Saturday, July 25, 2020

Applied Gamma-Ray Spectrometry: Is my Trinitite real?

Some time ago, I was purchasing Autunite specimens from a mineral dealer and after a couple of shipments from him, in one of the packages I received a small envelope marked as "Free / Thank you Gift". What was inside was a piece of rock with a card simply saying "Trinitite".

Trinitite is an artificial "mineral" (also known as "Atomic Bomb Glass") created during the first ever nuclear explosion test in the desert of New Mexico - "The Trinity Test" on July 16, 1945. During the nuclear explosion, desert sand was sucked into the fireball, melted while being infused with fission products and deposited back on the desert floor as "glassy" rocks.
The "classic" Trinitite has olive-greenish, often smooth. glassy surface with many inclusions and gas pockets - technically, the nuclear explosion created glass out of the desert sand by melting it while including radionuclides and droplets of iron and copper from the supporting tower and wiring. The structure and make of this man-made mineral is complex and diverse based on the exact location it came from.

After the end of the war, when the Manhattan project was revealed to the public, people started collecting Trinitite from the White Sands Missile Range. Once the US Government and Army caught a wind of this, in 1954 they forbade any  collection of samples from Ground Zero and made it illegal. The Trinity site is also off-limits to the public except for one day in the year with a guided tour, so real Trinitite has somewhat limited availability and it is difficult to find / buy nowadays, thus somewhat expensive.

My sample didn't look exactly as what I've seen on pictures of typical Trinitite but it was showing an activity of around 500-550 CPM on my LND7317 "pancake" detector equipped GQ GMC-600+ Geiger Counter. 
A lot of mildly radioactive rocks are often passed as "Trinitite" especially the ones with greenish tint (minerals with micro-crystals of Torbernite on the surface can somewhat resemble the "Trinitite look").
I was wondering if I got a "fake Trinitite" - uranium or thorium mineral, showing natural radioactivity or it was the "real deal" - it was a "free gift" anyways but it did spark my curiosity and I didn't have any other actual Trinitite in my collection. To be honest, I was a bit skeptical about it at first and thought it just might be an unidentified radioactive mineral.

Upon a close examination, a promising feature of my sample was notable - the extremely porous nature in cross-cut with many gas pockets but relatively smooth on the surface - a result from Trinitite being cooked by a giant gaseous fireball - the hot gases from the explosion melting and mixing the desert sand yield a porous, almost Vulcanic / magma looking rock.
The somewhat coarse and "frothy" surface is due to the shockwave of the blast, rolling pieces of melted sand across the desert floor - another evidence that it came from an area close to the epicenter of the explosion is the higher level of radioactivity.

The Trinitite specimen loaded into the lead-shielded sample chamber of my "Gamma-nator 2000" 😀 Gamma Spectrometer.
The sample is placed in a plastic bag in order to prevent contamination of the test chamber and then placed in a copper sample holder.

Theremino MCA displaying the Gamma Spectrum Analysis of my Trinitite specimen (22 hr. scan).

A 22 hours spectrum scan with the background (22 hours scan as well) subtracted. 
The sample is located <1cm from the front of the detector.

A Gamma Spectrum Analysis immediately reveals the true nature of my Trinitite - clearly visible are photopeaks from Cs-137, Am-241 and especially revealing is the Eu-152 peaks! 
The desert sand in New Mexico has traces or Eu-151 but the only way to get Eu-152 (which is not naturally occurring isotope) is by neutron activation and the neutron capture of Eu-151 . Caesium-137 and Americium-241 are also not naturally occurring isotopes. Americium-241 is created by the neutron bombardment of Pu-239 + 2(n) -> Pu-241  -> Am-241. Cs-137 is an infamous fission by-product of splitting Uranium and Plutonium nuclei. 
While Trinity was a Plutonium-based device, natural Uranium was used as tamper/pusher and lots of it too, which also explains the U-235 peak.
This gamma spectrum is the typical gamma "fingerprint" of  Trinitite - none of the observed activity is natural.

In conclusion - this specimen indeed is a piece of history and it was born under a giant Mushroom Cloud at the White Sands Missile Range, NM. 

I also purchased a sample of Trinitite from Scott over at the Atomic Rock Shop (www.atomicrockshop.com). This is a great resource for anyone who wants to own a piece of history and the buying experience was excellent. 
This sample exhibits the "typical" Trinitite look - much deeper olive-green color and very glassy-smooth surface. As suspected the activity is lower as this piece of Trinitite likely came from an area further away from the blast where the sand was just melted and cooled down with less disturbance.

No surprises in the spectrum (a 4.5 hrs scan with Gamma Spectacular GS-1525 NaI(Tl) detector) - the typical isotopic mixture of Cs-137, Am-241 and Eu-152. The amount of Europium-152 in this sample appears to be less than the first sample, again consistent with a greater distance to the epicenter and less neutrons.
The Geiger Counter activity measures approximately 170-200 CPM @ 5 mm from the surface (Alpha-particles shielded by plastic) using an LND 7317 tube, so it is quite safe to keep in a display box on your desk or shelf.

For a reference, this mineral from my collection - Meta-Zeunerite (Hydrated Copper-Uranium Arsenate - Cu(UO₂)₂(AsO₄)₂•H₂O) is a secondary Uranium mineral with natural radioactivity due to U-238, U-235 and a slew of daughter products.
This particular specimen is from the Clara Mine, Oberwolfach, Ortenaukreis, Freiburg Region, Baden-Württemberg, Germany. Activity is around 300 cpm.

Here is the Gamma-Spectrum of the Zeunerite sample- a classic natural Uranium spectrum with a number of daughter product peaks - Lead-214, Bismuth-214, Ra-226 etc.

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