In near-surface nuclear explosions, device materials mix with vaporized and melted soil in the fireball, forming aerodynamic glassy fallout. The analysis of device signatures in this fallout glass has been the subject of study for post-detonation nuclear forensics. The interpretation of the compositional variations observed in such glasses remains a challenge, however, and the physical processes that control fallout formation are not well understood. Fallout samples consisting of cm-scale glassy blebs with spherical, tenth-millimeter glassy spherules fused to their surfaces, were obtained from a historical US test. The goal of this study is to constrain the time and temperature intervals that control mass transport in these fallout glasses by studying compositional and U-isotope profiles observed in them. Backscattered electron images of carbon-coated polished sections of the samples were acquired by scanning electron microscopy (SEM).  The interfaces between small spherules and the larger host objects are enriched in elements heavier than potassium.  The interfaces exhibit what appear to be chemical diffusion profiles of Fe, Ca, and Mg.  Concentration profiles across the interfaces were measured using energy dispersive x-ray spectroscopy (EDS), wavelength dispersive x-ray spectroscopy (WDS), and nano-secondary ion mass spectrometry (NanoSIMS). While a better understanding of temperature variation as the fireball cooled and improved data on diffusivities in rhyolitic glasses are needed, our new results provide compelling evidence that the time and temperature of mass transport, and cooling rate in the fireball, can be constrained by studying these diffusion profiles. A thermal model incorporating the time scale and temperature ranges for mass transport in glassy fallout will be discussed.

Mass transport in aerodynamic fallout glass from a near-surface nuclear explosion

249th National Meeting (2015)

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