Shield idea

[sgt_bear]

I own a scionix detetor that is 190mm long and has 44mm diameter, and i want to build a tube shield for it. For the most inner tube i would choose a standart copper pipe of a with a inside diameter of 53.98mm, 1.78mm wall thickness. Around the copper pipe i would make a tin-layer of 2mm for x-ray shielding, giving the construction a outside diameter of 57.98mm.

This construction i would place inside a steel pipe with inside diameter of 108mm, and fill the gap between the steel pipe and the construction with lead. This would result in a lead thickness of 25mm.

The Shield would have 300mm inside lenght.

For this i would need around 21 kg of Lead and 0.5kg of Tin.

Some of you already have build shields, so i'm asking if anyone knows if this idea is good, bad, or over dimensioned?

[MartinM]

Parameters look OK, quite similar to my shield: viewtopic.php?f=10&t=165
There you can find some measurements too.

[sgt_bear]

Parameters look OK, quite similar to my shield: viewtopic.php?f=10&t=165
There you can find some measurements too.

Very nice shield, i'm glad someone tried out lead balls! I wanted to use these, but was not sure about the result, since balls have air between them, but i see your project went well!

Also this version avoids smelting and casting! Thanks

I'm now planning to make a similar shield.

I can either use 2.5mm lead balls or fine granulate of 0.4-0.8mm, since the small balls are very expensive, i might use 2.5mm and then use the small ones to fill the remaining air-gap.

[MartinM]

Indeed, those Nickel-coated lead pellets were very expensive.
But the advantage is that you're protected from the toxicity of lead. And due to their relatively small size, the air gaps are rather small too, so you can still achieve a good attenuation.

[sgt_bear]

Indeed, those Nickel-coated lead pellets were very expensive.
But the advantage is that you're protected from the toxicity of lead. And due to their relatively small size, the air gaps are rather small too, so you can still achieve a good attenuation.

I have found a local supplier that offers a 0.7-1mm mixture pure lead at a very attractive price, almost at the price normal lead ingots are. I tend to use this type since i can pick it up locally and toxicity is not that to worry about if the shield is well enclosed. So in the best case, you only handle the pellets one time.

[sgt_bear]

I now have made further material selection and made myself a little excel calculator.
The "efficiency Part" i took from this topic: viewtopic.php?f=10&t=38

I will use a standart copper water pipe that is around 2mm of wall thickness, giving excellent copper shield and is readily available. I will wrap 5 layers of 0.5mm tin sheets, giving a total of 2.5mm tin shield

Seems like my Shield will be around 30kg total and will have a 30mm lead ring filled with lead balls. This should be equivalent to 26mm of solid molten lead...

I would need around 25kg wich would be optiomal, because my seller only sells 25kg packs of lead...

Sure with a 160mm outer pipe i would get 45mm of lead, but this would raise the weight to almost 50kg. But i guess 30mm will be also fine for homemade gamma spectroscopy :)

[Sesselmann]

Jonathan,

Nice handy excel calculator, thanks for sharing...

Steven

[sgt_bear]

Hello!

I finally had time to build the shield, here is the story about it:


My main problems with a shield from molten lead were that i don't have a space to do such things, since i live in the center of a town, so no garden or so to smelt lead. Also since lead fumes are considered toxic, it would be illegal to smelt lead without proper air filtering. So i decided to go with the lead balls (0.8mm, very fine), the company who sold it (Scherrer Metec) told me the weight of one litre would be 7kg, so its about factor 0.79 in dense, compared to solid lead.

A cheap source for the outer body of the shield was standart waste water pipe with 125mm inner width, 118mm on the inside. For the inner space i have choosen a copper pipe with 2mm wall thickness and an outside diameter of 54mm. I soldered a 1.5mm copper circle to the bottom to close the pipe.

My idea was to wrap 4x0.5mm tin sheets (99.99%, from http://www.pewtersheet.co.uk/) around the pipe, but since i didn't had the ability to cut them precise, there was a small gap in the wrap around of each sheet. This is why i decided to use fifth layer of tin, to get exactly at least 2mm of tin. I was not sure how flexible the tin would be when i orderd, but if i would build the shield again, i would use 1mm sheets instead of 0.5mm to save work :)

I filled the bottom of the plastic pipe with 30mm lead balls, and then placed the tin-wrapped copper tube inside, and filled the gaps with lead, the amout of lead is a bit less than 30mm which would equal around 22-23mm of solid lead.

To secure the balls, i glued a cap on the pipe and used silicon to close the cut out for the copper tube.

My plans are to build a top-cap with a smaller tube and only let the bnc cable through, but i have tested the shield already!


Test Results
Unshielded detector at background : around 50-70cps
Detector in shield at background : 6cps

A 10minute recording of background, and then used that recording as background substraction, the spectrum almost displayed nothing!


This leaves me pretty happy with the shield, since i have not excepted such a high reduction, i was excepting around half of the counts, but i got 1/10th! :)

Below are some pictures of the build process:

Regards
Jonathan

[luuk]

Hi Jonathan,

That is a real nice shiled you build, great job!!
I would like to see the background result that you have now with this shield, over a longer period maybe 1-3hours or more
is better.

Luuk

[sgt_bear]

Thank you ! :)

I did a 2.5hr test run today, the results are attached, one image with CPS the oder with counts.

There is an interesting peak at 1240keV, i currently have no idea what this could be. One option would be K-40. I only have calibrated to Am-241 and Lu-176, since these are the only one i have available, so there is a possibility that the higher energies are not in line with the calibration. I hope i get my Eu-152 and Co-60 Sources soon, to calibrate full spectrum.

Regards,
Jonathan