Date of Award

8-2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemical and Biomolecular Engineering

Committee Member

Scott M. Husson, Committee Chair

Committee Member

Brian A. Powell

Committee Member

David A. Bruce

Committee Member

Douglas E. Hirt

Abstract

Thermal ionization mass spectrometry (TIMS) and alpha spectroscopy are powerful analytical techniques for the detection and characterization of Pu samples. These techniques are important for efforts in environmental monitoring, nuclear safeguards, and nuclear forensics. Measurement sensitivity and accuracy are imperative for these efforts. TIMS is internationally recognized as the “gold standard” for Pu isotopic analysis. Detection of ultra-trace quantities of Pu, on the order of femtograms, is possible with TIMS. Alpha spectroscopy has a long history of use in the detection and isotopic analysis of actinides and can be a simpler and less expensive alternative to mass spectrometer based techniques. The sensitivity and accuracy of both techniques is highly dependent upon the method of sample loading. High quality sample loading is often tedious, time consuming, and expensive. In this work, we sought to simplify and improve high quality sample loading for TIMS in an effort to expand the utility and improve the sensitivity of this technique. During these efforts a promising sample loading method for alpha spectroscopy was developed. Three improvements were developed for sample loading procedures for isotope ratio measurements of ultra-trace quantities of Pu using (TIMS). Firstly, a new filament geometry, the “dimple”, was developed. The bead loading method was used for these analyses. Beads were loaded with New Brunswick Laboratory certified reference material (NBL CRM) Pu128 (239Pu and 242Pu) from an 8M HNO3 matrix. Overall ion counts and isotopic ratios measured using the dimpled filament geometry were compared to those measured when using the established V-shaped filament geometry. The average number of Pu counts detected when using dimpled filaments was approximately 34% greater than ion counts detected using V-shaped filaments. The accuracy and precision of isotopic ratio measurements were unaffected by the use of dimpled filaments. The well-like geometry of dimpled filaments aids in sample loading and alignment. Additionally, the use of dimpled filaments was found to reduce sample losses inside the ion source. Over the course of 25 measurements, no sample losses were experienced on dimpled filaments, in contrast to 15% total sample loss experienced with v-shaped filaments. Secondly, a polymer fiber architecture for TIMS sample loading was developed using similar sample loading procedures as those used in bead loading. Fibers with diameter of approximately 100 μm were prepared from triethylamine-quaternized-poly(vinylbenzyl chloride) cross-linked with diazabicyclo[2.2.2]octane. Total ion counts (239Pu + 242Pu) and isotope ratios obtained from fiber-loaded filaments were compared to those measured bead loading. Fiber loading was found to improve sensitivity, accuracy, and precision of isotope ratio measurements of Pu compared to the established resin bead loading method, while maintaining its simplicity. The average number of detected Pu+ counts was 180% greater and there was a 72% reduction in standard deviation of ratio measurements when using fiber loading. An average deviation of 0.0003 (0.033%) from the certified isotope ratio value of NBL CRM Pu128 was measured when fiber loading versus a deviation of 0.0013 (0.133%) when bead loading. The fiber formation method can be extended to other anion-exchange polymer chemistries, and therefore offers a convenient platform to investigate the efficacy of novel polymer chemistries in sample loading for TIMS. Thirdly, a sample loading procedure was developed that is based on a polymer thin film architecture. Rhenium filaments were degassed, dip-coated with a thin (~180 nm) hydrophobic base layer of poly(vinylbenzyl chloride) (PVBC), and spotted with an aqueous solution of triethylamine-quaternized-PVBC and a cross-liking agent (diazabicyclo[2.2.2]octane). Spotting resulted in the formation of a toroidal, hydrophilic extractive polymer disk surrounded by the hydrophobic base polymer. Thin film coated filaments were direct loaded with NBL CRM Pu128 from a 9 M HCl matrix. Aqueous sample droplets adhered to the extractive polymer spot, facilitating sample loading. The influence of spot thickness upon ion production was investigated. Overall ion counts and isotopic ratios obtained from thin film coated filaments were compared to those produced by the established resin bead loading method. Isotopic ratios were within error of those measured using the bead loading method with few background interferences. The average number of detected Pu+ counts was 175% greater when using thin film coated filaments with 20-30 μm thick toroidal spots. The use of dimpled filaments further aided sample loading by providing a well-shaped substrate to deposit the sample droplet. No sample loss was experienced with the thin film loading method over the course of 65 sample analyses. Finally, thin films used in this design were found to slow filament aging under atmospheric conditions, facilitating the bulk production of filaments for future analyses. During this work, an unreported form of rhenium surface oxidation was discovered. Rhenium is the most common ionization filament material for Pu analysis by TIMS. Degassing is a common preparation technique for rhenium filaments and is performed to clean filaments before analysis. Degassing involves resistively heating the filaments under high vacuum to volatilize and degrade contaminants. Collaborators at Savannah River National Laboratory reported anecdotally that the use of excessively aged filaments (on the order of 2 months of aging in atmosphere after degassing) decreased the sensitivity and precision of TIMS analyses. Although optimization studies regarding degassing conditions have been reported, little work has been done to characterize filament aging after degassing. In this study, the effects of filament aging after degassing were explored to determine a “shelf-life” for degassed rhenium filaments, and methods to limit filament aging were investigated. Zone-refined rhenium filaments were degassed by resistance heating under high vacuum before exposure to ambient atmosphere for up to 2 months. After degassing, the nucleation and preferential growth of oxo-rhenium crystallites on the surface of polycrystalline rhenium filaments was observed by atomic force microscopy and scanning electron microscopy (SEM). Compositional analysis of the crystallites was conducted using SEM-Raman spectroscopy and SEM energy dispersive X-ray spectroscopy, and grain orientation at the metal surface was investigated by electron back-scatter diffraction mapping. Spectra collected by SEM-Raman suggest crystallites are composed primarily of perrhenic acid. The relative extent of growth and crystallite morphology were found to be grain dependent and affected by the dissolution of carbon into filaments during annealing (often referred to as carbonization or carburization). Crystallites were observed to nucleate in region specific modes and grow over time through transfer of material from the surface. The roles of atmospheric humidity and carburization on the oxidation characteristics (i.e. aging) of rhenium filaments were studied. Degassed and carburized filaments were aged for up to 79 days under dry and humid conditions, and the growth of oxo-rhenium crystallites was investigated intermittently by SEM to construct growth profiles. SEM images were analyzed to determine average crystallite size, number density, and percent surface coverage. Crystallite growth was found to be suppressed by both filament carburization and dry storage conditions (~13% relative humidity). Under humid conditions (75% relative humidity), crystallite growth progressed steadily over the investigatory period, reaching >2.3% surface coverage within 79 days of aging. Atomic ion production of Pu+ was suppressed by approximately 20% and the standard deviation of isotope ratio measurements was increased by 170% when filaments with 1% oxide surface coverage were used in sample loading. Measurement sensitivity and reproducibility are imperative for applications involving ultra-trace analysis of Pu by TIMS. These findings offer validation for observations regarding the detrimental effect of excessive filament aging post-degassing, improve the understanding of conditions that impel the oxidation of rhenium filaments, and provide practical means to suppress the growth of oxides. PVBC nanolayers were found to slow the growth of oxo-rhenium crystallites on the filament surfaces and may serve as an alternative carbon source for filament carburization. A novel substrate for the simultaneous concentration of actinides and sample preparation for alpha spectroscopy was developed using thin films originally intended for TIMS sample loading. Substrate preparation involved forming ultrathin films (10-180 nm) of quaternary amine anion-exchange polymers on glass and silicon by dip-coating. Samples were loaded by submerging the polymer-coated substrates into acidified solutions of Pu or natural water with elevated uranium concentrations. High resolution (25-30 keV) alpha spectra were acquired from these substrates under certain loading conditions indicating that through further development they may be a useful, inexpensive, and potentially field deployable platform serving national security and environmental sampling applications.

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