Modular Shielding. Mind the Gap?

[Mike S]

I'm looking at various designs for my upgraded shield, but I do want to go with a modular design to keep component weight down, allow different configurations, and allow more flexibility when upgrading. I'd also like to keep the lead inclosed (probably in 3D printed plastic cases), so no matter what I do, there will be some sort of gap between sections. I think I can easily design something that keeps the gaps such that they don't have a straight shot to the active region of the detector. Designing (well, mostly building) something with overlap that covers all gaps, is significantly more challenging.

Are those background zoomies going to bounce around and hit my detector once they squeeze through a gap, or are they generally going to be absorbed? I'm sure occasionally there will be a gamma that's at the perfect angle to get through, and then scatters such that it hits the active area of the detector, but is that really a practical concern?

Short question: How big of deal is a gap in the shielding that doesn't have a straight shot to the crystal?

Related: is it as important to have tin/copper for lead x-ray suppression as you get farther from the active volume of the detector? how about in the "gaps" between sections of modular shielding?

A crude conceptual sketch showing the cuttaway of my proposed design is attached.

Thanks!

Mike S.

[Sesselmann]

Mike,

How you construct your shield depends entirely on what level of background suppression you need for your experiment in order to get statistically significant results. There are online calculators that will give you attenuation for various materials at various energies. Keep in mind that most of the background counts are in the low energy range, so just a single sheet of lead or two will remove half the counts, and as you add more lead you get diminishing return, so it comes down to how much you want to spend and how much you need.

Proper lead bricks are designed with interlocking V shaped bricks to prevent those gaps you refer to above, but providing the gaps are not in line with the crystal the shine through will be minimal.

When it comes to the copper/tin lining, this is purely to suppress secondaries from the internal radiation from your source, not for the background, the lead does that.

best of luck and let us know how it goes.

Steven

[Mike S]

regarding background suppression, recently I ran a spectrum on a piece of trinitite that only increased my count by 5 cps. My normal background is 125 cps and my shield at the time brought that down to about 25 cps. I wanted get the background lower for working with low activity samples and also suppress the lead x-ray peak.

I've crudely assembled something on the way to what's shown in the whiteboard sketch, but without the top cap, less coverage for the bottom plate, and incomplete coverage for the top section, but the lead is not inclosed so the gaps are tight. That brought the background down to 18 cps, and importantly, got rid of the lead x-ray peak. I have 11 lbs of additional lead sheeting on the way so I can make the baseplate, top cap, and add full coverage to the top section. I'd love to get below 10 cps, but I think 15 cps is a reasonable goal.

Regarding copper and tin, what I'm trying to understand is if the need for lead x-ray suppression is less as you get farther from the active area of the detector. At this point I'm not planning to add (expensive) pewter to the upper section of the main tube (which will at least have the 2mm of copper pipe) or the top cap, but I will have 1mm on the bottom plate. Because I have copper available, the bottom plate will also get 3mm of copper and the top plate will get 2mm of copper.

Mike S.

[Mike S]

Here's a better drawing of what I have in mind. This is just the metal parts; I have not added any plastic enclosures in this drawing.

Mike S.

[Sesselmann]

Regarding copper and tin, what I'm trying to understand is if the need for lead x-ray suppression is less as you get farther from the active area of the detector.

Mike, there is actually an advantage to have the detector further away from the shield, but as you can imagine the cost of lead goes up dramatically as you increase the diameter. Professional lead enclosures often have an internal diameter of at least three times the detector. You can imagine how this reduces reflection.

Steven