NUCLEAR FORENSIC APPLICATIONS INVOLVING HIGH-SPATIAL-RESOLUTION ANALYSIS OF TRINITITE CROSS-SECTIONS

Nuclear proliferation and expanding access to nuclear technology has increased the potential of a nuclear incident or unauthorized detonation. Such an event can create post-detonation material (PDM) with mixed trace element and isotopic characteristics from both the nuclear device and impacted environment. Hence, developing methods for rapid and accurate source attribution may help to deter such incidents. However, sampling of PDMs may be limited and thus developing the most effective analytical strategies will be of utmost importance for attaining accurate forensic information. For example, what is an adequate sample size, and how would depth of analysis influence the interpretations? At what distance from ground zero can we adequately characterize the device’s signature? Here, these questions are addressed by adopting a high spatial resolution study of the vertical distribution of trace element abundances and isotopic compositions within Trinitite petrographic thin sections. Trinitite was chosen because it is a relatively well-characterized PDM – the bomb design and isotopic composition of the nuclear fuel used in the explosion are known – and it originated in a geologically simple environment. Although Trinitite is heterogeneous, its composition is influenced primarily by melting of the local arkosic sand and incorporation of anthropogenic components (e.g., blast tower, bomb material).

Trinitite thin sections were first thoroughly imaged by SEM, µ-XRF, and alpha radiography. Major and minor element concentrations were obtained by electron microprobe analysis along multiple vertical transects on each sample. Subsets of these points were subsequently analyzed for trace element compositions by laser ablation-ICP-MS. The latter indicate that a Pu-signature was most likely to be recorded in a thin surface layer (≤2 mm), and the lowest ²⁴⁰Pu/²³⁹Pu ratio was found in the top millimeter. Below this range, the partially melted sand is the primary contributor to compositional differences and the Pu isotopic signature, and there is also a gradual increase in Na₂O and K₂O contents with depth in many samples. LA-ICP-MS results from a sample collected 51±1 m from ground zero indicates a single region ­– at 1.13 mm depth – with Nb and Ta values consistent with an anthropogenic source.