DIY Radiation Shilded Chamber - for Gamma spectroscopy

I want to build a radiation shielded chamber for Gamma spectroscopy using my Radiacode 103. After batting some ideas about on another thread this is what I have so far:

• Outer case is 6mm steel (mild steel) - This could be a square tube section, or made from sheet.
• Inner case is the same as the outer and has 60mm x 60mm to 75mm x 75mm (thereabouts) space in the center for the Radiacode and source.
• Internal length of the chamber will be ~200mm.
• Space between the inner case and outer case will be filled with lead, ~25mm thick.
• Tabs/flange on both caps and outer case to put bolts through to fix the end caps in place when it is being used. The bolts could be thumb turn screws or similar.
• Caps made to slide over the outer case with ~25mm lead in each end.
• 0.5-1mm copper lining the chamber walls and end caps.

I should be able to knock this up fairly easily from scrap. If the outer and inner case need welding, the welds can be done on the inside of the outer case and the outside of the inner case as the distance from corner to corner is about 10mm more than the sides, so a few mm of weld wouldn't compromise anything. All welding should be completed before filling with lead. Once the outer case and the inner case are complete, the inner can be stood up on end and the outer case centered over that, then the void between can be filled with lead. A temporary bottom plate will be necessary and the two case parts (inner and outer) can be stuck down using silicone sealant or similar to stop molten lead escaping. Or just hold in place and let the lead seal any small gaps as it solidifies. Keeping the temporary plate cool should aid in this. Here are some 3D images I knocked up:

 
Complete chamber, assembled as if in use.

 
Chamber with both end caps detached.

 
Cut-away view of the chamber showing 6mm steel on the outside, then 25mm of lead, then another 6mm steel, and finally 0.5-1mm of copper sheet lining the chamber.

 
Exploded diagram of the chamber, for the nerdy.

 
Exploded diagram of an end cap, for the nerdy.

 
This picture shows the temporary plate on one end whilst filling with molten lead.

Whilst it is not lab grade, this should remove >90% of the background in the low energy range, which is ideal for sampling radioactive sources.

fabricating from flat plate seems a lot of work when you can buy rectangular tube fairly easily

Agreed, it will be a lot of work. I have just had a quick look on ebay and after looking at a few listings I am unable to find square section with a 6mm wall. However when I was looking at round rube section, there were plenty with 1/4", which is what I have based my 6mm on, as this all stemmed from using round tube. I need to go and see my metal working friend and he will probably find the best solution, but the following criteria have to be met: Cost as low as possible, preferably free. He has tones (I mean literally, tons) of scrap metal laying about all over. He also has the big tools I'd need - hefty welder, saw etc. I'll probably go over a couple of Saturdays and just make it from bits laying around. It is more work, but I can make it exactly as I want, and for little to no cost. I'm happy to work more to spend less. The walls would only need 3 or 4 tacks to hold it together. The molten lead will fill any large gaps. And if it all goes to s**t, then hopefully I'll learn something! 

Agreed also about big clamps to hold a temporary plate on the bottom. I don't have any metal clamps large enough, only plastic ones. They are sure to get damaged. I do have access to some sash cramps though. They may work. Or I can just use some glue/silicone/some other sticky shmoo. It is only whilst the first layer of lead is poured. After that it shouldn't be an issue.. I understand you wanting to use tube. That's probably the easiest for you, but I have set my heart on a square construction now, for myself. The shape is kind of academic, if it suits it's purpose.

If you are going to buy a camping stove, have a look at those large blow torches too (something like this) . It might be better for you than a camping stove. You could cobble together some kind of stand on steel or bricks or something, wedge the torch nozzle somehow and use that. It would be much more efficient than a camping stove, and have other uses. I already have a stove, so I don't need to buy one. And with a blowtorch heating the top, it doesn't fair too bad, for occasional use, which is, very occasional.

A few reasons I want to use thick steel.

• It does make a difference, albeit very small. 2mm extra lead would probably compensate for 6mm less steel.
• I'm not the best welder, and I'm way less liable to blow holes in, or warp 6mm steel compared to 2mm. A grinder and a sander can make anyone look like a welder.
• My metal working friend does mainly structural work, so most of his scrap is 6mm, 10mm, and just gets bigger. If I can use his scrap, it will be much cheaper for me.
• I want it to be like a brick-s**thouse!
• I may, possibly, one day, want to use it for a DIY X-Ray generator, but again, it wouldn't matter much for that either.

It's just about what is available to me, for cheap and what I can easily work with.