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.

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.

Advances in Nuclear Forensics: GSA 2014 Technical Session

The lunar basalts in my doctoral research were almost four billion years old, plus or minus a couple hundred million years. The rocks I study now were created on July 16th, 1945, at 05:29:45 AM (Mountain War Time). It’s a strange thing to know so precisely. But how can I pinpoint the exact second of creation? Because these rocks are trinitite, the glassy result of a sandy New Mexico desert experiencing the first atomic bomb blast.

Two views of a common green glass variety of trinitite. Image from the Simonetti Lab at Notre Dame.

The first nuclear bomb test, codenamed Trinity, was performed at the White Sands Proving Grounds (near Alamogordo, New Mexico). The device, Gadget, was an implosion-type design with a plutonium (Pu-239) core. The heat resulting from the 18 kiloton explosion melted the desert sand surface out to distances of 400 meters from ground zero. trinitite_thin_section The surface sand melted to form a glassy layer (1-2 cm) on top of incipiently melted desert sand together, these form trinitite (alternatively, Alamogordo glass). This post-detonation material is a valuable tool in nuclear forensics research. Trinitite incorporated pieces of Gadget and the blast tower, and one of our goals is to identify and characterize the distribution and composition of individual components through geochemical and radionuclide analysis. At right, a vertical cross-section of trinitite is shown in thin section.

The analysis of postdetonation material (like trinitite) is one arm of the nuclear forensics field. An effective nuclear forensic analysis requires technical information and relevant databases, and specialized skills and expertise to generate, analyze, and interpret the data. This analysis combined with law enforcement and intelligence data can provide valuable information on the provenance of such materials, and processing history so as to improve source attribution. Identifying the source(s) of stolen or illicitly trafficked nuclear materials will therefore prevent, or make more difficult, terrorist acts that would use material from these same sources. Moreover, effective forensic analysis of postdetonation materials in the unlikely event of a nuclear terrorist attack is also expected to deter individuals or groups involved, and provides incentives to countries to enhance their security and safeguards relative to their nuclear materials and facilities.

The microscopic and macroscopic appearance, as well as the elemental and isotopic composition of nuclear materials, i.e. its ‘signature’ reflects its entire history. The term ‘signature’ is used to describe material characteristics that may be used to link nuclear samples to people, places, and processes, much as a written signature can be used to link a document to a particular person. Forensic methods employed to establish signatures in nuclear materials typically combine physical and chemical (e.g. X-ray fluorescence, scanning electron microscopy, electron microprobe analysis, secondary ion mass spectrometry) characterization and radiometric measurements (e.g. alpha, beta and gamma spectroscopy). The methodologies and interpretation of forensic analyses are constantly being advanced and perfected.

gsa-logo_14CAt this year’s annual meeting of the Geological Society of America, the Notre Dame crew (Drs. Tony Simonetti, Sara Mana, and myself) are chairing a session to update the geoscience community on the latest developments of nuclear forensics. The cleverly-titled session, “Advances in Nuclear Forensics”, will emphasize analytical techniques, database development, and implications for our ability to identify and possibly prevent nuclear attacks and trafficking of illicit nuclear materials.

UPDATE (Aug 9, 2014): The session has been designated a poster session.

Note: A significant portion of this post was reused from our session proposal, which isn’t published by GSA.

Further Resources:

Conversation with a Microbiologist (audio)

Do you know how to complement a bacterium? What about the difference between flagellum and Type 4 pili (and why it matters)? Listen and learn! Headphones recommended.

This was an in-person chat with Morgen Anyan, PhD candidate at the University of Notre Dame (research page). Morgen is researching environmental and morphological effects on the behavior of the bacteria Pseudomonas aeruginosa.

For those few who listened to my previous interview with Ben the Beekeeper, you’ll be pleasantly surprised to find that the audio is much better thanks to a Zoom H2n recorder. I’ve also edited it a bit heavier to keep it clipping along (mostly cutting out myself as much as possible to let Morgen tell her story). Avery made the cut, though.


Remove a carbon coat from thin sections with methanol

tl;dr: methanol is great but don’t let it kill you

Thin sections, oh glorious thin sections! They are little slices of truth, windows into the processes that shape all rocks. And for a variety of reasons, geologists do terrible things to them. Thin sections are subjected to staining, acid etching, laser beams and more. In my case, my samples are all subjected to the electron microprobe for in-situ mineral analysis. The first step in this process is to coat each thin section in carbon. Samples are placed in a carbon evaporator, which creates a vacuum (down to at least ~10^-4 torr) and coats the sample in a ~20nm thick carbon layer. This is great for analytical work, but it dulls everything in plane polarized light, can mask birefringence colors, and just forget about trying reflect light microscopy.

Carbon coated lunar basalt 70135,64 looks mostly “normal” in plane-polarized light (though a bit dim). Most major phases are easily identifiable (plag = plagioclase; px = pyroxene), but reflected light is necessary to identify opaques…
Same area of lunar basalt 70135,64 shown in reflected light. Not only can you still not identify the opaque phases, but you’ve lost the boundaries between plagioclase and pyroxene!

It’s just a big gray mass of cracks. If you need to take a second look at your sample, that carbon coat will just have to go! Despite being only 20nm thick, the coating is surprisingly resilient. You must also take care not to damage the thin section, so what do you do? There are various ways to approach carbon coat removal, and I thought it would be useful to highlight two common methods: grit and methanol

Removing a carbon coating with Al-polish

Use a fine grit Al-oxide powder and water to polish off the carbon coat. This quote from a mineral forum post suggests several ways to remove a carbon coat, including:

The simplest way to remove a carbon coat or a gold coat is via a 1 micron Al2O3-H2O slurry (1 part Al2O3 to 8 parts H2O) on a Buehler Polishing Cloth ( Catalog No. 40-7218 Microcloth with adhesive for a 8 inch wheel) aka “moleskin”. Gentle rubbing of the thin section or grain mount by hand on the mole skin polishing cloth (on a flat surface) with a generous amount of the slurry will completely remove the coating both on the surface of the mineral grains as well as in between the cracks and grain boundaries. This is due to the the action of the very fine short hairs of the moleskin. The Al2O3 can be easily removed under a running tap (preferably distilled water) or else (if fussy) in distilled water for five minutes under ultrasound. Afterwards the thin section or grain mount should be dried using a soft cloth or a kleenex wipe.

I really like the idea of using a polishing cloth, as it is rather difficult to remove carbon from low areas (e.g., cracks) with only a kimwipe. Removing Al2O3 buildup in these areas can also prove difficult. In addition, this method gets messy, and I am always worried about over-polishing and losing the sample. That is why I prefer to use methanol.

Removing a carbon coating with methanol

Methanol is also an effective carbon coat remover, with one caveat: Methanol is extremely toxic! You should already be wearing gloves during cleaning to prevent your gross human oils from transferring to the thin sections, but in the case of methanol it is an absolute necessity to have proper hand protection. I also prefer to work under a fume hood to prevent inhalation (and because it smells wicked strong). Methanol is my preferred method because it is clear and evaporates rapidly, making for an easy assessment of the status of carbon removal and leaves almost nothing to clean up.

Before, during, and after carbon coat removal. Methanol preferentially removed carbon from the sample, so it was cleaned before the surrounding epoxy. The thin section is 1″ diameter.

How I removed this carbon coat with methanol:

  1. Wear safety gloves
  2. Place the thin section on a flat surface
  3. Moisten a kimwipe with a drop or two of methanol
  4. Hold the thin section in place and rub the kimwipe in a variably circular motion, applying gentle pressure
  5. Regularly change the face of the kimwipe being rubbed on the thin section. This minimizes the risk of loosened material scratching the surface.
  6. Keep the kimwipe moist but not too juicy with methanol
  7. Check your progress in reflected light. The image below is the nearly-cleaned thin section, with a couple trouble spots to finish up. Checking your progress early and often is the best way to get your eye in on carbon coat removal. You won’t know when it’s all gone if you have no idea what it looked like before you started!
A couple of minutes work on 70135,64 and the carbon coat is nearly gone. Now we can see the opaque phases in 70135,64 are mostly ilmenite, with some exsolution troilite (and unlabeled Fe-metal and possible melt inclusions) popping out that we would have missed previously.
70135,64 after methanol removal of the carbon coat. Notice the change from the previous picture – we uncovered a new exsolution feature in the lower left ilmenite grain. The ugly methanol droplets are also almost all gone, and the remaining “bubbles” are melt inclusions.

A few swipes with a methanol-dipped kimwipe is also a quick way to remove all those annoying loose particles surrounding laser ablation pits.

Four years of research poster design

Science conferences are ubiquitous components in research. What are you working on? What are you interested in? What do you want to tell us? Maybe you are allowed to read slides at us for twelve minutes (plus three for questions). Maybe you’ll bring twenty seven eight-by-ten color glossy pictures with circles and arrows and a paragraph on the back of each one explaining what each one is. More likely, however, you’ll have a dozen square feet of real estate on a tack-board. That is enough room for thirteen or so eight-by-ten color glossy photos with circles and arrows and a paragraph on the back of each one explaining what each one is, but the more commonly employed medium is that of the research poster.

The form and function of presentations and posters have their respective merits and drawbacks, and you can find ruminations extolling both of these somewhere else. I am a no preference kind of guy. To wit, I have one of each to present at this year’s Lunar and Planetary Science Conference (LPSC) in March. The LPSC is the main conference our entire research group attends every year. I have had at least one poster at each of the previous three LPSCs, plus two posters at other conferences. With three years under my belt, it should be easy to make a poster, right? Heck, you might say, after three years you should have a Masters of Poster Science! Well…no, it is not that simple.

substance without style is truth without beauty

Communicating science is hard, and only more difficult if conveyed boringly. Whenever it’s time to start making a new poster, my search history fills up with terms like “poster design”, “award science poster”, and “awesome research poster” (see here and here to start). The essence of a research story doesn’t change; My substance is the scientific method. But substance without style is truth without beauty. And with 700 other posters to choose from, would you stop to check this out?:

2009: Fear my wall of text! Size: 42″x34″; Title: Times New Roman 72pt; Body: Times New Roman 30pt

Nope. The color scheme is all right, but there is no hierarchy. What is important here? Graphs are all about the same size, there is no central point of focus, and look at all that text! This was made after 6 months of grad school, so I vowed to focus more on results the next time around, resulting in… Continue reading “Four years of research poster design”