Examining Bubbles as a Novel Nuclear Forensics Tool

The project began with a simple question prompted by this thin section. Notice anything particular about it? Ruminate for a bit while I fill in some details.

ppl Trinitite
Image mosaic of a Trinitite sample viewed in cross section. Some salient features annotated. The sample is ~1 inch wide.

That there is a little slice of nuclear post-detonation material called Trinitite. I spent 2014 working the stuff, which is a glassy product of the first atomic bomb detonation. On a July morning in 1945, the Plutonium-powered nuclear device was detonated at the Trinity site of the White Sands Proving Grounds in Alamagordo, New Mexico. The test yielded the equivalent of ~20 kilotons, the fireball exceeded 8000ºC, and it created a ~1 km wide (shallow) crater.

trinity_explosion_sequence
Stills from the Trinity test. (Source: US Gov)

Typical atomic bomb stuff. The terrible fireball is eye-catching, sure, but as a geologist my gaze is usually drawn to ground level. Fewer people appreciate what’s going on down there (until there’s a house in frame to obliterate). There’s a supersonic shock wave, and the upper surface gets scoured by heat and wind. It is a chaotic mixture of bomb material and local geology — and an interesting, unique mixture at that. Nuclear materials have certain fingerprints – chemically and isotopically – that can be tied to a source or sources. During the explosive process, this signature gets imprinted in post-detonation material (typically acronym’d as PDMs). Trinitite is the first such human-made material. It is not the first ever because there are natural fission reactors in some parts of the world where uranium has built up over time and “gone nuclear”.

 

trinitite whole rock
Top down view of the sample sectioned above

So back to the thin section that started it all. Notice anything unusual about the bubbles? Our question was simple: why are there so many bubbles along the bottom? Is that a real feature, or are we seeing patterns in randomness?

In looking at that and a couple additional samples, it was apparent that something was different between the top and bottom vesicles. With a simple hypothesis to test, it was time to measure. So I ran statistical analysis of the size and shape of bubbles in vertically cut trinitite sections. What we found agreed with our initial impressions, and our findings were published in a PLOS ONE paper earlier this year (and free to read).

It turned out there was more to see than just “more vesicles at greater depth”. In each of the samples we studied, the upper 2-3 mm was relatively devoid of bubbles. Below this region, there was an increase in the size, number, and elongation of vesicles. All this points to a different formation mode between the upper and lower zones. Our current sequence of events starts with the blast heat melting the desert surface to a few mm deep. There was enough moisture to cause extreme bubbling as the melt degassed (released water vapor and other volatiles to the atmosphere). As the fireball grew into the classic mushroom shape, it drew air inward. This cooler air quenched the upper surface of the glass, but there was enough heat and/or water in the glass to cause further bubbling while maintaining at least a semi-molten state. However, the bubbles were trapped and couldn’t degas at the surface. Thus the lower bubbles had time to grow together and flattened out below the quench zone, in the 2-3 mm deep region. There was also some “late” (a few seconds to minutes) contribution from fallback, where particles in the fireball settled out in a molten (or partially molten) rain. Our sequence matches well with previous interpretations of Trinitite formation. We hope it shows this type of textural analyses is a useful complement to other techniques in nuclear forensic analysis.

In the end, it was a fun little article to write, and reinforced the idea that it just takes an off-the-cuff observation to start an interesting research project.

Full Article: P. H. Donohue and A. Simonetti (2016) Vesicle Size Distribution as a Novel Nuclear Forensics Tool. PLOS ONE 11(9) e0163516.

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