1230 keV mystery peak from WWII era optical glass

[Bob-O-Rama]

I have a mildly active lens element I assumed was some REE, its about 4x the shielded background, 4 cps vs 1 cps. The glass is definitely a beta emitter. For gamma, it shows a peak at ~1235, ~225, and ~75. - they all disappear without the sample. The 1235 does not really correspond to anything plausible I could find except Lu-174 decay to Yb-174, and the 75 keV peak could be Yb x-rays ( or, you know, basically anything ). Not sure what the 225 is all about. Another much older reference from USGS showed that 1240 peak associated with Hf(n,2n)Lu-174. So I was thinking it could be Hf glass that is gobbling up the occasional neutron?

[Rob Tayloe]

I understand that thorium and lanthanum have been used in the manufacturing process for some optical lenses. Both of these elements contain naturally occurring radioisotopes. Links to some articles about the inclusion of radioactive materials in optical lenses follow -

https://orau.org/health-physics-museum/ ... -lens.html

https://en.wikipedia.org/wiki/Thoriated_glass

https://lenslegend.com/radioactive-lenses/

[Sesselmann]

Bob,

Compton backscatter E' from your 1230 mystery peak would cause a peak at 211 keV.

\[ E' = \frac{E}{1+\frac{E}{511 \ keV}(1 - cos\phi)} \]

\[ E' = \frac{1230\ keV}{1+\frac{1230 keV}{511\ keV \ keV}(1 - cos\phi)} = 211.70 keV \]


Steven

PS: Bob, I see you pasted two screen shots of your spectrum, do you know in PRA the [UP], [DOWN] arrow keys on your keyboard can adjust the width of your spectrum? It's worth learning the shortcuts in PRA, there are many.

A possible candidate for your glass lens might be 154Eu, apparently used in older military optics.

154Eu

[Bob-O-Rama]

Just as a practical matter, would finding either Eu-154 or Lu-174 by itself be unlikely without also seeing related isotopes? I let it bake some more hoping less prominent Eu-154 peaks will emerge. In all honesty, I dismissed Eu-154 owing to what I thought were "missing" peaks based on things like, especially the one near 1000:


( From: https://www.researchgate.net/publicatio ... _to_154_Gd )

But I don't know how that all "smooths out" to make a NaI(Tl) spectrum.

[Bob-O-Rama]

So I tabled the lens, and moved on. I have a collection of ore minerals, and a few of them are mildly radioactive. One unusual one is a halite type mineral, deliquescent, with what I think are AgCl inclusions, which I could not ID. Same as the lens! I had to double check to make sure it was not hallucinating or that the lens was still in the shield. I also checked to make sure the spectrum followed the sample, it did. Maybe the 1240 peak is an escape peak for some mundane higher energy thing the 2x2" cannot see? This is about 5 cps, BG is about 0.8 - 1.0, so that 1240 peak is because of the rock.



For comparison, this is a typical "nothing burger" low activity rock sample - a piece of slate - it has a tiny bit of NORM stuff. There is a hint of the 1240 peak, but in comparison with K-40 as a yard stick, not much. This was about 1.6 cps compared to 1.0



This is a background without a sample, instead using a dummy foam spacer to keep the probe at the same location. About 0.8 cps with 4" lead, inner liner of pewter, then Cu down from 37.



I also tried ordinary rock salt, similar sized sample, and it produced no elevated counts. I'm probably going to have to do a proper XRF on both the lens and the rock assuming I can schedule an audience with the keepers of the magical XRF device we
have.

[Lposter]

I still think your energy calibration might be a bit "off" but its of minor consequence.

To take a look I made two spectra with MCNP - both with 1000 Bq of U238 and 232Th in equilibrium with no 235U. One on HPGe and one on NaI under the same conditions. With 40K. And with the list of peaks MCNP thinks is reasonable to put in.

[attachment=0]rock.jpg[/attachment]

Please note - spectra are in Ortec .chn format. rename pdf files to .chn. Peak list is MeV first column and isotope last column. The middle one is some MCNP normalized emission so its a relative measure of probability.

In the image, green line is HPGe, black is NaI and the blue lines are reference lines from Interspec for 238U in equilibrium.



Make of it what you will.

Lars

[Lposter]

Sorry....forgot.......the line that Interspec seems to be present at 1240-ish is a Bi-214 line at 1238.1 keV. With possible contributions from Ac-228 at 1245 keV and 1246 keV.

Please note - these are in relation to this synthetic spectrum. Im not saying this is what you are seeing in yours.

But based on your information - it would seem the peak you see in some materials is either an emission from some natural isotope or a peak from some other process like escape or whatever due to a process involving a natural isotope.

Lars

[Bob-O-Rama]

Lars, that is super helpful! I don't have much calibration points above 600. 800 and below the peaks are very much spot on. The K-40 peak is about 30 Kev too high, the "1240" peak may be more like 1215. I let it bake for a few days ( about 2M counts from the source ) then subtracted a day or so of BG, its pretty much monochromatic - not other significant peaks - so that excludes a lot of things.

[Mike S]

Bob,

What a fun mystery. I spent a little time over the last week trying to figure out what it could be and I couldn't come up with a solid explanation. I have two theories that I'll share for what they are worth, and some additional information I gathered. As a hobbyist, take my input for what it's worth.

Theory 1: I think this theory is the most probable. If your spectrum and background are not aligned with each other, your background subtraction could be accentuating the ~1243keV Compton edge of the K40, either from your sample or background. It may be more apparent when the sample is in place due to more Compton scattered gammas seen by the detector because the sample is right in front of it. To see if this might be the case, can you provide your spectrum (of the lens, and showing the mystery peak) without background subtraction, and (separately) your background spectrum (if possible with both on the same plot)?

Theory 2: This seems much less probable, and more of a thought exercise, but what if... There is something in lens that would normally give off multiple peaks but the majority of the low energy gammas are being blocked by the lens material. That scenario would have to be combined with the ~1230 and ~250 peaks being at sort of sweet spots for lens material absorption, relative peak height, detector response, and obscuration by background, so it doesn't seem terribly plausible.

I've have some spectra form someone that tested a lens and it does not show the same peak you are seeing. I need to step away, so I'll have to post that later.

[Mike S]

In addition to my attempts to figure it out myself, I also contacted the person who runs allradiactive.com because they had an article (https://allradioactive.com/why-are-vint ... dioactive/) about radioactive lenses that included a gamma spectrum. Because the spectrum they posted is cut off about 1MeV I asked if they could share the full spectrum including the higher energies. They were kind enough to respond with the following plot, but it does not show any peaks around 1230 keV:

They also reviewed this thread and provided the comments and two K40 spectra below:
" This looks very interesting! I'd say that the spectrum [in this thread] resembles K-40 gamma spectrum but that peak at ~1230keV is definitely an outlier. I made two gamma spectra of my KCl powder, one with RAYSID <7% (opened in Interspec) and the other with my Gamma Spectacular with a Bicron 2M2/2 NaI(Tl) and didn’t see any peaks around that energy."

Still nothing conclusive, but I wanted to share what I found.