Pb-210 Spectrum - No, Really, I Promise

[Bob-O-Rama]

This is Pb-210 collected on plastic cling wrap lining the insides of a radon box - which contains uranium ore. Radon decay products build up on the wrap, and after about 2 weeks, its very close to a stead state radon concentration. The wrap is carefully doubed over so the contaminated surface is "inside" and its folded over a few more times to get it to a convenient size. Then laminated. Fresh from the box, its about 4 Kcps. Letting the cling wrap accumulate Pb-210 for a month, and then "resting" to burn off radon and other short lived decay products gives this:



The ~47 keV and ~15 keV peak are there. But the rate is about 0.05 cps. From the initial activity and this result, the long term radon concentration is about 25,000x the action threshold for radon. Accounting for the whole surface area, its more like 125,000x - about 1 ppm of radon in air.

[ColoRad-o]

Hello Bob. As you know, the decay of 222Rn is a tad complex and almost all of the radiation dose from Rn is in fact from its decay daughters. Can you fill in the steps that lead you to the conclusion above? I attach (no background subtracted yet) what I saw for a radon+daughters count.

[Mike S]

Bob,

That's an interesting technique that I have not heard of before; thanks for sharing.

The references I've seen for Pb-210 show the 47keV peak, but nothing (specific) at 15 keV. I did find some references showing Bi x-rays in that region, so I assume that's what is causing that peak. Is that your conclusion or do you think the 15 keV peak is from something else?

[Bob-O-Rama]

Hello Bob. As you know, the decay of 222Rn is a tad complex and almost all of the radiation dose from Rn is in fact from its decay daughters. Can you fill in the steps that lead you to the conclusion above? I attach (no background subtracted yet) what I saw for a radon+daughters count.

This spectra is for a sample of radon decay plateout aged for about a month to let the short lived RDPs to burn out entirely leaving just the Pb-210. Prior to letting it rest. it lined the lid of a radon box, which I use to concentrate radon "as one does." This radon box reachs as stead state concentration of > 1 uCi/L of Rn-222 - 250,000 times the EPA action limit. So basically it is the "baloon experiment" with 250,000x the radon concentration and 1500x the exposure interval. And even with that, the Pb-210 is hard to detect without 4" of lead and a Z-graded shield. Its also a pain because it is a low probability gamma emitter.

[Bob-O-Rama]

Bob,

That's an interesting technique that I have not heard of before; thanks for sharing.

The references I've seen for Pb-210 show the 47keV peak, but nothing (specific) at 15 keV. I did find some references showing Bi x-rays in that region, so I assume that's what is causing that peak. Is that your conclusion or do you think the 15 keV peak is from something else?

Good question. In the USGS gamma / xray photopeak manual, it has an entry at 17 keV for Pb-210 - so I stupidly believed the US government. But nobody else seemed to have that, and on looking at R. L. Heath's compendium, Pb-210 has a low probability alpha decay route to Hg-206, which produces a lot of Hg K x-rays.



While the alpha decay to Hg is unlikely, its chances of producing x-rays is high? But its worth noting that the 15 kev peak is entirely absent in background measurements, or spectra of other thing I test. If it were XRF from the shield / probe it would be seen it on other things.

[ColoRad-o]

Bob--thanks for all the clarifications and, as others have said, the description of the technique.

[Sparky]

Pb210 decays to Bi210 by beta emission. I am thinking maybe this 15 keV line is a decay x-ray for this reaction. My x-ray tables for Bi shows characteristic X-rays in the L shell at 10.8, 10.7, 13.0, and 15.2 keV. I have only seen K shell decay x-rays, but maybe this is not energetic enough to produce them. ????

[Lposter]

I dont want to barge in...but does 210Pb not have a strongish peak at 15.5 keV or so?

If as the picture shows 47 is being identified at 48 point something...then taking 1.5 keV from 15 pyts you within shouting distance of 12.5 keV?

Lars

[Mike S]

Good question. In the USGS gamma / xray photopeak manual, it has an entry at 17 keV for Pb-210 - so I stupidly believed the US government. But nobody else seemed to have that, and on looking at R. L. Heath's compendium, Pb-210 has a low probability alpha decay route to Hg-206, which produces a lot of Hg K x-rays.

While the alpha decay to Hg is unlikely, its chances of producing x-rays is high? But its worth noting that the 15 kev peak is entirely absent in background measurements, or spectra of other thing I test. If it were XRF from the shield / probe it would be seen it on other things.

Thanks for the thoughtful response. It seems like there's consensus that the ~15keV peak is from x-rays. Since Hg, Pb, and Bi can all produce x-rays in that general range (particularly if we assume some non-linearity or calibration offset of the spectrum at that low energy) so that makes sense.

I don't know if I've seen the USGS gamma/x-ray photopeak manual before you mentioning it here (or maybe I just forgot about it). The only version I could find is from 1978, so it seems possible Pb210 understanding has improved since then. Even though it appears to have let you down in this instance, it's a new resource for me; I can see myself using it to identify possible sources for a mystery peak and then drilling down on the candidate peaks using other data sources. Maybe I'm the only one that didn't have this already, but just in case anyone else doesn't have this potentially useful document (if you keep in mind its age), it can be found here: https://www.usgs.gov/publications/a-tab ... geophysics

Mike S.

[Bob-O-Rama]

The only version I could find is from 1978, so it seems possible Pb210 understanding has improved since then.

I would be eager to find a better modern resource, preferably one I can print out and mark up. InterSpec has the data, from what I can tell. But not easy to do what-iffing.