Shield Lining for X-Ray Suppression

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Steven Sesselmann
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Shield Lining for X-Ray Suppression

Post by Steven Sesselmann » 22 Sep 2019, 20:56

I often get asked about copper lining of lead shields and how thick to make the lining, so with a bit of time on my hands today I ran a couple of experiments to determine the effect of copper and tin on the unwanted Pb x-rays that show up in our spectra.

So just a quick recap for those new to this:
When a gamma ray strikes lead, some of the energy from the photon interacts with the electrons of the Pb atom, causing it to fluoresce or emit x-rays at around 80 keV in our spectrum (As a general rule atoms with higher Z emit higher energy x-rays). This can be a problem if we are looking for other gamma rays or x-rays in this region.

The solution is to apply one or more layers of shielding to attenuate the 80 keV Pb x-rays, but since all materials emit x-rays the objective is to find a high Z material that emit's x-rays at lower energies, which is in itself a contradiction in terms.

The popular solution to this problem is to add a layer of Tin (Sn), tin has a relatively high Z and is pretty good at attenuating the 80 keV Pb x-rays, but Tin emit's its own x-rays at 28 keV, so adding a further layer of copper can attenuate the Sn x-rays.

Copper also emits x-rays, but those x-rays are in the 5 keV region and outside the range of most gamma detectors.

X-Ray Fluorescence energies can be looked up here:
http://nucleardata.nuclear.lu.se/toi/xr ... list&el=Pb
http://nucleardata.nuclear.lu.se/toi/xr ... list&el=Sn
http://nucleardata.nuclear.lu.se/toi/xr ... list&el=Cu

Tin in the form of Pewter can be obtained in sheets, I found a supplier in the United Kingdom here: https://www.wentworth-pewter.com (No association)

The Experiment

I set up a low energy X=Ray detector facing a sample sheet of lead at 45˚ angle with a Cs137 source perpendicular to the detector and shielded so there were no incident gamma directly hitting the detector, only the Pb plate, and recorded a 1 hour spectrum.

The experiment was then repeated using a sheet of 0.7 mm copper in front of the lead, and again with 0.7 mm copper and 0.7 mm ti, and again with two sheets of tin and finally three sheets of tin.

Conclusion
What I discovered was that copper alone is a poor attenuator of 80 keV x-rays and with one layer of 0.7 mm Cu, I achieved an 11% reduction.
Adding just one layer of Sn (Pewter) 0.7 mm between the copper and the lead gave a much better 33% reduction in x-rays
A second layer of tin improved it further and reduced the x-rays by 45%.

Based on this experiment I would project that a reasonable lining to aim for would be around 2-3 mm of Pewter and 1-2 mm of copper.

Copper and Tin are not the only materials you can use, ordinary iron for example has a low energy x-ray but you would need a much thicker layer to give the same attenuation.

Steven
Shield-lining.png
Shield Lining Experiment

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Go-Figure
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Re: Shield Lining for X-Ray Suppression

Post by Go-Figure » 23 Sep 2019, 02:35

That's brilliant Steven!
I am about to report about an experiment where I tested the first version of my shield and, with copper in place, fluorescence from the shield is loud and clear.
I am already moving on pewter :)

Thanks for the tip.

Massimo

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