Casting a Lead shield for Scionix 38B37 /w Red Bull can

How to build the perfect shield for your measurements? Discuss it here!
kotarak
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Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by kotarak » 09 Jul 2020, 20:34

I just cast a lead shield for my spectroscopy setup (Scionix 38B37 probe)

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These are the tools I used for the casting process in addition to about 6kg of "soft Lead ingots"

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The key component is the "5kg Casting Clay Graphite crucible" purchased on eBay for $30.

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The Lead ingots were melted using a propane "Weed torch". I constructed simple furnace out of concrete blocks. After all of the lead melted, I removed the floating oxides with a steel spoon and turned off the flame. The crucible keeps the lead as liquid for at least 7-8 min with no flame so there was a plenty of time to work with it.

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To make the opening for the probe, I used an 8oz can of Red Bull, tightly packed with fine sand and clay. The can was inserted and centered into the molten Lead and held in place until the Lead solidified. A word of caution (aside from the dangers of working with a large volume of molten metal :) - nearly everything floats in molten lead so inserting the can all the way to the bottom of the crucible, centering it and holding it in place until the lead cools down - all, while fighting with the can's buoyancy was a bit of a chore - thick casting gloves and a steel rod helped a lot. (it took 2 attempts until I got it right in terms of procedure and Lead quantity). The bottom of the aluminum can had to be submerged initially at an angle to allow the air to escaped from the concave cavity. This picture is from my first attempt - l kept this first cast and turned it into a "lead pig".

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The cast shield, after emptying the can of sand and peeling it off the internal wall of the cast. A few taps with a small hammer punched the hole where the bottom of the can was - the lead was super thin due to the concave can bottom and it worked perfectly.
The thickness of the shield wall where the crystal of the probe resides is more than 22mm and it tapers off (after about 60mm height) to 6mm wall thickness at the very top.
As the lead cools down, it shrinks before solidifying, leaving an uneven void on the top of the cast. I used a propane torch to melt another 500g ingot and "top it off" filling all of the gaps and smoothing it out while the cast was fairly hot.

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All it took is just a few gentle taps for the cast to come out of the graphite crucible and the result is nearly perfect!
The inside diameter is 55mm and overall height is 110mm (the dimensions of the crucible would have probably allowed me to add yet another 10mm of height)

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Here is the shield, in place on top of the sample holder.
The outside diameter of the shield matches perfectly the aluminum flange of my sample holder. I'll be using a strip of rolled lead sheet (1/8" thinkness) as a 25mm thick "collar" overlapping the aluminum flange and the bottom 50mm of the main shield.

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This is the final result.
The whole sample holder will be placed in a small lead castle made out of 1" thick lead bricks - all around, overlapping a bit the bottom of the probe shield.
Start to finish it took about an hour to complete the cast.
There is a plenty of space for an aluminum or copper insert around the probe.

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I used the cast from my first attempt and filled the bottom of the hole with about 500g of molten lead, creating an improvised "lead pig" for larger samples.

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this is what it looks when both pieces are put together.
Last edited by kotarak on 12 Jul 2020, 22:39, edited 1 time in total.
Andrey E. Stoev
Brookfield, Connecticut, USA

Mike S
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by Mike S » 11 Jul 2020, 01:54

I love it! That's a great way to make a very useful shield. I like how it is already optimized to put more lead around the crystal.

Thanks for sharing, I enjoy seeing creative shield concepts and implementation.

Mike S.
Mike Sullivan
Central Coast of California, USA

kotarak
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by kotarak » 24 Jul 2020, 01:40

Here are the Shield Components and the complete setup:

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1.3 cm of Lead sheet (4 turns of 1/8" thickness) rolled onto a 6 inch section of 4" PVC Schedule 40 pipe to serve as an outer sleeve for the 2 lead shells. About 0.5mm of copper foil was rolled first onto the PVC and then the lead was rolled on top of the copper. Some cosmetics were in order so it doesn't look so boring and trivial.

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I built a wooden box and placed a base of 1" thick lead bricks on which the bottom shell is situated. The two lead shells were lined on the inside with copper and additional 1mm copper inserts are also placed.

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All components of the shield - a total of 23+ kg Lead. The box makes it convenient to move the shield components around.

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This is the complete setup as it stands today. Total Lead thickness around - sides is about 32mm. On the bottom, under the detector, the thickness is approx 75mm.
Last edited by kotarak on 24 Aug 2020, 09:15, edited 1 time in total.
Andrey E. Stoev
Brookfield, Connecticut, USA

Conor Whyte
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by Conor Whyte » 16 Aug 2020, 16:22

Very nice. This is definitely going to be my next project. I really do need a large lead shield --
Great pictures and construction details!

Well done sir.

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Svilen
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by Svilen » 16 Aug 2020, 16:54

Great Job, Andrey. Bravo! For safety you better cover or paint the inner lead component too, but I guess you are gonna do that already.
How much is the background reduction ratio inside the castle?
Svilen

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GigaBecquerel
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by GigaBecquerel » 16 Aug 2020, 17:41

I really like your shield, it's simple and easy to DIY!
My only "complaint" is that the copper inside won't do much, the 80 keV XRF from lead has a halve value layer of around 1 mm in copper, so to get appreciable shielding you'd need 2 to 3 mm of it.
Most hardware stores have copper tubing for rain gutters, maybe line your shield with a few layers of those.

kotarak
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by kotarak » 17 Aug 2020, 01:32

Svilen wrote:
16 Aug 2020, 16:54
Great Job, Andrey. Bravo! For safety you better cover or paint the inner lead component too, but I guess you are gonna do that already.
How much is the background reduction ratio inside the castle?
Hey Svilen,
With my first version of the shield (which is what I posted above) the count rate dropprd from an average of 105-115 cps down to less than 10 - usually between 5 and 12 cps. I would say it is a factor of 10 reduction.
The total lead thickness during this test was 3cm.
In my final version, which I will test probably tomorrow, the thickness is close to 5 cm.


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Here is what the count rate looks at the moment of the insertion of the detector into the shield.
Andrey E. Stoev
Brookfield, Connecticut, USA

kotarak
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by kotarak » 17 Aug 2020, 01:50

GigaBecquerel wrote:
16 Aug 2020, 17:41
I really like your shield, it's simple and easy to DIY!
My only "complaint" is that the copper inside won't do much, the 80 keV XRF from lead has a halve value layer of around 1 mm in copper, so to get appreciable shielding you'd need 2 to 3 mm of it.
Most hardware stores have copper tubing for rain gutters, maybe line your shield with a few layers of those.
Hi GigaBecquerel,
Since my last post I made a number of improvements while still keeping it simple.
I made inserts for the bottom and top portion of the cast, comprised of a about 1mm of tin /pewter foil and 1 mm of copper foil. The inserts are actually a "sandwich" - copper-tin-copper. In addition, I used a copper pipe end-cap with a thickness of 1mm for holding the actual sample into the lower lead cast.

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You can see the secondary shielding insert. I added lead brick in order to raise the lead sleeve up.

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The top of the lead sleeve now sits flush with the top cast. I also added added a bit more lead to the sleeve so total thickness of the lead sleeve is 1/2" (12.7mm)

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I made a collar out of 4" PVC pipe and 8 lead ingots jammed inside for the top of the detector.

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The collar is placed on top of the sleeve shielding the top portion of the detector protruding from the cast.

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This is the final shield configuration - a lead cap on the top with a hole for the cable and lead bricks all around the outer sleeve. Not sure if the bricks make a huge difference but generally you can't go wrong with more lead. Still, the setup is pretty modular and components can be used in different configurations. It is more convenient this way than having one huge, massive barely movable castle - I've already taken it on the road.
Andrey E. Stoev
Brookfield, Connecticut, USA

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GigaBecquerel
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by GigaBecquerel » 17 Aug 2020, 04:38

Nothing left to add to it!
Simply looks good and will perform perfectly, I'm sure.
Well done!

kotarak
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Re: Casting a Lead shield for Scionix 38B37 /w Red Bull can

Post by kotarak » 18 Aug 2020, 02:26

Seems that the improvements I made by adding more lead bought me 1-2 cps lower background.

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12 Hour shield background

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Count rate.

The average background inside the shield dropped down to 3.2 cps. It used to be around 5cps. Still pretty good reduction from 120cps unshielded.

A peculiar and unexplained event occurred 2 hours after the beginning of the recording and it was the only anomaly during the 12 hr scan - there was a weird excursion of 143cps inside the shield!

Any ideas what this could be? I've seen such anomaly only once before again in the same scenario - a lengthy background scan with a shielded detector.
Is it possible that this is a some sort of cosmic ray burst?
Andrey E. Stoev
Brookfield, Connecticut, USA

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