a technique for screening U-bearing particles within a complex matrix of other components. Typically performed as a two-step process, an initial pre-screening pass acquires images of up to 2500 fields of view (FOV) across the sample. These FOVs are then analyzed to identify U-bearing particles within the matrix such that a second, more accurate, spot mode analysis can be performed to determine isotopic ratios. Identification of the particles between the two analysis stages, however, is often hindered by considerable background signal and variable intensity across images in a data set.

Correlative Isotopic Analysis by Image Fusion of Electron Microscopy and Secondary Ion Mass Spectrometry Data

Image fusion is a technique to combine complementary information from two or more input images into a final image which, by definition, better describes the scene than any of the original images could alone. Capable of presenting valuable information from multiple sources in a single visualization, image fusion has been successfully applied to a variety of disciplines, including satellite [1] and medical imaging [2]. Recently, the technique has been developed for application to new analytical techniques, most significantly to combine chemical information from secondary ion mass spectrometry (SIMS) images with images acquired using electron microscopy [3, 4].

FY15 Progress Report for PL14-Lg Radius SIMS-PD1Ea: Large Radius SIMS Support / Large Radius SIMS for Nuclear Materials Analysis and Characterization

PNNL has been procured a Cameca 1280 Large Radius Secondary Ions Mass Spectrometer (LRSIMS) from the Amtek corporation out of France. This state-of-the-art instrument is aligning PNNL to deliver to NNSA the ability to address issues from proliferation detection to nuclear archeology of reactor operation and cascade enrichment history verification pushing beyond the limits of currently available methods and instrumentation at PNNL.

FY14 Progress Report for PL12-LaserSPec SIMS-PD08. Laser Photoionization of Sputtered Neutral atoms in PNNL SIMS and Applications in Nuclear Materials and Environmental Analyses

A continuous wave (CW) Ar ion laser producing photons at 244 nm (doubled from the fundamental wavelength at 488nm) was used to ionize neutrals sputtered from representative lanthanide (neodymium oxide, Nd2O3) and actinide (uranium oxide, U3O8) containing materials in the modified Cameca ims-4f at PNNL.