MICROANALYSIS OF TRINITITE

Recent advances in microanalysis allow re-evaluation of nuclear fallout debris at unprecedented scales. Fine-scale (tens of microns) elemental and isotopic heterogeneities contradict the traditional concept that debris is largely homogeneous and provide additional constraints on the incorporation (location, timing) of radionuclides in glassy debris. We have analyzed aerodynamic fallout glass from Trinity by autoradiography, SEM, EPMA, and SIMS. Trinitite is composed of three glassy phases and in some samples, whole minerals that have experienced minimal, if any, melting. The glass phases comprise an alkali-rich silicate glass, a Ca-Mg-Fe-bearing silicate glass, and SiO₂ (unmelted to partially melted quartz). Uranium is present only in the Ca-Mg-Fe glass. We measured the U isotope composition of the Ca-Mg-Fe glass using a Cameca IMS 1280 SIMS. Most ²³⁵U/²³⁸U measurements are natural (~0.0072), reflecting incorporation of the natural U metal tamper and fused earth. Depleted ²³⁵U/²³⁸U is less common and reflects fission of the natural U. Enriched ²³⁵U/²³⁸U was also measured, and the origin of this signature is under investigation. We calculate U abundance (7-55 μg/g) using a sensitivity factor based on NIST 611 and a glass doped with NBL CRM-U500. The 239.05 amu peak is deconvolved into ²³⁸UH and ²³⁹Pu, and ²³⁹Pu abundances (<5-160 ng/g) are estimated assuming an RSF_U:Pu of 2.5.^{1, 2} Regions of higher U and/or Pu do not directly correlate with more radioactive regions identified by autoradiography, suggesting that other species (e.g. fission products) contribute to the observed radioactivity. Decay correction of ²³⁵U/²³⁸U using estimated ²³⁹Pu contents is negligible. We will show that microanalytical interrogation of debris reveals a greater range of elemental and isotopic heterogeneities and textural information than bulk analysis. These data facilitate targeting of specific device-bearing phases for compositional analysis and constrain the relative timing of incorporation of radioactive species into debris.