Mechanisms for the Incorporation and Distribution of Radionuclides in Near-~Surface Fallout
Fallout is the radioactive glass formed from a mixture of vaporized anthropogenic materials with proximate environmental material. These glassy byproducts constitute a compositional record of environmental materials such as soil and vapor precursors that are responsible for chemical heterogeneity in the glasses. The work of this dissertation is to untangle these different sources of compositional heterogeneity, distinguishing low-abundance vaporized anthropogenic-rich material from natural compositions, in order examine soil behavior and chemical evolution during fallout formation. Unfortunately, mixing convolutes the multivariate elemental relationships in these melts due to overlapping element abundances that might distinguish different source materials and chemical behaviors, making bivariate analysis difficult or impossible to interpret. For these reasons historical and modern studies do not thoroughly investigated how soil affects deposition and incorporation of vaporized material in fallout.
This work employs a two-step strategy to overcome these challenges and more effectively understand preserved heterogeneity. First (1), three prospective multivariate approaches are compared: classical least squares (CLS) and principal component analysis (PCA), which are commonly employed in the literature, with Multivariate Curve Resolution Alternating Least Squares (MCR-ALS) that is novel in this application. This analysis shows that a closure and equality constrained MCR-ALS approach succeeds in identifying unrecognized precursor components and is a suitable alternative to PCA, or CLS-style approaches in situations distinguished by limited a priori information of precursor compositions. Second (2), the MCR-ALS method is applied to unbiased spatial analyses from 13 Trinity fallout glasses to resolve the contribution and composition of the environmental or vaporized sources. This numerical approach reveals the importance of environmental precursors such as alkali feldspar, calcite, and quartz, to chemical heterogeneity in most glasses. In contrast to previous studies, however, the work resolves a high Al-Si-rich precursor constituting evidence for volatile loss in the melts. Likewise, an Ca- Fe-Mg-rich composition is also resolved, constituting possible evidence for a vapor source term. Correlation of spatial activity distributions, measured by autoradiography, with the compositional precursors from MCR-ALS in clear evidence supporting surface-driven condensation, wherein molten soil components serve as heterogeneous nucleation sites for the vapor source, agglomeration of small melts onto the surface of larger melts, and physical mixing of the vapor source into the interior of molten objects. This is the first report to provide quantitative model-supported evidence for key mechanisms affecting the composition of the vapor phase in a nuclear event and highlighting the impact of local materials on resultant fallout compositions.