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 a 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, infused with fission products and deposited back on the desert floor.
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 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, but once the US Government and Army caught a wind of this, in 1954 they forbade any collection and made collecting of samples from ground zero 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, thus somewhat expensive.

My sample didn't look quite exactly as what I've seen on pictures of real Trinitite but it was showing an activity of around 500-550 cpm on my LND7311 "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 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.

A promising feature of my sample was the extremely porous nature in cross-cut with many gas pockets but very smooth on the surface - just like the real Trinitite cooked by a giant gaseous fireball - the hot gases from the explosion melting and mixing the desert sand yields a porous, almost Vulcanic / magma looking rock.

PRA displaying the Gamma Spectrum Analysis of the Trinitite specimen.

This is an 8 hour spectrum scan with the Trinitite sample at a greater distance from the detector (~4cm) and background scan  (3 hours) subtracted.

A Gamma Spectrum Analysis immediately reveals the true nature of my Trinitite - clearly visible are peaks 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 is by neutron bombardment and 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 product of Uranium. While Trinity was a plutonium based device, natural Uranium was used a tamper/pusher and lots of it.
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. 

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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