My first Mystery Source (Ra226)

[Mike S]

A friend dropped off some sources he's been collecting over the years. Many of them were physically identifiable (lantern mantles, fiestaware) but there was one small piece of thin metal that had a tag with unitless source check values and no other identifying information. I really wanted to figure out what it was. My pancake detector gave 120k counts, a little less on the other side. This is probably the hottest source I have dealt with so far and the large countrate really opened up the possibilities. I got the thing in a small plastic bag and in my shield setup and was seeing over 1000 cps with my 2x2" NaI detector. I pulled back the detector to get the counts down and got a clear spectrum quickly. I looked through various online sources and I'm confident it is Ra226. I have a small Ra226 source that I have previously tested and the spectra overlay nicely.

I did my best identifying the isotopes and adjusting the energy. The x-rays from my shield were very prominent, so I used the height equalizer to specifically knock that peak down so everything else was more visible.

It was pretty exciting to me, since this is my first genuinely unknown source that I used gamma spec to identify. Cool stuff.

--Mike

[Mike S]

After testing various other samples, I'm thinking the large lead peak is only partially from my shield. I ran the same sample using just the copper inner shielding and the lead x-ray peak is notably smaller, but still dominates the spectrum. I also did better low end calibration using Cs137 and Am241 sources. Now the big peak falls right on the Pb x ray energies (I think).

Are there any characteristics that can confirm this is Ra226 and not something else? I'm thinking that the lack of Thorium peaks supports the Ra226 conclusion, but I'm new to this.

Note that the large lead peak in the previous images has been reduced by using the height equalizer; that was not done on the image attached to this message. Also, I went to 10x bins to see if I could pick up more detail, but I'd need a much longer run to smooth it out (and I didn't wait).

--Mike

[Peter-1]

hello,
I have a small sample of fluorescent powder that used to be used to coat the hands of a watch. Ra226 was used in these substances to stimulate ZnS. The spectrum for Ra226 can also be found in old luminous hands on clocks.
I am showing such a spectrum here that I have recorded. The shield is built with 2.5cm lead and 8mm copper.

Peter

[Pavel M.]

Are there any characteristics that can confirm this is Ra226 and not something else? I'm thinking that the lack of Thorium peaks supports the Ra226 conclusion, but I'm new to this.

Hi Mike,
as you might already know, those typical peaks from approximately 150keV to 400keV (Ra226+Pb214) are sometimes called "The four fingers of uranium-238", because that part of the spectrum somewhat looks like a hand (the Bi214 peak at 609keV being a thumb). When you see it, you can be sure that there is some Ra226 in your sample. If you only saw "3 fingers" (with the Ra226 peak missing), it would mean that you are probably looking at the Rn222 decay chain.
Then you can start searching for other isotopes (U238, Th234, Pa234), to see if you are dealing with uranium or pure radium. If it's uranium, it might be depleted uranium (no significant U235 peaks), or natural uranium (mostly U238 with some U235 in it). Theoretically, there could also be enriched uranium, but I doubt that you will see such samples very often :-)
But please don't take me too seriously, because I'm also just starting out with gamma spectroscopy...
Good luck with your measurements!

[Sparky]

The only other spectrum that resembles Ra226 that I am aware of is Uranium in equilibrium (unlikely in this case). The emissions from Ra 226 and her daughters are so strong it can drown out that part of the spectrum above Radium. The "tell" for U in equilibrium are the Th 234 peaks at 63 and 92 keV. They kind of grow out as horns from that central lead peak at around 77 keV. The attached spectrum is of a Mosasaur tooth with and without shielding. It is a weak source and the Th peaks just merge with the lead x-rays in the un-shielded spectrum.

I was confused about the source of those lead x-rays at first also. My first spectrums were from a watch hand (unshielded) and I just figured there was some lead in the metal. I then tried a CDV-700 Ra check source, which is on an aluminum case, and the lead peaks were just as strong. I now think these lines are caused by beta decays that produce Bismuth. Linearity below 100 keV is tricky and I place the "lead" peak at about 77 keV (Bi Ka1) and the right-side "hump" at 87 (Bi Kb1). I use Bismuth x-ray lines because a friend with a high resolution detector assured me that they were Bismuth and there are multiple beta decays to Bi in the decay chain. Anyone have thoughts about this? Gordon Gilmore's "Appendix C" lists both. They are so close to lead it doesn't matter much, but placing these x-ray lines correctly helps you look for those weak Th234 peaks.

Clean DU and NU without progeny don't have Lead or Bismuth or Radium and their spectrums look very different.

BTW, FWI, I am amateur also and frequently wrong. Wrong AF.

That is an interesting source. I wonder how well sealed it is? Maybe you should place it in a plastic bag for a few days and then do a wipe test with a pancake.

Mike L.

[Mike S]

Thanks everyone!

That's all very helpful. I've been testing some other samples that are labeled as uranium ore and I couldn't figure out why I wasn't seeing thorium. Well, I thought I might be seeing thorium buried in the lead peak, and now that seems likely. I'll attach a couple plots of the ore samples and if you look hard you can see some inflection around the area where I would expect to see thorium.

[EM-OS-01 was 3000 cpm and done without a lead shield, just the copper inner shielding and background subtraction was used. EM-OS-02 is 500 CPM and was done in my full shield without background subtraction.]

Any suggestions on what bump at about 150 keV might be? I want to say it's U235, but it doesn't quite line up. I'm wondering if it is a backscatter peak.

Right now I only have a 2mm copper inner shield, so with my setup I can either have a lot of background or a lot of lead x ray; both of which make a mess trying to pick up the thorium peaks. I'm working on obtaining materials for improving my inner shield.

Mike L. this is an odd source. I don't think it is sealed at all unless it has been coated with clear paint or something similar. When I took it out of the box, I picked it up by the tag and dropped it in a plastic bag. My 120k CPM reading is only on one side (I think the other side was 10k-20k CPM), so I'm guessing they painted some radium on one side of a piece of metal. If I have a chance I'll do some experiments to determine roughly how much of the count was from alpha, betta, and gamma. When you say do a wipe test, are you saying to do a wipe test of the bag after the source has been stored in there awhile? Is the intent to see if radon is transporting daughters off the source and onto surrounding surfaces? Will a plastic bag keep radon in?

Thanks for all the help!

Mike S.

[Sparky]

Mike S

If you look closely at the fat peak centered at 77 keV, there is also an inflection point on the low energy side at about 60 kev. Th234 has another peak at about 63.

I like your idea about the peak at about 150 keV being a backscatter peak. I labeled it as Pb214 on the Mosasaur tooth Spectrum, but that was wrong. The calculator below indicates that 150 KeV is about right for a 180⁰ backscatter photon from the big Pb214 peak at 352 kev, and your source is mounted on a metal plate that would be conducive for this. I went back and looked at the spectrum I did with a Radium watch hand vs a CDV-700 check source. I figured the CDV-700 case is a thicker metal backstop than the watch hand and should have a bigger peak at 150. It does. I might experiment with this some more.

Regarding the wipe test, I would check both the plate and the inside of the bag. The idea is that a few days should be enough to accumulate Radon decay (Pb214) products and also find out if it is shedding Ra226. That thing is about like 10 or 12 watch hands, worthy of respect.

http://www.sciencecalculators.org/nucle ... cattering/

[Sparky]

Regarding that Peak at 150 keV, I think it is from 180 degree back scatter photons from the Pb214 peak at 352keV. I did a little experiment with a Radium Watch Hand, with and without a copper back scatter target. I oriented the watch hand pointing away from the probe (to maximize the difference in spectrums), and then gathered about 500,000 counts with and without the target. It looks pretty convincing to me.

[Mike S]

That's a really cool result. Thanks for sharing your experiment.

Is there any way to reduce the effect of backscatter on a spectrum? I suppose if there's no nearby targets it would help (as your experiment shows). Is there a way to shield or absorb the backscatter with different materials, or is it something that you just have to live with? (Remember I'm new to all this)

--Mike

[Sparky]

A bigger shield (no nearby targets) is the only suggestion in the Gordon Gilmore book (sec 2.5.2) to reduce backscatter noise. Not a very cheap option. If you don't have his book already you can download it at the link below.

https://epdf.pub/queue/practical-gamma- ... scopy.html