This post is about what’s been my longest experiment so far.
I’ve been trying to extract a nice spectrum from Trinitite for a while now, I got better overtime but my past attempts left me pretty unsatisfied. Cs137 and Am241 were always easy to spot, but I never found anything more than a hint of Eu152, so I got myself a couple more sample and gave it another go.
This time my supplier was Mineralogical Research Company (http://www.minresco.com/).
They have a number of Trinitite samples for sale and other very interesting radioactive minerals. As for the Trinitite they have “ordinary” specimens, but what caught my attention was what they call “Type 5” ones, which are from a single collector and apparently come from pretty close to the blast, where the neutron flux was more intense. Activation by neutron capture was the process from which Eu152 (and Eu154) originated, therefore Europium abundance in the Type 5 samples is supposed to be higher than in “ordinary” samples.
The problem with Trinitite is that you can only judge the samples you pick by their appearance. This time I asked the seller to test the specimens with a Geiger counter so I could at least have an idea of the samples’ activity, but I know by now this test can be highly misleading since what a Geiger counter gets is mostly betas (and alphas), besides there’s no way to know what radionuclide those counts come from without a spectrometer.
So in the end I got myself the sample with the highest count-rate (an ordinary “Type 1”) and the most active “Type 5”.
I tested them both, in this first post I present the result from the “Type 5”, the one with the less counts but also with the richer spectrum.
So here it is. As usual it’s glassy on one side and pretty sandy on the other one. The weight is 5,90 grams.
The sandy side gave me a little more than 200 CPM.
My ordinary background is in the region of 55 CPM.
The 2’’x2’’ NaI(Tl) typically gives me roughly five times as many counts as the PDS so I projected those figures to result in a 7-8 CPS difference between the two samples, which in the end resulted pretty accurate.
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I decided early on that this was going to be at least a 5 days measurement, but in the end I extended it to 14 days in order to make the spectrum look smooth enough (with this level of activity you record little more than 700,000 counts per day from the sample, and we know you need several million counts to have a smooth results).
Of course as the accumulation time got longer every further day of accumulation added less and less improvement and after two weeks I decided there was little point in keep on going, by that time the background+sample spectrum had reached 84 million counts with 10 millions of them coming from the sample.
Still I was entirely happy with the result and I figured out the problem was that my initial background wasn’t smooth enough. In terms of counts my 24 hours shielded background was the equivalent of a 6 hours unshielded one, not much.
The sample's spectrum is the difference between two spectra, every “asperity” in one of the two will show in the difference, and when the difference is small compared with the two spectra you start with, those asperities will be magnified in the final result, while if the difference is “big” (as it’s the case with a very active sample) they will practically disappear.
So, once my two weeks measurement with the sample were over I decided to add another day to my background. That also looked like a good choice methodologically, because the two spectra I was going to subtract from each other would then had the same “temporal center”. In the end I decided to add 48 hours instead of 24 so the two temporal centers don’t coincide anymore, but they are still closer than they were originally going to be.
Making the background measurement even longer would have certainly brought further improvement and in the end I could have done that, but you have to stop somewhere, the whole experiment lasted 17 days.
So here’s the result, both in linear and logarithmic view.
Cs137 is expected: fission product, long half-life time (30 years), it’s always easy to spot.
Am241 is expected too. The “gadget” used Plutonium (Pu239) as a fuel and only part of it fissioned. The rest was bombarded with neutrons and some of it underwent double neutron capture becoming Pu241 which then beta-decayed to Am241. It’s half-life of 433 years means it’s still very present and easy to see.
Eu152 is much more of a challenge. I only got a hint of its presence in previous samples. It comes from activation by neutrons of stable Eu151, naturally occurring in Trinity’s desert sand. Its abundance seems to be heavily dependent on the proximity of the sample to the center of the blast (which in turns influenced the intensity of the neutron flux). This time the peak at 121 keV is very strong and unmistakable and other peaks at higher energy are visible as well. The ones at 244 keV and 344 keV are possibly mixed up with Pb212 and Pb214, but I learned from previous samples that when Eu152 is weak those peaks barely show so I must assume their contribution here is very small. I didn’t put them in the labels.
The Eu152 peak at 1408 keV doesn’t seem to be visible. It’s there, albeit very weak, subtracting my first day background only, but disappears when you subtract the whole three days background, leaving what appears to be a solitary K40 peak
Eu154 is supposed to be there as well, coming from the activation of Eu153 (same process as above), but it’s signature peak at 123 keV tends to be mixed up with that of Eu152 at 121 keV so its inclusion in the peak’s label has no direct evidence for it. It’s certainly there but how big or small its contribution is I can’t tell, only a HPGe detector could tell us. Anyway, it’s shorter half-life time (8.8 years versus 13.3 of Eu152) provides an indication that its residual activity must be a lot lower than that of Eu152.
Samarium. I wasn’t prepared to deal with it, but when there’s Europium 152 there’s Samarium as well. I’ve got a pretty strong peak at 40 keV, pretty close to that from Am241 and Cs137 in that same region and I could only explain it as a product of X-Rays from Samarium which is linked to the presence of Eu152 (“Practical Gamma Spectrometry” by Gordon Gilmore, Appendix C, page 359).
Also in that region there could be a weak gamma peak from Pu239. As above for Eu154 I have no direct evidence of its presence, but a few measurements with HPGe detectors accessible online found it in Trinitite samples, since only part of the Pu239 used as fuel actually underwent fission. Its contribution to the peak must be very small so I wasn’t sure it was worth adding it to the label, but we know it’s there and it’s half-life of 24000 years means whatever its original activity was, it stayed pretty much the same 75 years later.
Ba133. I didn’t see this coming. When a 80 keV peak started to form I didn’t know what to make of it. I first considered the most uninteresting and conventional options: an X-Rays peak from naturally occurring Pb212 or Pb214? It’s possible, but I tested several Trinitite samples already and that peak never showed before. An X-Rays fluorescence peak from the shielding? Again, not likely, XRF is kept pretty low by the pewter in the shielding and experience tells me it doesn’t leave a visible mark on the final spectrum once the background is subtracted.
In the end Barium 133 seemed to be the most likely option. The bomb had an explosive lens system containing Baratol, which in turn contained stable Ba132. Activation due to neutron capture created Ba133, visible in other Trinitite spectra.
Ba133 has another strong peak at 356 keV and that appeared to be a good explanation to the fact that the 344 keV peak from Eu152 looked a bit wide as if there was something else to the right.
I also have a very low energy peak at 12-15 keV. That’s likely an L-shell X-rays from Uranium.
Lastly there’s a hint of another peak which I left unlabeled. Right beside the 80 keV peak which I attributed to Ba133 there’s something else which seems to be centered around 85-86 keV. The only potential explanation I found for it so far are Eu155 and Np237 but neither of them seems convincing.
Eu155 has an half-life of just 4.76 years, it halved almost 16 times since 1945, I find hard to believe there’s still enough of it to produce something that I can clearly detect.
Np237 is the decay product of Am241, but the latter has an half-life of 432 years, so again I am not sure there’s enough of it for me to clearly detect it.
X-rays from naturally occurring Lead was another option, but, again, it never showed before.
So in the end I left that peak unlabeled.
I have some degree of uncertainty on some of the “calls” above, but it should be mostly right I think.
And finally the quantitative analysis.
Until next time.
Massimo
