Tin, and Pewter, and Copper, and Iron, Oh MY!

[Mike S]

Short question: Is pewter (tin alloy with antimony and copper) an acceptable substitute for pure tin for use between copper and lead? Any other good options.

Background:
I'm putting together a basic shield and I'm struggling with choosing and buying the inner portion to reduce lead x-ray.

At first I thought the extra effort to put copper and tin as inner layers under the lead was probably more than I needed at this stage in my hobby. So I built a simple lead-only partial shield and it knocked down the background nicely from 125 CPS to 25 CPS. I thought that would do what I wanted until I saw that the spectrum now had a very large lead x-ray peak. No problem, I'll just buy some copper pipe and some tin sheet and knock this thing out...

First, copper pipe large enough for my detector was quite expensive and not available from a normal hardware store. I did find someone selling one foot lengths on ebay and have a piece on order. MUCH more expensive than I expected, but, well, moving on...

I really got stumped with the tin sheet:

- Tin sheet is very expensive and hard to find.
- I looked into using all copper, zinc, or steel but they have a much thicker HVL and the price starts getting expensive in the thickness I want. Also, the larger the inner shield, the more lead I need to keep the same thickness. If I had access to short cut-off sections of material, steel could be a good way to go, but even a short section of steel pipe with a 3 inch (~7.62 cm)ID with 10mm wall thickness is hard to find, and longer sections get expensive fast.
- I considered buying tin shot because it is much cheaper than tin sheet. My thought was to 3D print a cylindrical container that would hold the tin shot. But tin shot tends to be large enough that I'd have to have a large volume to assure it could pack efficiently and have sufficient effective thickness (back to more and more lead, plus it's not that much cheaper).

I have seen posts where folks talk about using pewter, which has significant antimony and a little copper (usually?). Does the antimony cause any issues? Is this really what people are using, and not purer tin? Pewter is still expensive (it's still mostly tin), but at least there's a few more purchasing options (Contenti is the best option I've found so far).

If there's a better source, method, or material, please let me know. Otherwise, I'll brace myself and move forward.

Mike S.

[sgt_bear]

What do you consider expensive?
I've used a ~50mm copper tube for my shield, and added 4 sheets of tin (320x200x0.5mm). The sheets were 30£ in total. 2mm tin is enough for shielding pretty good.

I got the sheets from pewtersheet.co.uk

[Mike S]

For reference, I've only spent $20 USD on lead at this point, so I'm still adjusting to the cost ratio for the inner layers.

The pure tin solutions I've found so far are $120 USD or more.

Conteni has a 2mm pewter sheet that will work for me for $47 (plus unknown shipping, but in USA). That's still more than I was expecting, but if that's the best I can do, then I can accept that. I still want to be sure that pewter is acceptable in place of pure tin.

Mike S.

[iRad]

I usually order from a seller called greenstuffworld, their tin sheet found here... https://www.ebay.com/itm/282104195838
The company is located in Spain and their price and service are the best I have found yet outside of China.

[Sesselmann]

Mike,

I buy my pewter from a company in the UK, they respond quickly and the sheets come nicely packed, the price is not too bad.

https://www.wentworth-pewter.com

Steven

[Mike S]

Thanks for the input folks!

Nobody has explicitly said that pewter is OK/almost as good/just as good as tin, but since that seems to be what folks are using, I'm going to assume pewter is OK and the antimony doesn't cause problems.

My copper pipe came in yesterday and I was able to run a test using it as the inner layer. The pipe has a little over 2mm wall thickness. I'll post some images comparing the background with and without the copper as well as with no shielding (CPS used to keep comparison even and I did my best on the energy scaling). Pretty interesting stuff. I calculated the copper is about 1.3 HVL for the lead x-ray and the reduction is pretty close to 1/2. Considering there's no copper on the lead that's below the detector (it's only around it), that seems pretty close to prediction for my napkin math.

The slight increase between 125 and 350 is interesting. Anyone have any idea on the cause, or is it just measurement variation?

Mike S.

[Peter-1]

Since I don't get pure tin, I decided to use copper. So I have a first shield with lead and then a layer with 8mm copper. To get an overview of how good the shielding is, I wrote a small program for myself.

I'm sorry that it is in German.

Peter

[iRad]

Why not just order the TIN sheet from my reference above...
The vendor told me it is pure tin, not alloy or pewter, although they use "pewter" as a search word in their listing. I'm not a chemist or metallurgist to know for sure, but the metal I receive from them is pliable like tin and shiny like tin, whereas pewter is usually more grey and darker color. The price is very cheap and they always shipped very quickly in the past, however I don't know what will happen with an order placed with them now. I'm sure if you live outside of EU it could take more time to reach you than when ordered in better times.

[Mike S]

Peter, that looks handy. Maybe I will make a spreadsheet or see if there's an online calculator somewhere.

Tom, the ebay seller you linked is one of the sources I am considering; I just haven't finished "shopping" yet.

****

Today's "learn by doing" lesson: While the lead x-ray from background was significantly knocked down by the copper pipe I added, when I put a fairly hot source (120k cpm measured with a pancake) in the cavity, the lead x-ray became dominant in my signal (also, I don't have copper coverage everywhere). I probably would have been better off without a shield since I had plenty of counts, but in this case I actually wanted to keep the dose down since I sit close to the detector. I'll post my spectrum in the spectrum section.

--Mike

[Sparky]

I just rolled up .007 inch X 6 in wide X 100 inch long brass shim stock and dropped it into my copper tube core an let it expand outward. .100 (2.5 mm) thick. Not cheap ($40-$50), but the fit is perfect and it seems to work well.

Mike L